HP HP 830 Series User manual
HP 830 Series PoE+ Unified Wired-WLAN
Switch Switching Engine
Layer 3 Configuration Guide
Part number: 5998-3931
Software version: 3308P26
Document version: 6W101-20130628
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Contents
Configuring ARP··························································································································································· 1
Overview············································································································································································1
ARP message format ················································································································································1
ARP operation···························································································································································1
ARP table···································································································································································2
Configuring a static ARP entry·········································································································································3
Configuring the maximum number of dynamic ARP entries for an interface······························································4
Setting the aging timer for dynamic ARP entries ···········································································································4
Enabling dynamic ARP entry check ································································································································4
Displaying and maintaining ARP·····································································································································5
Static ARP entry configuration example··························································································································5
Network requirements··············································································································································5
Configuration procedure ·········································································································································6
Configuring gratuitous ARP ········································································································································· 7
Overview············································································································································································7
Enabling learning of gratuitous ARP packets ········································································································7
Configuring periodic sending of gratuitous ARP packets ····················································································7
Configuration guidelines ··················································································································································7
Configuration procedure··················································································································································8
Configuring IP addressing··········································································································································· 9
Overview············································································································································································9
IP address classes·····················································································································································9
Special IP addresses ············································································································································· 10
Subnetting and masking ······································································································································· 10
Assigning an IP address to an interface ······················································································································ 11
Displaying and maintaining IP addressing ················································································································· 11
DHCP overview ··························································································································································12
DHCP address allocation ·············································································································································· 12
Allocation mechanisms ········································································································································· 12
Dynamic IP address allocation process··············································································································· 12
IP address lease extension···································································································································· 13
DHCP message format··················································································································································· 14
DHCP options ································································································································································· 15
Common DHCP options········································································································································ 15
Custom options ······················································································································································ 15
Protocols and standards ················································································································································ 19
Configuring the DHCP server····································································································································20
Overview········································································································································································· 20
DHCP address pool··············································································································································· 20
IP address allocation sequence···························································································································· 21
DHCP server configuration task list ······························································································································ 21
Configuring an address pool on the DHCP server····································································································· 22
Configuration task list ··········································································································································· 22
Creating a DHCP address pool··························································································································· 22
Configuring address allocation mode for a common address pool ································································ 23
Configuring dynamic address allocation for an extended address pool························································ 24
Configuring a domain name suffix for the client ······························································································· 25
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Configuring DNS servers for the client ··············································································································· 25
Configuring WINS servers and NetBIOS node type for the client ·································································· 26
Configuring BIMS server information for the client···························································································· 26
Configuring gateways for the client ···················································································································· 27
Configuring Option 184 parameters for the client with voice service ···························································· 27
Configuring the TFTP server and bootfile name for the client··········································································· 28
Configuring self-defined DHCP options ·············································································································· 28
Enabling DHCP ······························································································································································ 29
Enabling the DHCP server on an interface·················································································································· 29
Configuration guidelines ······································································································································ 29
Configuration procedure ······································································································································ 30
Applying an extended address pool on an interface ································································································ 30
Configuring the DHCP server security functions ········································································································· 31
Configuration prerequisites ·································································································································· 31
Enabling unauthorized DHCP server detection ·································································································· 31
Configuring IP address conflict detection ··········································································································· 31
Enabling handling of Option 82·································································································································· 32
Configuration prerequisites ·································································································································· 32
Enabling Option 82 handling······························································································································ 32
Specifying the threshold for sending trap messages·································································································· 32
Configuration prerequisites ·································································································································· 32
Configuration procedure ······································································································································ 33
Displaying and maintaining the DHCP server ············································································································ 33
DHCP server configuration examples ·························································································································· 34
Static IP address assignment configuration example························································································· 34
Dynamic IP address assignment configuration example··················································································· 35
Self-defined option configuration example········································································································· 37
Troubleshooting DHCP server configuration ··············································································································· 38
Symptom································································································································································· 38
Analysis ·································································································································································· 38
Solution··································································································································································· 38
Configuring the DHCP relay agent···························································································································39
Overview········································································································································································· 39
Fundamentals························································································································································· 39
DHCP relay agent support for Option 82 ·········································································································· 40
DHCP relay agent configuration task list····················································································································· 40
Enabling DHCP ······························································································································································ 41
Enabling the DHCP relay agent on an interface ········································································································ 41
Correlating a DHCP server group with a relay agent interface················································································ 41
Configuring the DHCP relay agent security functions ································································································ 42
Configuring address check ·································································································································· 42
Configuring periodic refresh of dynamic client entries ····················································································· 43
Enabling unauthorized DHCP server detection ·································································································· 43
Enabling DHCP starvation attack protection ······································································································ 44
Enabling client offline detection ··································································································································· 45
Configuring the DHCP relay agent to release an IP address ···················································································· 45
Configuring the DHCP relay agent to handle Option 82·························································································· 45
Displaying and maintaining the DHCP relay agent ··································································································· 47
DHCP relay agent configuration examples)················································································································ 47
DHCP relay agent configuration example·········································································································· 47
DHCP relay agent Option 82 support configuration example········································································· 48
Troubleshooting DHCP relay agent configuration······································································································ 49
Symptom································································································································································· 49
Analysis ·································································································································································· 49
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Solution··································································································································································· 49
Configuring DHCP client ···········································································································································50
Introduction to DHCP client ··········································································································································· 50
Enabling the DHCP client on an interface··················································································································· 50
Displaying and maintaining the DHCP client·············································································································· 50
DHCP client configuration example ····························································································································· 51
Network requirements··········································································································································· 51
Configuration procedure ······································································································································ 51
Verifying the configuration··································································································································· 52
Configuring DHCP snooping·····································································································································53
Overview········································································································································································· 53
Application of trusted and untrusted ports·········································································································· 53
DHCP snooping support for Option 82·············································································································· 54
DHCP snooping configuration task list ························································································································ 55
Configuring DHCP snooping basic functions·············································································································· 55
Configuring DHCP snooping to support Option 82··································································································· 56
Configuring DHCP snooping entries backup ·············································································································· 57
Enabling DHCP starvation attack protection ··············································································································· 58
Enabling DHCP-REQUEST message attack protection ······························································································· 58
Displaying and maintaining DHCP snooping ············································································································· 59
DHCP snooping configuration example ······················································································································ 60
Network requirements··········································································································································· 60
Configuration procedure ······································································································································ 60
DHCP snooping Option 82 support configuration example····················································································· 60
Network requirements··········································································································································· 60
Configuration procedure ······································································································································ 61
Configuring BOOTP client·········································································································································62
BOOTP application························································································································································ 62
Obtaining an IP address dynamically ························································································································· 62
Protocols and standards ················································································································································ 62
Configuring an interface to dynamically obtain an IP address through BOOTP ···················································· 63
Displaying and maintaining BOOTP client configuration·························································································· 63
BOOTP client configuration example ·························································································································· 63
Network requirements··········································································································································· 63
Configuration procedure ······································································································································ 63
Configuring IPv4 DNS ···············································································································································64
Overview········································································································································································· 64
Static domain name resolution····························································································································· 64
Dynamic domain name resolution······················································································································· 64
DNS proxy ····························································································································································· 65
DNS spoofing ························································································································································ 66
Configuring the IPv4 DNS client ·································································································································· 67
Configuring static domain name resolution········································································································ 67
Configuring dynamic domain name resolution·································································································· 68
Configuring the DNS proxy ·········································································································································· 69
Configuring DNS spoofing ··········································································································································· 69
Displaying and maintaining IPv4 DNS························································································································ 69
IPv4 DNS configuration examples ······························································································································· 70
Static domain name resolution configuration example ····················································································· 70
Dynamic domain name resolution configuration example ··············································································· 71
DNS proxy configuration example······················································································································ 74
Troubleshooting IPv4 DNS configuration ···················································································································· 75
iv
Configuring IPv6 DNS ···············································································································································76
Configuring the IPv6 DNS client ·································································································································· 76
Configuring static domain name resolution········································································································ 76
Configuring dynamic domain name resolution·································································································· 76
Displaying and maintaining IPv6 DNS························································································································ 77
IPv6 DNS configuration examples ······························································································································· 77
Static domain name resolution configuration example······························································································ 77
Network requirements··········································································································································· 77
Configuration procedure ······································································································································ 78
Dynamic domain name resolution configuration example ························································································ 78
Network requirements··········································································································································· 78
Configuration procedure ······································································································································ 79
Verifying the configuration··································································································································· 82
Optimizing IP performance ·······································································································································84
Enabling receiving and forwarding of directed broadcasts to a directly connected network······························· 84
Enabling receiving of directed broadcasts to a directly connected network·················································· 84
Enabling forwarding of directed broadcasts to a directly connected network··············································· 84
Receiving and forwarding directed broadcasts configuration example) ························································ 85
Configuring TCP attributes ············································································································································ 86
Configuring the TCP send/receive buffer size ··································································································· 86
Configuring TCP timers ········································································································································· 86
Configuring ICMP to send error packet······················································································································· 87
Advantages of sending ICMP error packets······································································································· 87
Disadvantages of sending ICMP error packets·································································································· 88
Configuration procedure ······································································································································ 88
Displaying and maintaining IP performance optimization ························································································ 88
Configuring UDP helper·············································································································································90
Overview········································································································································································· 90
Configuring UDP helper ················································································································································ 90
Configuration procedure··············································································································································· 90
Displaying and maintaining UDP helper ····················································································································· 91
UDP helper configuration example ······························································································································ 91
Network requirements··········································································································································· 91
Configuration procedure ······································································································································ 91
Configuring IPv6 basics·············································································································································93
Overview········································································································································································· 93
IPv6 features··························································································································································· 93
IPv6 addresses······················································································································································· 94
IPv6 neighbor discovery protocol························································································································ 97
IPv6 path MTU discovery······································································································································ 99
IPv6 transition technologies································································································································100
Protocols and standards ·····································································································································101
IPv6 basics configuration task list·······························································································································101
Configuring basic IPv6 functions································································································································102
Enabling IPv6·······················································································································································102
Configuring an IPv6 global unicast address ····································································································102
Configuring an IPv6 link-local address ·············································································································103
Configure an IPv6 anycast address···················································································································104
Configuring IPv6 ND ···················································································································································105
Configuring a static neighbor entry ··················································································································105
Setting the maximum number of dynamic neighbor entries············································································105
Configuring parameters related to RA messages ····························································································106
Configuring the maximum number of attempts to send an NS message for DAD·······································108
v
Configuring ND snooping··································································································································109
Configuring path MTU discovery ·······························································································································110
Configuring a static path MTU for a specific IPv6 address············································································110
Configuring the aging time for dynamic path MTUs·······················································································110
Configuring IPv6 TCP properties ································································································································111
Configuring IPv6 FIB forwarding································································································································111
Configuring ICMPv6 packet sending·························································································································112
Configuring the maximum ICMPv6 error packets sent in an interval ····························································112
Enabling replying to multicast echo requests ···································································································112
Enabling sending ICMPv6 time exceeded messages······················································································113
Enabling sending ICMPv6 destination unreachable messages······································································113
Displaying and maintaining IPv6 basics configuration····························································································114
IPv6 basics configuration example ····························································································································115
Network requirements·········································································································································115
Configuration procedure ····································································································································115
Verifying the configuration·································································································································117
Troubleshooting IPv6 basics configuration················································································································120
Symptom·······························································································································································120
Solution·································································································································································120
IP routing basics ······················································································································································ 121
Routing table·································································································································································121
Route preference ··························································································································································122
Route backup································································································································································122
Displaying and maintaining a routing table ·············································································································122
Configuring static routing······································································································································· 124
Configuring a static route············································································································································124
Displaying and maintaining static routes ··················································································································125
Basic static route configuration example···················································································································125
Configuring IPv6 static routing······························································································································· 128
Overview·······································································································································································128
Configuring IPv6 static routing ···································································································································128
Displaying and maintaining IPv6 static routes··········································································································128
IPv6 static routing configuration example ·················································································································129
Configuring RIP ······················································································································································· 131
Overview·······································································································································································131
RIP route entries ···················································································································································131
RIP timers ······························································································································································131
Routing loop prevention······································································································································131
RIP operation························································································································································132
RIP versions ··························································································································································132
Protocols and standards ·····································································································································132
RIP configuration task list·············································································································································133
Configuring basic RIP ··················································································································································133
Enabling RIP ·························································································································································133
Configuring the interface behavior ···················································································································134
Configuring a RIP version···································································································································134
Configuring RIP route control······································································································································135
Configuring an additional routing metric ·········································································································135
Configuring RIPv2 route summarization············································································································136
Disabling host route reception ···························································································································137
Advertising a default route ·································································································································137
Configuring inbound/outbound route filtering·································································································138
vi
Configuring a preference for RIP·······················································································································138
Configuring RIP route redistribution···················································································································138
Tuning and optimizing RIP networks ··························································································································139
Configuration prerequisites ································································································································139
Configuring RIP timers·········································································································································139
Configuring split horizon and poison reverse··································································································139
Configuring the maximum number of ECMP routes ························································································140
Enabling zero field check on incoming RIPv1 messages ················································································140
Enabling source IP address check on incoming RIP updates··········································································141
Configuring RIPv2 message authentication ······································································································141
Specifying a RIP neighbor ··································································································································142
Configuring RIP-to-MIB binding··························································································································142
Configuring the RIP packet sending rate ··········································································································142
Displaying and maintaining RIP ·································································································································143
RIP configuration examples·········································································································································143
Configuring RIP version ······································································································································143
Configuring RIP route redistribution···················································································································145
Configuring an additional metric for a RIP interface·······················································································147
Troubleshooting RIP······················································································································································148
No RIP updates received ····································································································································148
Route oscillation occurred ··································································································································149
Configuring RIPng ··················································································································································· 150
Overview·······································································································································································150
RIPng working mechanism··································································································································150
RIPng packet format ············································································································································150
RIPng packet processing procedure ··················································································································151
Protocols and standards ·····································································································································152
RIPng configuration task list ········································································································································152
Configuring RIPng basic functions······························································································································152
Configuration prerequisites ································································································································152
Configuration procedure ····································································································································153
Configuring RIPng route control ·································································································································153
Configuring an additional routing metric ·········································································································153
Configuring RIPng route summarization ···········································································································154
Advertising a default route ·································································································································154
Configuring a RIPng route filtering policy·········································································································154
Configuring a priority for RIPng·························································································································154
Configuring RIPng route redistribution ··············································································································155
Tuning and optimizing RIPng networks······················································································································155
Configuration prerequisites ································································································································155
Configuring RIPng timers ····································································································································155
Configuring split horizon and poison reverse··································································································156
Configuring zero field check on RIPng packets ·······························································································156
Configuring the maximum number of ECMP routes ························································································157
Displaying and maintaining RIPng ·····························································································································157
RIPng configuration examples·····································································································································158
Configure RIPng basic functions ························································································································158
Configuring RIPng route redistribution ··············································································································160
Support and other resources ·································································································································· 164
Contacting HP ······························································································································································164
Subscription service ············································································································································164
Related information······················································································································································164
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Conventions ··································································································································································165
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1
Configuring ARP
This chapter describes how to configure the Address Resolution Protocol (ARP).
Overview
ARP resolves IP addresses into physical addresses such as MAC addresses. On an Ethernet LAN, a
device uses ARP to get the MAC address of the target device for a packet.
ARP message format
ARP uses two types of messages, ARP request and ARP reply. Figure 1 shows the format of the ARP
request/reply. 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 type of the 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 operation
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 its ARP table for an ARP entry for Host B. If an entry is found, Host A uses the
MAC address in the entry to encapsulate the IP packet into a data link layer frame and 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 sends an ARP request to the gateway. The target IP address in the ARP request 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 MAC address to encapsulate the packet and sends the packet to the
gateway.
4. If the gateway has the ARP entry for Host B, it forwards the packet to Host B directly. If not, it
broadcasts an ARP request, in which the target IP address is the IP address of Host B.
5. After obtaining the MAC address of Host B, the gateway 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, and it 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 a 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 can be classified into long and short ARP entries.
•To configure a long static ARP entry, specify the IP address, MAC address, VLAN, and output
interface. A long static ARP entry is directly used for forwarding matching packets. 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.
•To configure a short static ARP entry, you only need to specify 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 receiving the ARP reply to the short static ARP
entry, and then uses the resolved entry to forward the matching IP packets.
To communicate with a host by using a fixed IP-to-MAC mapping, configure a short 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,
all long static ARP entries in the VLAN are deleted, and all resolved short static ARP entries in the VLAN
becomes unresolved.
Follow these guidelines when you configure a long 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:
Ste
p
Command
1. Enter system view. system-view
2. Configure a static ARP entry.
•Configure a long static ARP entry:
arp static ip-address mac-address vlan-id interface-type
interface-number
•Configure a short static ARP entry:
arp static ip-address mac-address
4
Configuring the maximum number of dynamic ARP
entries for an interface
Ste
p
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 that the interface can
learn.
arp max-learning-num
number
Optional.
By default, a Layer 2 interface does
not limit the number of dynamic ARP
entries. A Layer 3 interface can learn
a maximum of 128 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 an aging timer. The aging timer
of a dynamic ARP entry is reset each time the dynamic ARP entry is updated. Dynamic ARP entries that
are not updated before their aging timers expire are deleted from the ARP table.
To set the aging timer for dynamic ARP entries:
Ste
p
Command
Remarks
1. Enter system view. system-view N/A
2. Set the aging timer for
dynamic ARP entries. arp timer aging aging-time Optional.
20 minutes by default.
Enabling dynamic ARP entry check
The dynamic ARP entry check function controls whether the device supports dynamic ARP entries with
multicast MAC addresses.
When dynamic ARP entry check is enabled, the device cannot learn dynamic ARP entries containing
multicast MAC addresses.
When dynamic ARP entry check is disabled, the device can learn dynamic ARP entries containing
multicast MAC addresses.
To enable dynamic ARP entry check:
Ste
p
Command
Remarks
1. Enter system view. system-view N/A
5
Ste
p
Command
Remarks
2. Enable dynamic ARP entry check. arp check enable Optional.
Enabled by default.
Displaying and maintaining ARP
CAUTION:
Clearing ARP entries from the ARP table might cause communication failures.
Task Command
Remarks
Display ARP entries in the ARP
table.
display arp [ [ all | dynamic | static ] [ slot
slot-number ] | vlan vlan-id | interface
interface-type interface-number ] [ count ] [ |
{ begin | exclude | include }
regular-expression ]
Available in any view.
Display the ARP entry for a
specific IP address.
display arp ip-address [slot slot-number ] [ |
{begin |exclude |include }
regular-expression ]
Available in any view.
Display the aging timer of
dynamic ARP entries.
display arp timer aging [ |{ begin | exclude |
include } regular-expression ] Available in any view.
Clear ARP entries from the ARP
table.
reset arp { all | dynamic | static | slot
slot-number | interface interface-type
interface-number }
Available in user view.
Static ARP entry configuration example
Network requirements
As shown in Figure 3, hosts are connected to the switch, which is connected to the router through
interface GigabitEthernet 1/0/1 in VLAN 10. The IP and MAC addresses of the router are
192.168.1.1/24 and 00e0-fc01-0000, respectively.
To prevent malicious users from attacking the switch and enhance security for communications between
the router and switch, configure a static ARP entry for the router on the switch.
6
Figure 3 Network diagram
Configuration procedure
# Create VLAN 10.
<Switch> system-view
[Switch] vlan 10
[Switch-vlan10] quit
# Add interface GigabitEthernet 1/0/1 to VLAN 10.
[Switch] interface gigabitethernet 1/0/1
[Switch-GigabitEthernet1/0/1] port link-type trunk
[Switch-GigabitEthernet1/0/1] port trunk permit vlan 10
[Switch-GigabitEthernet1/0/1] quit
# Create interface 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 GigabitEthernet 1/0/1 in VLAN 10.
[Switch] arp static 192.168.1.1 00e0-fc01-0000 10 gigabitethernet 1/0/1
# Display information about static ARP entries.
[Switch] display arp static
Type: S-Static D-Dynamic A-Authorized
IP Address MAC Address VLAN ID Interface Aging Type
192.168.1.1 00e0-fc01-0000 10 GE1/0/1 N/A S
7
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 change of its MAC address.
Enabling learning of gratuitous ARP packets
This feature enables a device to create or update ARP entries by using the sender IP and MAC addresses
in received gratuitous ARP packets.
With this feature disabled, the device uses the received gratuitous ARP packets to update only existing
ARP entries.
Configuring periodic sending of gratuitous ARP packets
Enabling a device to periodically send gratuitous ARP packets helps downstream devices update their
corresponding ARP entries or MAC entries in time. You can use this feature 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 a host's CPU usage is high, received ARP packets might be
discarded or might not be processed in time. 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.
Configuration guidelines
Follow these guidelines when you configure gratuitous ARP:
8
•You can enable periodic sending of gratuitous ARP packets on a maximum of 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 might be much lower than the sending interval set
by the user if this function is enabled on multiple interfaces, if each interface is configured with
multiple secondary IP addresses, or if a small sending interval is configured when the previous two
conditions exist.
Configuration procedure
To configure gratuitous ARP:
Ste
p
Command
Remarks
1. Enter system view. system-view N/A
2. Enable learning of gratuitous ARP
packets.
gratuitous-arp-learning
enable
Optional.
Enabled by default.
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, this feature is disabled.
9
Configuring IP addressing
This chapter describes IP addressing basic and manual IP address assignment for interfaces. Dynamic IP
address assignment (BOOTP and DHCP) is beyond the scope of this chapter.
Overview
This section describes the IP addressing basics.
IP addressing uses a 32-bit address to identify each host on a network. To make addresses easier to read,
they are written in dotted decimal notation, each address being four octets in length. For example,
address 00001010000000010000000100000001 in binary is written as 10.1.1.1.
IP address classes
Each IP address breaks down into the following parts:
•Net ID—Identifies a network. The first several bits of a net ID, known as the class field or class bits,
identify the class of the IP address.
•Host ID—Identifies a host on a network.
IP addresses are divided into five classes, as shown in Figure 4. The shaded areas represent the address
class. The first three classes are widely used.
Figure 4 IP address classes
Table 1 IP address classes and ranges
Class Address ran
g
e Remarks
A 0.0.0.0 to
127.255.255.255
The IP address 0.0.0.0 is used by a host at startup for temporary
communication. This address is never a valid destination address.
Addresses starting with 127 are reserved for loopback test.
Packets destined to these addresses are processed locally as input
packets rather than sent to the link.
B 128.0.0.0 to
191.255.255.255 N/A
C 192.0.0.0 to
223.255.255.255 N/A
10
Class Address ran
g
e Remarks
D 224.0.0.0 to
239.255.255.255 Multicast addresses.
E 240.0.0.0 to
255.255.255.255
Reserved for future use except for the broadcast address
255.255.255.255.
Special IP addresses
The following IP addresses are for special use and cannot be used as host IP addresses.
•IP address with an all-zero net ID—Identifies a host on the local network. For example, IP address
0.0.0.16 indicates the host with a host ID of 16 on the local network.
•IP address with an all-zero host ID—Identifies a network.
•IP address with an all-one host ID—Identifies a directed broadcast address. For example, a packet
with the destination address of 192.168.1.255 is broadcast to all the hosts on the network
192.168.1.0.
Subnetting and masking
Subnetting divides a network into smaller networks called subnets by using some bits of the host ID to
create a subnet ID.
Masking identifies the boundary between the host ID and the combination of net ID and subnet ID.
(When subnetting is not adopted, a mask identifies the boundary between the net ID and the host ID.)
Each subnet mask is made up of 32 bits, which correspond to the bits in an IP address. In a subnet mask,
consecutive ones represent the net ID and subnet ID, and consecutive zeros represent the host ID.
Before being subnetted, Class A, B, and C networks use default masks (also called natural masks)
255.0.0.0, 255.255.0.0, and 255.255.255.0, respectively.
Figure 5 shows how a Class B network is subnetted.
Figure 5 Subnetting a Class B network
Subnetting increases the number of addresses that cannot be assigned to hosts. After being subnetted,
a network can accommodate fewer hosts.
For example, a Class B network without subnetting can accommodate 1022 more hosts than the same
network subnetted into 512 subnets.
•Without subnetting—65534 hosts (216 – 2). (The two deducted addresses are the broadcast
address, which has an all-one host ID, and the network address, which has an all-zero host ID.)
•With subnetting—Using the first 9 bits of the host-id for subnetting provides 512 (29) subnets.
However, only 7 bits remain available for the host ID. This allows 126 (27– 2) hosts in each subnet,
a total of 64512 hosts (512 × 126).
11
Assigning an IP address to an interface
You can assign an interface one primary address and multiple secondary addresses.
Generally, you only need to assign the primary address to an interface. In some cases, you must assign
secondary IP addresses to the interface. For example, if the interface connects to two subnets, to enable
the device to communicate with all hosts on the LAN, assign a primary IP address and a secondary IP
address to the interface.
To assign an IP address to an interface:
Ste
p
Command
Remarks
1. Enter system view. system-view N/A
2. Enter interface view. interface interface-type interface-number N/A
3. Assign an IP address to
the interface.
ip address ip-address { mask-length |
mask }
By default, no IP address is
assigned to any interface.
Displaying and maintaining IP addressing
Task Command
Remarks
Display IP configuration information
about a specific Layer 3 interface or
all Layer 3 interfaces.
display ip interface [ interface-type
interface-number ] [ |{ begin | exclude |
include } regular-expression ]
Available in any view.
Display brief IP configuration
information about a specific Layer 3
interface or all Layer 3 interfaces.
display ip interface [ interface-type
[ interface-number ] ] brief [ |{ begin |
exclude | include } regular-expression ]
Available in any view.
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