HP 5920 User manual
HP 5920 & 5900 Switch Series
Layer 3 - IP Services
Configuration Guide
Part number: 5998-2894
Software version: Release2207
Document version: 6W100-20121130
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Contents
Configuring ARP··························································································································································· 1
Overview············································································································································································1
ARP message format ················································································································································1
ARP operating mechanism ······································································································································1
ARP table···································································································································································2
Configuring a static ARP entry·········································································································································3
Configuring a multiport ARP entry···································································································································4
Configuring the maximum number of dynamic ARP entries ·························································································4
Setting the aging timer for dynamic ARP entries ···········································································································5
Enabling dynamic ARP entry check ································································································································5
Displaying and maintaining ARP·····································································································································6
Static ARP configuration example ···································································································································6
Network requirements··············································································································································6
Configuration procedure ·········································································································································7
Multiport ARP entry configuration example····················································································································7
Network requirements··············································································································································7
Configuration procedure ·········································································································································8
Configuring gratuitous ARP ·······································································································································10
Overview········································································································································································· 10
Gratuitous ARP packet learning··························································································································· 10
Periodic sending of gratuitous ARP packets ······································································································· 10
Configuration procedure··············································································································································· 11
Configuring proxy ARP··············································································································································12
Enabling common proxy ARP ······································································································································· 12
Enabling local proxy ARP·············································································································································· 12
Displaying proxy ARP···················································································································································· 12
Common proxy ARP configuration example··············································································································· 13
Network requirements··········································································································································· 13
Configuration procedure ······································································································································ 13
Configuring ARP snooping········································································································································14
Configuration procedure··············································································································································· 14
Displaying and maintaining ARP snooping ················································································································ 14
Configuring IP addressing·········································································································································15
Overview········································································································································································· 15
IP address classes·················································································································································· 15
Special IP addresses ············································································································································· 16
Subnetting and masking ······································································································································· 16
Assigning an IP address to an interface ······················································································································ 17
Configuration guidelines ······································································································································ 17
Configuration procedure ······································································································································ 17
Configuring IP unnumbered ·········································································································································· 17
Configuration guidelines ······································································································································ 17
Configuration prerequisites ·································································································································· 18
Configuration procedure ······································································································································ 18
Displaying and maintaining IP addressing ················································································································· 18
IP address configuration example ································································································································ 18
Network requirements··········································································································································· 18
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Configuration procedure ······································································································································ 19
Verifying the configuration··································································································································· 19
DHCP overview ··························································································································································21
DHCP address allocation ·············································································································································· 21
Allocation mechanisms ········································································································································· 21
Dynamic IP address allocation process··············································································································· 22
IP address lease extension···································································································································· 22
DHCP message format··················································································································································· 23
DHCP options ································································································································································· 24
Common DHCP options········································································································································ 24
Custom DHCP options··········································································································································· 24
Protocols and standards ················································································································································ 26
Configuring the DHCP server····································································································································27
Overview········································································································································································· 27
DHCP address pool··············································································································································· 27
IP address allocation sequence···························································································································· 28
DHCP server configuration task list ······························································································································ 29
Configuring an address pool on the DHCP server····································································································· 29
Configuration task list ··········································································································································· 29
Creating a DHCP address pool··························································································································· 30
Specifying IP address ranges for a DHCP address pool ·················································································· 30
Configuring gateways for the client ···················································································································· 33
Configuring a domain name suffix for the client ······························································································· 34
Configuring DNS servers for the client ··············································································································· 34
Configuring WINS servers and NetBIOS node type for the client ·································································· 34
Configuring BIMS server information for the client···························································································· 35
Configuring the TFTP server and boot file name for the client ········································································· 35
Specifying a server for the DHCP client·············································································································· 36
Configuring Option 184 parameters for the client···························································································· 36
Configuring self-defined DHCP options ·············································································································· 37
Enabling DHCP ······························································································································································ 38
Enabling the DHCP server on an interface·················································································································· 38
Applying an address pool on an interface ················································································································· 38
Configuring IP address conflict detection···················································································································· 39
Enabling handling of Option 82·································································································································· 39
Configuring DHCP server compatibility······················································································································· 40
Configuring the DHCP server to broadcast all responses················································································· 40
Configure the DHCP server to ignore BOOTP requests ···················································································· 40
Configuring the DHCP server to send BOOTP responses in the format specified in RFC 1048 ·················· 40
Displaying and maintaining the DHCP server ············································································································ 41
DHCP server configuration examples ·························································································································· 41
Static IP address assignment configuration example························································································· 42
Dynamic IP address assignment configuration example··················································································· 43
DHCP user class configuration example············································································································· 44
Self-defined option configuration example········································································································· 46
Troubleshooting DHCP server configuration ··············································································································· 46
Symptom································································································································································· 46
Analysis ·································································································································································· 47
Solution··································································································································································· 47
Configuring the DHCP relay agent···························································································································48
Overview········································································································································································· 48
Operation······························································································································································· 48
DHCP relay agent support for Option 82 ·········································································································· 49
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DHCP relay agent configuration task list····················································································································· 49
Enabling DHCP ······························································································································································ 50
Enabling the DHCP relay agent on an interface ········································································································ 50
Specifying DHCP servers on a relay agent················································································································· 50
Configuring the DHCP relay agent security functions ································································································ 51
Enabling the DHCP relay agent to record relay entries ···················································································· 51
Enabling periodic refresh of dynamic relay entries··························································································· 51
Enabling DHCP starvation attack protection ······································································································ 52
Configuring the DHCP relay agent to release an IP address ···················································································· 52
Configuring Option 82 ················································································································································· 53
Displaying and maintaining the DHCP relay agent ··································································································· 53
DHCP relay agent configuration examples················································································································· 54
DHCP relay agent configuration example·········································································································· 54
Option 82 configuration example······················································································································· 55
Troubleshooting DHCP relay agent configuration······································································································ 55
Symptom································································································································································· 55
Analysis ·································································································································································· 55
Solution··································································································································································· 56
Configuring the DHCP client ·····································································································································57
Enabling the DHCP client on an interface··················································································································· 57
Configuring a DHCP client ID for an interface ··········································································································· 57
Enabling duplicated address detection ······················································································································· 58
Displaying and maintaining the DHCP client·············································································································· 58
DHCP client configuration example ····························································································································· 58
Network requirements··········································································································································· 58
Configuration procedure ······································································································································ 59
Verifying the configuration··································································································································· 60
Configuring DHCP snooping·····································································································································61
Overview········································································································································································· 61
Application of trusted and untrusted ports·········································································································· 61
DHCP snooping support for Option 82·············································································································· 62
DHCP snooping configuration task list ························································································································ 63
Configuring basic DHCP snooping······························································································································ 63
Configuring Option 82 ················································································································································· 64
Saving DHCP snooping entries ···································································································································· 65
Enabling DHCP starvation attack protection ··············································································································· 66
Enabling DHCP-REQUEST attack protection ··············································································································· 66
Configuring DHCP packet rate limit····························································································································· 67
Displaying and maintaining DHCP snooping ············································································································· 67
DHCP snooping configuration examples····················································································································· 68
Basic DHCP snooping configuration example ··································································································· 68
Option 82 configuration example······················································································································· 69
Configuring the BOOTP client···································································································································71
BOOTP application························································································································································ 71
Obtaining an IP address dynamically ························································································································· 71
Protocols and standards ················································································································································ 71
Configuring an interface to use BOOTP for IP address acquisition·········································································· 71
Displaying and maintaining BOOTP client ················································································································· 72
BOOTP client configuration example ·························································································································· 72
Network requirements··········································································································································· 72
Configuration procedure ······································································································································ 72
iv
Configuring DNS ·······················································································································································73
Overview········································································································································································· 73
Static domain name resolution····························································································································· 73
Dynamic domain name resolution······················································································································· 73
DNS proxy ····························································································································································· 74
DNS spoofing ························································································································································ 75
DNS configuration task list············································································································································ 76
Configuring the IPv4 DNS client ·································································································································· 76
Configuring static domain name resolution········································································································ 76
Configuring dynamic domain name resolution·································································································· 77
Configuring the IPv6 DNS client ·································································································································· 77
Configuring static domain name resolution········································································································ 77
Configuring dynamic domain name resolution·································································································· 78
Configuring the DNS proxy ·········································································································································· 79
Configuring DNS spoofing ··········································································································································· 79
Specifying the source interface for DNS packets ······································································································· 80
Configuring the DNS trusted interface················································································································ 80
Displaying and maintaining IPv4 DNS························································································································ 81
IPv4 DNS configuration examples ······························································································································· 81
Static domain name resolution configuration example ····················································································· 81
Dynamic domain name resolution configuration example ··············································································· 82
DNS proxy configuration example······················································································································ 84
IPv6 DNS configuration examples ······························································································································· 86
Static domain name resolution configuration example ····················································································· 86
Dynamic domain name resolution configuration example ··············································································· 86
DNS proxy configuration example······················································································································ 91
Troubleshooting IPv4 DNS configuration ···················································································································· 92
Symptom································································································································································· 92
Solution··································································································································································· 92
Troubleshooting IPv6 DNS configuration ···················································································································· 92
Symptom································································································································································· 92
Solution··································································································································································· 92
Configuring DDNS·····················································································································································93
Overview········································································································································································· 93
DDNS application ················································································································································· 93
DDNS client configuration task list······························································································································· 94
Configuring a DDNS policy·········································································································································· 94
Configuration prerequisites ·································································································································· 95
Configuration procedure ······································································································································ 95
Applying the DDNS policy to an interface·················································································································· 96
Displaying DDNS··························································································································································· 97
DDNS configuration examples ····································································································································· 97
DDNS configuration example 1 ·························································································································· 97
DDNS configuration example 2 ·························································································································· 98
Basic IP forwarding on the device ························································································································· 100
FIB table ········································································································································································100
Displaying FIB table entries·········································································································································100
Optimizing IP performance ···································································································································· 102
Enabling an interface to receive and forward directed broadcasts destined for the directly connected network
·······················································································································································································102
Configuration procedure ····································································································································102
Configuration example ·······································································································································102
Configuring MTU for an interface ······························································································································103
v
Configuring TCP MSS for an interface ······················································································································103
Configuring TCP path MTU discovery ·······················································································································104
Enabling TCP SYN Cookie··········································································································································105
Configuring the TCP buffer size··································································································································105
Configuring TCP timers················································································································································105
Enabling sending ICMP error packets ·······················································································································106
Functions of sending ICMP error packets ·········································································································106
Disadvantages of sending ICMP error packets································································································107
Configuration procedure ····································································································································107
Disabling forwarding ICMP fragments ······················································································································108
Displaying and maintaining IP performance optimization ······················································································108
Configuring UDP helper·········································································································································· 109
Overview·······································································································································································109
Configuration guidelines ·············································································································································109
Configuration procedure·············································································································································109
Displaying and maintaining UDP helper ···················································································································110
UDP helper configuration example ····························································································································110
Network requirements·········································································································································110
Configuration procedure ····································································································································110
Verifying the configuration·································································································································111
Configuring basic IPv6 settings······························································································································ 112
Overview·······································································································································································112
IPv6 features·························································································································································112
IPv6 addresses·····················································································································································113
IPv6 ND protocol·················································································································································115
IPv6 path MTU discovery····································································································································117
IPv6 transition technologies·········································································································································118
Dual stack·····························································································································································118
Tunneling······························································································································································118
Protocols and standards ··············································································································································118
IPv6 basics configuration task list·······························································································································119
Assigning IPv6 addresses to interfaces······················································································································119
Configuring an IPv6 global unicast address ····································································································120
Configuring an IPv6 link-local address ·············································································································120
Configuring an IPv6 anycast address ···············································································································121
Configuring IPv6 ND ···················································································································································122
Configuring a static neighbor entry ··················································································································122
Configuring the maximum number of neighbors dynamically learned ·························································122
Setting the aging timer for ND entries in stale state························································································123
Minimizing link-local ND entries························································································································123
Setting the hop limit ············································································································································123
Configuring parameters for RA messages········································································································124
Configuring the maximum number of attempts to send an NS message for DAD·······································126
Configuring path MTU discovery ·······························································································································126
Configuring the interface MTU ··························································································································126
Configuring a static path MTU for a specific IPv6 address ············································································127
Configuring the aging time for dynamic path MTUs·······················································································127
Controlling sending ICMPv6 packets·························································································································127
Enabling replying to multicast echo requests ···································································································127
Enabling sending ICMPv6 destination unreachable messages······································································128
Enabling sending ICMPv6 time exceeded messages ······················································································128
Enabling sending ICMPv6 redirect messages ··································································································129
Displaying and maintaining IPv6 basics ···················································································································129
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IPv6 basics configuration example ····························································································································130
Network requirements·········································································································································130
Configuration procedure ····································································································································131
Verifying the configuration·································································································································131
Troubleshooting IPv6 basics configuration················································································································135
Symptom·······························································································································································135
Solution·································································································································································135
DHCPv6 overview··················································································································································· 136
DHCPv6 address/prefix assignment··························································································································136
Rapid assignment involving two messages·······································································································136
Assignment involving four messages·················································································································136
Address/prefix lease renewal ····································································································································137
Stateless DHCPv6·························································································································································138
Protocols and standards ··············································································································································138
Configuring the DHCPv6 server····························································································································· 139
Overview·······································································································································································139
IPv6 address assignment ····································································································································139
IPv6 prefix assignment········································································································································139
Concepts·······························································································································································140
DHCPv6 address pool ········································································································································141
IPv6 address/prefix allocation sequence ·········································································································142
Configuration task list ··················································································································································142
Configuring IPv6 prefix assignment ···························································································································142
Configuration guidelines ····································································································································142
Configuration procedure ····································································································································143
Configuring IPv6 address assignment························································································································143
Configuration guidelines ····································································································································144
Configuration procedure ····································································································································144
Configuring network parameters assignment ···········································································································145
Configuring the DHCPv6 server on an interface ······································································································146
Configuration guidelines ····································································································································146
Configuration procedure ····································································································································146
Displaying and maintaining the DHCPv6 server ······································································································146
DHCPv6 server configuration examples····················································································································147
Dynamic IPv6 prefix assignment configuration example ················································································147
Dynamic IPv6 address assignment configuration example·············································································150
Configuring tunneling ············································································································································· 152
Overview·······································································································································································152
IPv6 over IPv4 tunneling ·····································································································································152
IPv4 over IPv4 tunneling ·····································································································································154
IPv4 over IPv6 tunneling ·····································································································································155
IPv6 over IPv6 tunneling ·····································································································································156
Protocols and standards ·····································································································································157
Tunneling configuration task list ·································································································································157
Configuring a tunnel interface····································································································································157
Configuring an IPv6 over IPv4 manual tunnel···········································································································158
Configuration example ·······································································································································159
Configuring a 6to4 tunnel···········································································································································161
6to4 tunnel configuration example ···················································································································162
Configuring an ISATAP tunnel ····································································································································164
Configuration example ·······································································································································165
Configuring an IPv4 over IPv4 tunnel ························································································································167
Configuration example ·······································································································································168
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Configuring an IPv4 over IPv6 tunnel ························································································································170
Configuration example ·······································································································································171
Configuring an IPv6 over IPv6 tunnel ························································································································173
Configuration example ·······································································································································175
Displaying and maintaining tunneling configuration ·······························································································177
Troubleshooting tunneling configuration ···················································································································177
Symptom·······························································································································································177
Analysis ································································································································································177
Solution·································································································································································177
Configuring GRE····················································································································································· 178
Overview·······································································································································································178
GRE encapsulation format··································································································································178
GRE encapsulation and de-encapsulation ········································································································179
Protocols and standards ·····································································································································179
Configuring a GRE over IPv4 tunnel ··························································································································179
Configuration prerequisites ································································································································180
Configuration procedure ····································································································································180
Configuring a GRE over IPv6 tunnel ··························································································································182
Configuration prerequisites ································································································································182
Configuration procedure ····································································································································182
Displaying and maintaining GRE·······························································································································183
GRE configuration examples ······································································································································184
GRE over IPv4 configuration example ··············································································································184
GRE over IPv6 configuration example ··············································································································187
Troubleshooting GRE ···················································································································································190
Support and other resources ·································································································································· 191
Contacting HP ······························································································································································191
Subscription service ············································································································································191
Related information······················································································································································191
Documents····························································································································································191
Websites·······························································································································································191
Conventions ··································································································································································192
Index ········································································································································································ 194
1
Configuring ARP
This chapter describes how to configure the Address Resolution Protocol (ARP).
Overview
ARP resolves IP addresses into MAC addresses on Ethernet networks.
ARP message format
ARP uses two types of messages: ARP request and ARP reply. Figure 1 shows the format of ARP
request/reply messages. Numbers in the figure refer to field lengths.
Figure 1 ARP message format
•Hardware type—Hardware address type. The value 1 represents Ethernet.
•Protocol type—Type of the protocol address to be mapped. The hexadecimal value 0x0800
represents IP.
•Hardware address length and protocol address length—Length, in bytes, of a hardware address
and a protocol address. For an Ethernet address, the value of the hardware address length field is
6. For an IPv4 address, the value of the protocol address length field is 4.
•OP—Operation code, which describes the type of ARP message. Value 1 represents an ARP request,
and value 2 represents an ARP reply.
•Sender hardware address—Hardware address of the device sending the message.
•Sender protocol address—Protocol address of the device sending the message.
•Target hardware address—Hardware address of the device to which the message is being sent.
•Target protocol address—Protocol address of the device to which the message is being sent.
ARP operating mechanism
As shown in Figure 2, Host A and Host B are on the same subnet. Host A sends a packet to Host B as
follows:
1. Host A looks through the ARP table for an ARP entry for Host B. If one entry is found, Host A uses
the MAC address in the entry to encapsulate the IP packet into a data link layer frame. Then Host
A sends the frame to Host B.
2
2. If Host A finds no entry for Host B, Host A buffers the packet and broadcasts an ARP request. The
payload of the ARP request comprises the following information:
{Sender IP address and sender MAC address—Host A's IP address and MAC address
{Target IP address—Host B's IP address
{Target MAC address—An all-zero MAC address
All hosts on this subnet can receive the broadcast request, but only the requested host (Host B)
processes the request.
3. Host B compares its own IP address with the target IP address in the ARP request. If they are the
same, Host B:
a. Adds the sender IP address and sender MAC address into its ARP table.
b. Encapsulates its MAC address into an ARP reply.
c. Unicasts the ARP reply to Host A.
4. After receiving the ARP reply, Host A:
a. Adds the MAC address of Host B into its ARP table.
b. Encapsulates the MAC address into the packet and sends the packet to Host B.
Figure 2 ARP address resolution process
If Host A and Host B are on different subnets, Host A sends a packet to Host B as follows:
1. Host A broadcasts an ARP request where the target IP address is the IP address of the gateway.
2. The gateway responds with its MAC address in an ARP reply to Host A.
3. Host A uses the gateway's MAC address to encapsulate the packet, and then sends the packet to
the gateway.
4. If the gateway has an ARP entry for Host B, it forwards the packet to Host B directly. If not, the
gateway broadcasts an ARP request, in which the target IP address is the IP address of Host B.
5. After the gateway gets the MAC address of Host B, it sends the packet to Host B.
ARP table
An ARP table stores dynamic and static ARP entries.
Dynamic ARP entry
ARP automatically creates and updates dynamic entries. A dynamic ARP entry is removed when its aging
timer expires or the output interface goes down. In addition, a dynamic ARP entry can be overwritten by
a static ARP entry.
3
Static ARP entry
A static ARP entry is manually configured and maintained. It does not age out and cannot be overwritten
by any dynamic ARP entry.
Static ARP entries protect communication between devices because attack packets cannot modify the
IP-to-MAC mapping in a static ARP entry.
Static ARP entries include long, short, and multiport ARP entries.
•A long static ARP entry comprises the IP address, MAC address, VLAN, and output interface. It is
directly used for forwarding packets.
•A short static ARP entry comprises only the IP address and MAC address. The device first sends an
ARP request whose target IP address is the IP address of the short entry. If the sender IP and MAC
addresses in the received ARP reply match the IP and MAC addresses of the short static ARP entry,
the device adds the interface that received the ARP reply to the short static ARP entry, and uses the
resolved short static ARP entry to forward IP packets.
•A multiport ARP entry comprises the IP address, MAC address, and VLAN. If a multiport ARP entry
has the same MAC address and VLAN as a multicast or multiport unicast MAC address entry, the
device can use the multiport ARP entry to send IP packets. A multiport ARP entry is manually
configured. It does not age out and cannot be overwritten by any dynamic ARP entry. For more
information about multicast MAC, see IP Multicast Configuration Guide.
To communicate with a host by using a fixed IP-to-MAC mapping, configure a short static ARP entry on
the device. To communicate with a host by using a fixed IP-to-MAC mapping through a specific interface
in a specific VLAN, configure a long static ARP entry on the device.
Configuring a static ARP entry
A static ARP entry is effective when the device works normally. If a VLAN or VLAN interface is deleted,
any long static ARP entry in the VLAN is deleted, and any resolved short static ARP entry in the VLAN
becomes unresolved.
A resolved short static ARP entry becomes unresolved upon certain events. For example, it becomes
unresolved when the resolved output interface goes down.
A long static ARP entry is ineffective if the IP address in the entry conflicts with a local IP address, or no
local interface has an IP address in the same subnet as the IP address in the ARP entry. An ineffective long
static ARP entry cannot be used to forward packets.
Follow these guidelines when you configure a static ARP entry:
•The vlan-id argument must be the ID of an existing VLAN where the ARP entry resides. The specified
Ethernet interface must belong to that VLAN. The VLAN interface of the VLAN must be created.
•The IP address of the VLAN interface of the VLAN specified by the vlan-id argument must belong to
the same subnet as the IP address specified by the ip-address argument.
To configure a static ARP entry:
Step Command Remarks
1. Enter system view. system-view N/A
4
Step Command Remarks
2. Configure a static ARP
entry.
•Configure a long static ARP entry:
arp static ip-address mac-address
vlan-id interface-type interface-number
[ vpn-instance vpn-instance-name ]
•Configure a short static ARP entry:
arp static ip-address mac-address
[ vpn-instance vpn-instance-name ]
Use either command.
By default, no static ARP entry is
configured.
Configuring a multiport ARP entry
A multiport ARP entry comprises an IP address, MAC address, and VLAN ID. To use the multiport ARP
entry, you must also configure a multicast or multiport unicast MAC address entry that has the same MAC
address and VLAN ID as the multiport ARP entry to specify multiple output interfaces. In addition, the IP
address in the multiport ARP entry must reside on the same subnet as the virtual interface of the specified
VLAN (VLAN interface). Otherwise, the multiport ARP entry does not take effect. For more information
about multiport unicast MAC addresses, see the mac-address multiport command in Layer 2—LAN
Switching Command Reference. For more information about multicast MAC addresses, see the
mac-address multicast command in IP Multicast Command Reference.
A multiport ARP entry can overwrite a dynamic, short static or long static ARP entry. A short static or long
static ARP entry can also overwrite a multiport ARP entry.
To configure a multiport ARP entry:
Step Command Remarks
1. Enter system view. system-view N/A
2. Configure a multicast or
multiport unicast MAC
address entry.
•Configure a multiport unicast MAC
address entry:
mac-address multiport mac-address
interface interface-list vlan vlan-id
•Configure a multicast MAC address
entry:
mac-address multicast mac-address
interface interface-list vlan vlan-id
Use either command.
By default, no multicast or
multiport unicast MAC address
entries are configured.
3. Configure a multiport ARP
entry.
arp multiport ip-address mac-address
vlan-id [ vpn-instance
vpn-instance-name ]
By default, no multiport ARP
entries are configured.
Configuring the maximum number of dynamic ARP
entries
An interface can dynamically learn ARP entries. To prevent an interface from holding too many ARP
entries, you can set the maximum number of dynamic ARP entries that the interface can learn. When the
maximum number is reached, the interface stops learning ARP entries.
5
The Layer-2 interface can learn an ARP entry only when both its maximum number and the VLAN
interface's maximum number are not reached.
To set the maximum number of dynamic ARP entries that an interface can learn:
Step Command Remarks
1. Enter system view. system-view N/A
2. Enter interface view. interface interface-type
interface-number N/A
3. Set the maximum number of
dynamic ARP entries. arp max-learning-num number
By default, an interface can learn a
maximum of 16384 dynamic ARP
entries.
If the value of the number argument is set
to 0, the interface is disabled from
learning dynamic ARP entries.
Setting the aging timer for dynamic ARP entries
Each dynamic ARP entry in the ARP table has a limited lifetime, called aging timer. The aging timer of a
dynamic ARP entry is reset each time the dynamic ARP entry is updated. A dynamic ARP entry that is not
updated before its aging timer expires is deleted from the ARP table.
To set the aging timer for dynamic ARP entries:
Step Command Remarks
1. Enter system view. system-view N/A
2. Set the aging timer for dynamic ARP
entries. arp timer aging aging-time By default, the aging time for
dynamic ARP entries is 20 minutes.
Enabling dynamic ARP entry check
The dynamic ARP entry check function controls whether the device supports dynamic ARP entries
containing multicast MAC addresses.
When dynamic ARP entry check is enabled, the device cannot learn dynamic ARP entries containing
multicast MAC addresses, and you cannot manually add static ARP entries containing multicast MAC
addresses.
When dynamic ARP entry check is disabled, the device can learn dynamic ARP entries containing
multicast MAC addresses obtained from the ARP packets sourced from a unicast MAC address. You can
also manually add static ARP entries containing multicast MAC addresses.
To enable dynamic ARP entry check:
Step Command Remarks
1. Enter system view. system-view N/A
2. Enable dynamic ARP entry check.
arp check enable By default, dynamic ARP entry check is
enabled.
6
Displaying and maintaining ARP
IMPORTANT:
Clearing ARP entries from the ARP table might cause communication failures. Make sure the entries to be
cleared do not affect current communications.
Execute display commands in any view and reset commands in user view.
Task Command
Display ARP entries.
display arp [ [ all | dynamic | multiport | static ] [ slot
slot-number ] | vlan vlan-id | interface interface-type
interface-number ] [ count | verbose ]
Display the ARP entry for a specific IP
address. display arp ip-address [slot slot-number ] [ verbose ]
Display the ARP entries for a specific VPN
instance. display arp vpn-instance vpn-instance-name [ count ]
Display the aging timer for dynamic ARP
entries. display arp timer aging
Clear ARP entries from the ARP table. reset arp { all | dynamic | interface interface-type
interface-number |multiport | slot slot-number | static }
Static ARP configuration example
Network requirements
As shown in Figure 3, hosts are connected to the switch, which is connected to the router through
interface Ten-GigabitEthernet 1/0/1 in VLAN 10.
To ensure secure communications between the router and switch, configure a static ARP entry for the
router on the switch.
7
Figure 3 Network diagram
Configuration procedure
# Create VLAN 10.
<Switch> system-view
[Switch] vlan 10
[Switch-vlan10] quit
# Add interface Ten-GigabitEthernet 1/0/1 to VLAN 10.
[Switch] interface Ten-GigabitEthernet 1/0/1
[Switch-Ten-GigabitEthernet1/0/1] port access vlan 10
[Switch-Ten-GigabitEthernet1/0/1] quit
# Create VLAN-interface 10 and configure its IP address.
[Switch] interface vlan-interface 10
[Switch-vlan-interface10] ip address 192.168.1.2 8
[Switch-vlan-interface10] quit
# Configure a static ARP entry that has IP address 192.168.1.1, MAC address 00e0-fc01-0000, and
output interface Ten-GigabitEthernet 1/0/1 in VLAN 10.
[Switch] arp static 192.168.1.1 00e0-fc01-0000 10 Ten-GigabitEthernet 1/0/1
# Display information about static ARP entries.
[Switch] display arp static
Type: S-Static D-Dynamic M-Multiport I-Invalid
IP Address MAC Address VLAN ID Interface Aging Type
192.168.1.1 00e0-fc01-0000 10 XGE1/0/1 N/A S
Multiport ARP entry configuration example
Network requirements
As shown in Figure 4, a switch connects to three servers through interfaces Ten-GigabitEthernet 1/0/1,
Ten-GigabitEthernet 1/0/2, and Ten-GigabitEthernet 1/0/3 in VLAN 10. The servers share the IP
address 192.168.1.1/24 and MAC address 00e0-fc01-0000.
8
Configure a multiport ARP entry to send IP packets with destination IP address 192.168.1.1 to the three
servers.
Figure 4 Network diagram
Configuration procedure
# Create VLAN 10.
<Switch> system-view
[Switch] vlan 10
[Switch-vlan10] quit
# Add Ten-GigabitEthernet 1/0/1, Ten-GigabitEthernet 1/0/2, and Ten-GigabitEthernet 1/0/3 to
VLAN 10.
[Switch] interface Ten-GigabitEthernet 1/0/1
[Switch-Ten-GigabitEthernet1/0/1] port access vlan 10
[Switch-Ten-GigabitEthernet1/0/1] quit
[Switch] interface Ten-GigabitEthernet 1/0/2
[Switch-Ten-GigabitEthernet1/0/2] port access vlan 10
[Switch-Ten-GigabitEthernet1/0/2] quit
[Switch] interface Ten-GigabitEthernet 1/0/3
[Switch-Ten-GigabitEthernet1/0/3] port access vlan 10
[Switch-Ten-GigabitEthernet1/0/3] quit
# Create VLAN-interface 10 and specify its IP address.
[Switch] interface vlan-interface 10
[Switch-vlan-interface10] ip address 192.168.1.2 24
[Switch-vlan-interface10] quit
# Configure a multiport unicast MAC address entry that has MAC address 00e0-fc01-0000, and output
interfaces Ten-GigabitEthernet 1/0/1, Ten-GigabitEthernet 1/0/2, and Ten-GigabitEthernet 1/0/3 in
VLAN 10.
[Switch] mac-address multiport 00e0-fc01-0000 interface Ten-GigabitEthernet 1/0/1 to
Ten-GigabitEthernet 1/0/3 vlan 10
# Configure a multiport ARP entry with IP address 192.168.1.1 and MAC address 00e0-fc01-0000.
[Switch] arp multiport 192.168.1.1 00e0-fc01-0000 10
Swtich
XGE1/0/1 XGE1/0/3
XGE1/0/2
Server group
192.168.1.1/24
00e0-fc01-0000
Server ServerServer
9
# Display ARP information.
[Switch] display arp
Type: S-Static D-Dynamic M-Multiport I-Invalid
IP Address MAC Address VLAN Interface Aging Type
192.168.1.1 00e0-fc01-0000 10 N/A N/A M
10
Configuring gratuitous ARP
Overview
In a gratuitous ARP packet, the sender IP address and the target IP address are the IP address of the
sending device.
A device sends a gratuitous ARP packet for either of the following purposes:
•Determine whether its IP address is already used by another device. If the IP address is already used,
the device is informed of the conflict by an ARP reply.
•Inform other devices of a MAC address change.
Gratuitous ARP packet learning
This feature enables a device to create or update ARP entries by using the sender IP and MAC addresses
in received gratuitous ARP packets.
When this feature is disabled, the device uses received gratuitous ARP packets to update existing ARP
entries only.
Periodic sending of gratuitous ARP packets
Enabling a device to periodically send gratuitous ARP packets helps downstream devices update ARP
entries or MAC entries in a timely manner. This feature can be used to prevent gateway spoofing, prevent
ARP entries from aging out, and prevent the virtual IP address of a VRRP group from being used by a host.
•Prevent gateway spoofing.
An attacker can use the gateway address to send gratuitous ARP packets to the hosts on a network,
so that the traffic destined for the gateway from the hosts is sent to the attacker instead. As a result,
the hosts cannot access the external network.
To prevent such gateway spoofing attacks, you can enable the gateway to send gratuitous ARP
packets containing its primary IP address and manually configured secondary IP addresses at a
specific interval, so hosts can learn correct gateway address information.
•Prevent ARP entries from aging out.
If network traffic is heavy or if the host CPU usage is high, received ARP packets can be discarded
or are not promptly processed. Eventually, the dynamic ARP entries on the receiving host age out
and the traffic between the host and the corresponding devices is interrupted until the host
re-creates the ARP entries.
To prevent this problem, you can enable the gateway to send gratuitous ARP packets periodically.
The gratuitous ARP packets contain the gateway's primary IP address or one of its manually
configured secondary IP addresses, so the receiving hosts can update ARP entries in time.
•Prevent the virtual IP address of a VRRP group from being used by a host.
The master router of a VRRP group can periodically send gratuitous ARP packets to the hosts on the
local network, so that the hosts can update local ARP entries and avoid using the virtual IP address
of the VRRP group. For more information about VRRP, see High Availability Configuration Guide.
11
{If the virtual IP address of the VRRP group is associated with a virtual MAC address, the sender
MAC address in the gratuitous ARP packet is the virtual MAC address of the virtual router.
{If the virtual IP address of the VRRP group is associated with the real MAC address of an
interface, the sender MAC address in the gratuitous ARP packet is the MAC address of the
interface on the master router in the VRRP group.
Configuration procedure
The following conditions apply to the gratuitous ARP configuration:
•You can enable periodic sending of gratuitous ARP packets on up to 1024 interfaces.
•Periodic sending of gratuitous ARP packets takes effect only when the link of the enabled interface
goes up and an IP address has been assigned to the interface.
•If you change the interval for sending gratuitous ARP packets, the configuration is effective at the
next sending interval.
•The frequency of sending gratuitous ARP packets may be much lower than the sending interval set
by the user in any of the following circumstances:
{This function is enabled on multiple interfaces.
{Each interface is configured with multiple secondary IP addresses.
{A small sending interval is configured when the previous two conditions exist.
To configure gratuitous ARP:
Step Command Remarks
1. Enter system view. system-view N/A
2. Enable learning of gratuitous
ARP packets. gratuitous-arp-learning enable By default, learning of gratuitous
ARP packets is enabled.
3. Enable the device to send
gratuitous ARP packets upon
receiving ARP requests whose
sender IP address belongs to
a different subnet.
gratuitous-arp-sending enable
By default, a device does not send
gratuitous ARP packets upon
receiving ARP requests whose
sender IP address belongs to a
different subnet.
4. Enter interface view. interface interface-type
interface-number N/A
5. Enable periodic sending of
gratuitous ARP packets and
set the sending interval.
arp send-gratuitous-arp [ interval
milliseconds ]
By default, periodic sending of
gratuitous ARP packets is disabled.
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