HP Enterprise HP 3100-8 v2 User manual

HP 3100 v2 Switch Series
IP Multicast
Configuration Guide
HP 3100-8 v2 SI Switch (JG221A)
HP 3100-16 v2 SI Switch (JG222A)
HP 3100-24 v2 SI Switch (JG223A)
HP 3100-8 v2 EI Switch (JD318B)
HP 3100-16 v2 EI Switch (JD319B)
HP 3100-24 v2 EI Switch (JD320B)
HP 3100-8-PoE v2 EI Switch (JD311B)
HP 3100-16-PoE v2 EI Switch (JD312B)
HP 3100-24-PoE v2 EI Switch (JD313B)
Part number: 5998-5994
Software version: Release 5203P05
Document version: 6W100-20140603

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The information contained herein is subject to change without notice.
HEWLETT-PACKARD COMPANY MAKES NO WARRANTY OF ANY KIND WITH REGARD TO THIS
MATERIAL, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY
AND FITNESS FOR A PARTICULAR PURPOSE. Hewlett-Packard shall not be liable for errors contained
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herein.

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Contents
Multicast overview ······················································································································································· 1
Introduction to multicast····················································································································································1
Information transmission techniques·······················································································································1
Multicast features······················································································································································3
Common notations in multicast ·······························································································································4
Multicast advantages and applications ·················································································································4
Multicast models································································································································································5
Multicast architecture························································································································································5
Multicast addresses ··················································································································································6
Multicast protocols ···················································································································································9
Multicast packet forwarding mechanism····················································································································· 11
Configuring IGMP snooping ·····································································································································12
Overview········································································································································································· 12
Basic concepts in IGMP snooping······················································································································· 12
How IGMP snooping works ································································································································· 14
IGMP snooping proxying ····································································································································· 15
Protocols and standards ······································································································································· 17
IGMP snooping configuration task list························································································································· 17
Configuring basic IGMP snooping functions ·············································································································· 18
Enabling IGMP snooping ····································································································································· 18
Specifying the version of IGMP snooping ·········································································································· 18
Configuring static multicast MAC address entries ····························································································· 19
Configuring IGMP snooping port functions················································································································· 20
Setting aging timers for dynamic ports ··············································································································· 20
Configuring static ports········································································································································· 21
Configuring a port as a simulated member host ······························································································· 21
Enabling IGMP snooping fast-leave processing································································································· 22
Disabling a port from becoming a dynamic router port ··················································································· 23
Configuring IGMP snooping querier ··························································································································· 24
Enabling IGMP snooping querier ························································································································ 24
Configuring parameters for IGMP queries and responses ··············································································· 24
Configuring the source IP addresses for IGMP queries····················································································· 25
Configuring IGMP snooping proxying ························································································································ 26
Enabling IGMP snooping proxying····················································································································· 26
Configuring a source IP address for the IGMP messages sent by the proxy·················································· 26
Configuring an IGMP snooping policy························································································································ 27
Configuring a multicast group filter····················································································································· 27
Configuring multicast source port filtering·········································································································· 28
Enabling dropping unknown multicast data······································································································· 29
Configuring IGMP report suppression ················································································································ 29
Setting the maximum number of multicast groups that a port can join ··························································· 30
Enabling multicast group replacement················································································································ 30
Setting the 802.1p precedence for IGMP messages ························································································ 31
Configuring a multicast user control policy (available only on the HP 3100 v2 EI)······································ 32
Enabling the IGMP snooping host tracking function ························································································· 32
Setting the DSCP value for IGMP messages······································································································· 33
Displaying and maintaining IGMP snooping·············································································································· 33
IGMP snooping configuration examples ····················································································································· 34

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Group policy and simulated joining configuration example············································································ 34
Static port configuration example ······················································································································· 36
IGMP snooping querier configuration example································································································· 40
IGMP snooping proxying configuration example······························································································ 42
Multicast source and user control policy configuration example (available only on the HP 3100 v2 EI)··· 44
Troubleshooting IGMP snooping ·································································································································· 49
Layer 2 multicast forwarding cannot function ···································································································· 49
Configured multicast group policy fails to take effect ······················································································· 49
Configuring multicast VLANs·····································································································································51
Overview········································································································································································· 51
Multicast VLAN configuration task list ························································································································· 52
Configuring a port-based multicast VLAN··················································································································· 52
Configuration prerequisites ·································································································································· 52
Configuring user port attributes ··························································································································· 53
Configuring multicast VLAN ports ······················································································································· 53
Displaying and maintaining multicast VLAN··············································································································· 54
Multicast VLAN configuration examples······················································································································ 54
Configuring MLD snooping (available only on the HP 3100 v2 EI) ······································································58
Overview········································································································································································· 58
Basic concepts in MLD snooping························································································································· 58
How MLD snooping works ··································································································································· 60
MLD snooping proxying ······································································································································· 61
Protocols and standards ······································································································································· 62
MLD snooping configuration task list ··························································································································· 62
Configuring basic MLD snooping functions ················································································································ 63
Enabling MLD snooping ······································································································································· 64
Specifying the version of MLD snooping ············································································································ 64
Configuring IPv6 static multicast MAC address entries····················································································· 65
Configuring MLD snooping port functions··················································································································· 65
Configuring aging timers for dynamic ports ······································································································ 66
Configuring static ports········································································································································· 66
Configuring a port as a simulated member host ······························································································· 67
Enabling fast-leave processing····························································································································· 68
Disabling a port from becoming a dynamic router port ··················································································· 68
Configuring MLD snooping querier ····························································································································· 69
Enabling MLD snooping querier ·························································································································· 69
Configuring parameters for MLD queries and responses ················································································· 70
Configuring the source IPv6 addresses for MLD queries ·················································································· 71
Configuring MLD snooping proxying ·························································································································· 71
Enabling MLD snooping proxying······················································································································· 71
Configuring the source IPv6 addresses for the MLD messages sent by the proxy ········································· 72
Configuring an MLD snooping policy·························································································································· 72
Configuring an IPv6 multicast group filter ·········································································································· 72
Enabling dropping unknown IPv6 multicast data ······························································································ 73
Configuring MLD report suppression ·················································································································· 74
Setting the maximum number of multicast groups that a port can join ··························································· 74
Enabling IPv6 multicast group replacement ······································································································· 75
Setting the 802.1p precedence for MLD messages ·························································································· 76
Configuring an IPv6 multicast user control policy······························································································ 76
Enabling the MLD snooping host tracking function ··························································································· 77
Setting the DSCP value for MLD messages········································································································· 78
Displaying and maintaining MLD snooping················································································································ 78
MLD snooping configuration examples ······················································································································· 79

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IPv6 group policy and simulated joining configuration example ···································································· 79
Static port configuration example ······················································································································· 81
MLD snooping querier configuration example··································································································· 84
MLD snooping proxying configuration example································································································ 86
IPv6 multicast source and user control policy configuration example ····························································· 89
Troubleshooting MLD snooping ···································································································································· 93
Layer 2 multicast forwarding cannot function ···································································································· 93
Configured IPv6 multicast group policy fails to take effect··············································································· 94
Configuring IPv6 multicast VLANs (available only on the HP 3100 v2 EI) ···························································95
Overview········································································································································································· 95
IPv6 multicast VLAN configuration task list ················································································································· 96
Configuring a port-based IPv6 multicast VLAN ·········································································································· 96
Configuration prerequisites ·································································································································· 96
Configuring user port attributes ··························································································································· 97
Configuring IPv6 multicast VLAN ports ··············································································································· 97
Displaying and maintaining IPv6 multicast VLAN ······································································································ 98
IPv6 multicast VLAN configuration examples·············································································································· 98
Support and other resources ·································································································································· 102
Contacting HP ······························································································································································102
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Related information······················································································································································102
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Conventions ··································································································································································103
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1
Multicast overview
Introduction to multicast
As a technique that coexists with unicast and broadcast, the multicast technique effectively addresses the
issue of point-to-multipoint data transmission. By enabling high-efficiency point-to-multipoint data
transmission over a network, multicast greatly saves network bandwidth and reduces network load.
By using multicast technology, a network operator can easily provide new value-added services, such as
live webcasting, web TV, distance learning, telemedicine, web radio, real time video conferencing, and
other bandwidth-critical and time-critical information services.
The term "router " in this document refers to both routers and Layer 3 switches.
Unless otherwise stated, the term "multicast" in this document refers to IP multicast.
Information transmission techniques
The information transmission techniques include unicast, broadcast, and multicast.
Unicast
In unicast transmission, the information source must send a separate copy of information to each host that
needs the information.
Figure 1 Unicast transmission
In Figure 1, assume that Host B, Host D and Host E need the information. A separate transmission channel
must be established from the information source to each of these hosts.
Source
Receiver
Receiver
Receiver
Host A
Host B
Host C
Host D
Host E
Packets for Host B
Packets for Host D
Packets for Host E
IP network

2
In unicast transmission, the traffic transmitted over the network is proportional to the number of hosts that
need the information. If a large number of hosts need the information, the information source must send
a separate copy of the same information to each of these hosts. Sending many copies can place a
tremendous pressure on the information source and the network bandwidth.
Unicast is not suitable for batch transmission of information.
Broadcast
In broadcast transmission, the information source sends information to all hosts on the subnet, even if
some hosts do not need the information.
Figure 2 Broadcast transmission
In Figure 2, assume that only Host B, Host D, and Host E need the information. If the information is
broadcast to the subnet, Host A and Host C also receive it. In addition to information security issues,
broadcasting to hosts that do not need the information also causes traffic flooding on the same subnet.
Broadcast is disadvantageous in transmitting data to specific hosts. Moreover, broadcast transmission is
a significant waste of network resources.
Multicast
Unicast and broadcast techniques cannot provide point-to-multipoint data transmissions with the
minimum network consumption.
Multicast transmission can solve this problem. When some hosts on the network need multicast
information, the information sender, or multicast source, sends only one copy of the information.
Multicast distribution trees are built through multicast routing protocols, and the packets are replicated
only on nodes where the trees branch.

3
Figure 3 Multicast transmission
The multicast source sends only one copy of the information to a multicast group. Host B, Host D and Host
E, which are receivers of the information, must join the multicast group. The routers on the network
duplicate and forward the information based on the distribution of the group members. Finally, the
information is correctly delivered to Host B, Host D, and Host E.
To summarize, multicast has the following advantages:
•Advantages over unicast—Because multicast traffic flows to the farthest-possible node from the
source before it is replicated and distributed, an increase in the number of hosts does not increase
the load of the source or remarkably add to the usage of network resources.
•Advantages over broadcast—Because multicast data is sent only to the receivers that need it,
multicast uses network bandwidth reasonably and enhances network security. In addition, data
broadcast is confined to the same subnet, but multicast is not.
Multicast features
•A multicast group is a multicast receiver set identified by an IP multicast address. Hosts join a
multicast group to become members of the multicast group before they can receive the multicast
data addressed to that multicast group. Typically, a multicast source does not need to join a
multicast group.
•An information sender is called a "multicast source". A multicast source can send data to multiple
multicast groups at the same time, and multiple multicast sources can send data to the same
multicast group at the same time.
•All hosts that have joined a multicast group become members of the multicast group. The group
memberships are dynamic. Hosts can join or leave multicast groups at any time. Multicast groups
are not subject to geographic restrictions.
•Routers or Layer 3 switches that support Layer 3 multicast are called "multicast routers" or "Layer 3
multicast devices". In addition to providing the multicast routing function, a multicast router can also
manage multicast group memberships on stub subnets with attached group members. A multicast
router itself can be a multicast group member.

4
For a better understanding of the multicast concept, you can compare multicast transmission to the
transmission of TV programs.
Table 1 Comparing TV program transmission and multicast transmission
TV transmission
Multicast transmission
A TV station transmits a TV program through a
channel.
A multicast source sends multicast data to a multicast
group.
A user tunes the TV set to the channel. A receiver joins the multicast group.
The user starts to watch the TV program transmitted
by the TV station via the channel.
The receiver starts to receive the multicast data that the
source is sending to the multicast group.
The user turns off the TV set or tunes to another
channel.
The receiver leaves the multicast group or joins another
group.
Common notations in multicast
The following notations are commonly used in multicast transmission:
•(*, G)—Indicates a rendezvous point tree (RPT), or a multicast packet that any multicast source sends
to multicast group G. Here, the asterisk represents any multicast source, and "G" represents a
specific multicast group.
•(S, G)—Indicates a shortest path tree (SPT), or a multicast packet that multicast source S sends to
multicast group G. Here, "S" represents a specific multicast source, and "G" represents a specific
multicast group.
Multicast advantages and applications
Multicast advantages
Advantages of the multicast technique include the following:
•Enhanced efficiency—Reduces the processor load of information source servers and network
devices.
•Optimal performance—Reduces redundant traffic.
•Distributed application—Enables point-to-multipoint applications at the price of minimum network
resources.
Multicast applications
The scenarios in which the multicast technique can be effectively applied are:
•Multimedia and streaming applications, such as web TV, web radio, and real time video/audio
conferencing
•Communication for training and cooperative operations, such as distance learning and
telemedicine
•Data warehouse and financial applications (stock quotes)
•Any other point-to-multipoint application for data distribution

5
Multicast models
Based on how the receivers treat the multicast sources, the multicast models include any-source multicast
(ASM), source-filtered multicast (SFM), and source-specific multicast (SSM).
ASM model
In the ASM model, any sender can send information to a multicast group as a multicast source, and
receivers can join a multicast group (identified by a group address) and obtain multicast information
addressed to that multicast group. In this model, receivers do not know the positions of the multicast
sources in advance. However, they can join or leave the multicast group at any time.
SFM model
The SFM model is derived from the ASM model. To a sender, the two models appear to have the same
multicast membership architecture.
The SFM model functionally extends the ASM model. The upper-layer software checks the source address
of received multicast packets and permits or denies multicast traffic from specific sources. Therefore,
receivers can receive the multicast data from only part of the multicast sources. To a receiver, multicast
sources are not all valid; they are filtered.
SSM model
Users might be interested in the multicast data from only certain multicast sources. The SSM model
provides a transmission service that enables users to specify the multicast sources that they are interested
in at the client side.
The main difference between the SSM model and the ASM model is that in the SSM model, receivers
have already determined the locations of the multicast sources by some other means. In addition, the
SSM model uses a multicast address range that is different from that of the ASM/SFM model, and
dedicated multicast forwarding paths are established between receivers and the specified multicast
sources.
Multicast architecture
IP multicast addresses the following questions:
•Where should the multicast source transmit information to? (Multicast addressing.)
•What receivers exist on the network? (Host registration.)
•Where is the multicast source that will provide data to the receivers? (Multicast source discovery.)
•How should information be transmitted to the receivers? (Multicast routing.)
IP multicast is an end-to-end service. The multicast architecture involves the following parts:
•Addressing mechanism—A multicast source sends information to a group of receivers through a
multicast address.
•Host registration—Receiver hosts can join and leave multicast groups dynamically. This mechanism
is the basis for management of group memberships.
•Multicast routing—A multicast distribution tree (a forwarding path tree for multicast data on the
network) is constructed for delivering multicast data from a multicast source to receivers.
•Multicast applications—A software system that supports multicast applications, such as video
conferencing, must be installed on multicast sources and receiver hosts. The TCP/IP stack must
support reception and transmission of multicast data.

6
Multicast addresses
Network-layer multicast addresses (multicast IP addresses) enables communication between multicast
sources and multicast group members. In addition, a technique must be available to map multicast IP
addresses to link-layer multicast MAC addresses.
IP multicast addresses
•IPv4 multicast addresses
Internet Assigned Numbers Authority (IANA) assigned the Class D address space (224.0.0.0 to
239.255.255.255) to IPv4 multicast.
Table 2 Class D IP address blocks and description
Address block Descri
p
tion
224.0.0.0 to 224.0.0.255
Reserved permanent group addresses. The IP address 224.0.0.0 is
reserved. Other IP addresses can be used by routing protocols and
for topology searching, protocol maintenance, and so on. Table 3
lists common permanent group addresses. A packet destined for an
address in this block will not be forwarded beyond the local subnet
regardless of the Time to Live (TTL) value in the IP header.
224.0.1.0 to 238.255.255.255
Globally scoped group addresses. This block includes the following
types of designated group addresses:
•232.0.0.0/8—SSM group addresses.
•233.0.0.0/8—Glop group addresses.
239.0.0.0 to 239.255.255.255
Administratively scoped multicast addresses. These addresses are
considered locally unique rather than globally unique, and can be
reused in domains administered by different organizations without
causing conflicts. For more information, see RFC 2365.
NOTE:
"Glop" is a mechanism for assi
g
nin
g
multicast addresses between different autonomous systems (ASs). By
filling an AS number into the middle two bytes of 233.0.0.0, you
g
et 255 multicast addresses for that AS.
For more information, see RFC 2770.
Table 3 Some reserved multicast addresses
Address Descri
p
tion
224.0.0.1 All systems on this subnet, including hosts and routers
224.0.0.2 All multicast routers on this subnet
224.0.0.3 Unassigned
224.0.0.4 Distance Vector Multicast Routing Protocol (DVMRP) routers
224.0.0.5 Open Shortest Path First (OSPF) routers
224.0.0.6 OSPF designated routers and backup designated routers
224.0.0.7 Shared Tree (ST) routers
224.0.0.8 ST hosts
224.0.0.9 Routing Information Protocol version 2 (RIPv2) routers
224.0.0.11 Mobile agents

7
Address Descri
p
tion
224.0.0.12 Dynamic Host Configuration Protocol (DHCP) server/relay agent
224.0.0.13 All Protocol Independent Multicast (PIM) routers
224.0.0.14 Resource Reservation Protocol (RSVP) encapsulation
224.0.0.15 All Core-Based Tree (CBT) routers
224.0.0.16 Designated Subnetwork Bandwidth Management (SBM)
224.0.0.17 All SBMs
224.0.0.18 Virtual Router Redundancy Protocol (VRRP)
•IPv6 multicast addresses
Figure 4 IPv6 multicast format
The following describes the fields of an IPv6 multicast address:
{0xFF—The most significant eight bits are 11111111, which indicates that this address is an IPv6
multicast address.
{Flags—The Flags field contains four bits.
Figure 5 Flags field format
Table 4 Flags field description
Bit Descri
p
tion
0 Reserved, set to 0.
R
•When set to 0, it indicates that this address is an IPv6 multicast
address without an embedded RP address.
•When set to 1, it indicates that this address is an IPv6 multicast
address with an embedded RP address. (The P and T bits must
also be set to 1.)
P
•When set to 0, it indicates that this address is an IPv6 multicast
address not based on a unicast prefix.
•When set to 1, it indicates that this address is an IPv6 multicast
address based on a unicast prefix. (The T bit must also be set to
1. )
T
•When set to 0, it indicates that this address is an IPv6 multicast
address permanently-assigned by IANA.
•When set to 1, it indicates that this address is a transient, or
dynamically assigned IPv6 multicast address.

8
{Scope—The Scope field contains four bits, which indicate the scope of the IPv6 internetwork for
which the multicast traffic is intended.
Table 5 Values of the Scope field
Value Meanin
g
0, F Reserved
1 Interface-local scope
2 Link-local scope
3 Subnet-local scope
4 Admin-local scope
5 Site-local scope
6, 7, 9 through D Unassigned
8 Organization-local scope
E Global scope
{Group ID—The Group ID field contains 112 bits. It uniquely identifies an IPv6 multicast group in
the scope that the Scope field defines.
Ethernet multicast MAC addresses
A multicast MAC address identifies a group of receivers at the data link layer.
•IPv4 multicast MAC addresses
As defined by IANA, the most significant 24 bits of an IPv4 multicast MAC address are 0x01005E.
Bit 25 is 0, and the other 23 bits are the least significant 23 bits of a multicast IPv4 address.
Figure 6 IPv4-to-MAC address mapping
The most significant four bits of a multicast IPv4 address are 1110, which indicates that this
address is a multicast address. Only 23 bits of the remaining 28 bits are mapped to a MAC
address, so five bits of the multicast IPv4 address are lost. As a result, 32 multicast IPv4 addresses
map to the same IPv4 multicast MAC address. Therefore, in Layer 2 multicast forwarding, a switch
might receive some multicast data destined for other IPv4 multicast groups. The upper layer must
filter such redundant data.
•IPv6 multicast MAC addresses
The most significant 16 bits of an IPv6 multicast MAC address are 0x3333. The least significant
32 bits are the least significant 32 bits of a multicast IPv6 address.

9
Figure 7 An example of IPv6-to-MAC address mapping
Multicast protocols
Generally, Layer 3 multicast refers to IP multicast working at the network layer. The corresponding
multicast protocols are Layer 3 multicast protocols, which include IGMP, MLD, PIM, IPv6 PIM, MSDP,
MBGP, and IPv6 MBGP. Layer 2 multicast refers to IP multicast working at the data link layer. The
corresponding multicast protocols are Layer 2 multicast protocols, which include IGMP snooping, MLD
snooping, PIM snooping, IPv6 PIM snooping, multicast VLAN, and IPv6 multicast VLAN.
IGMP snooping, PIM snooping, multicast VLAN, IGMP, PIM, MSDP, and MBGP are for IPv4, and MLD
snooping, IPv6 PIM snooping, IPv6 multicast VLAN, MLD, IPv6 PIM, and IPv6 MBGP are for IPv6.
This section provides only general descriptions about applications and functions of the Layer 2 and Layer
3 multicast protocols in a network. For more information about these protocols, see the related chapters.
Layer 3 multicast protocols
Layer 3 multicast protocols include multicast group management protocols and multicast routing
protocols.
Figure 8 Positions of Layer 3 multicast protocols
•Multicast group management protocols
Typically, the Internet Group Management Protocol (IGMP) or Multicast Listener Discovery Protocol
(MLD) is used between hosts and Layer 3 multicast devices that directly connect to the hosts. These

10
protocols define the mechanism of establishing and maintaining group memberships between
hosts and Layer 3 multicast devices.
•Multicast routing protocols
A multicast routing protocol runs on Layer 3 multicast devices to establish and maintain multicast
routes and forward multicast packets correctly and efficiently. Multicast routes constitute loop-free
data transmission paths from a data source to multiple receivers, namely, a multicast distribution
tree.
In the ASM model, multicast routes include intra-domain routes and inter-domain routes.
{An intra-domain multicast routing protocol discovers multicast sources and builds multicast
distribution trees within an AS to deliver multicast data to receivers. Among a variety of mature
intra-domain multicast routing protocols, Protocol Independent Multicast (PIM) is most widely
used. Based on the forwarding mechanism, PIM has dense mode (often referred to as
"PIM-DM"), and sparse mode (often referred to as "PIM-SM").
{An inter-domain multicast routing protocol is used for delivery of multicast information between
two ASs. So far, mature solutions include Multicast Source Discovery Protocol (MSDP) and
Multicast Border Gateway Protocol (MBGP). MSDP propagates multicast source information
among different ASs. MBGP is an extension of the Multiprotocol Border Gateway Protocol
(MP-BGP) for exchanging multicast routing information among different ASs.
For the SSM model, multicast routes are not divided into intra-domain routes and inter-domain
routes. Because receivers know the position of the multicast source, channels established through
PIM-SM are sufficient for the transport of multicast information.
Layer 2 multicast protocols
Layer 2 multicast protocols include IGMP snooping, MLD snooping, PIM snooping, IPv6 PIM snooping,
multicast VLAN, and IPv6 multicast VLAN.
Figure 9 Positions of Layer 2 multicast protocols
•IGMP snooping and MLD snooping
IGMP snooping and MLD snooping are multicast constraining mechanisms that run on Layer 2
devices. They manage and control multicast groups by monitoring and analyzing IGMP or MLD
messages exchanged between the hosts and Layer 3 multicast devices, effectively controlling the
flooding of multicast data in a Layer 2 network.

11
•PIM snooping and IPv6 PIM snooping
PIM snooping and IPv6 PIM snooping run on Layer 2 devices. They determine which ports are
interested in multicast data by analyzing the received IPv6 PIM messages, and add the ports to a
multicast forwarding entry to make sure that multicast data can be forwarded to only the ports that
are interested in the data.
•Multicast VLAN and IPv6 multicast VLAN
In the traditional multicast-on-demand mode, when users in different VLANs on a Layer 2 device
need multicast information, the upstream Layer 3 device must forward a separate copy of the
multicast data to each VLAN of the Layer 2 device. When the multicast VLAN or IPv6 multicast
VLAN feature is enabled on the Layer 2 device, the Layer 3 multicast device sends only one copy
of multicast to the multicast VLAN or IPv6 multicast VLAN on the Layer 2 device. This approach
avoids waste of network bandwidth and extra burden on the Layer 3 device.
Multicast packet forwarding mechanism
In a multicast model, a multicast source sends information to the host group identified by the multicast
group address in the destination address field of IP multicast packets. To deliver multicast packets to
receivers located at different positions of the network, multicast routers on the forwarding paths usually
need to forward multicast packets that an incoming interface receives to multiple outgoing interfaces.
Compared with a unicast model, a multicast model is more complex in the following aspects:
•To ensure multicast packet transmission in the network, unicast routing tables or multicast routing
tables (for example, the MBGP routing table) specially provided for multicast must be used as
guidance for multicast forwarding.
•To process the same multicast information from different peers received on different interfaces of the
same device, every multicast packet undergoes a reverse path forwarding (RPF) check on the
incoming interface. The result of the RPF check determines whether the packet will be forwarded or
discarded. The RPF check mechanism is the basis for most multicast routing protocols to implement
multicast forwarding.

12
Configuring IGMP snooping
Overview
Internet Group Management Protocol (IGMP) snooping is a multicast constraining mechanism that runs
on Layer 2 devices to manage and control multicast groups.
By analyzing received IGMP messages, a Layer 2 device that runs IGMP snooping establishes mappings
between ports and multicast MAC addresses, and forwards multicast data based on these mappings.
As shown in Figure 10, without IGMP snooping enabled, the Layer 2 switch floods multicast packets to
all devices at Layer 2. With IGMP snooping enabled, the Layer 2 switch forwards multicast packets for
known multicast groups to only the receivers that require the multicast data at Layer 2. This feature
improves bandwidth efficiency, enhances multicast security, and helps per-host accounting for multicast
users.
Figure 10 Before and after IGMP snooping is enabled on the Layer 2 device
Basic concepts in IGMP snooping
IGMP snooping related ports
As shown in Figure 11, Router A connects to the multicast source, IGMP snooping runs on Switch A and
Switch B, and Host A and Host C are receiver hosts as members of a multicast group.

13
Figure 11 IGMP snooping related ports
The following describes the ports involved in IGMP snooping:
•Router port—Layer 3 multicast device-side port. Layer 3 multicast devices include designated
routers (DRs) and IGMP queriers. In Figure 11, Ethernet 1/0/1 of Switch A and Ethernet 1/0/1 of
Switch B are router ports. The switch registers all its router ports in its router port list.
Do not confuse the "router port" in IGMP snooping with the "routed interface" commonly known as
the "Layer 3 interface." The router port in IGMP snooping is the Layer 2 interface.
•Member port—Multicast receiver-side port. In Figure 11, Ethernet 1/0/2 and Ethernet 1/0/3 of
Switch A and Ethernet 1/0/2 of Switch B are member ports. The switch registers all its member
ports in its IGMP snooping forwarding table.
Unless otherwise specified, router ports and member ports in this document include both static and
dynamic router ports and member ports.
NOTE:
A
n IGMP-snooping-enabled switch deems that all its ports on which IGMP
g
eneral queries with the source
IP address other than 0.0.0.0 or that receive PIM hello messages are received are dynamic router ports.
Aging timers for dynamic ports in IGMP snooping and related messages and actions
Timer Descri
p
tion Messa
g
e before ex
p
ir
y
Action after ex
p
ir
y
Dynamic router port
aging timer
For each dynamic router
port, the switch starts an
aging timer. When the
timer expires, the
dynamic router port ages
out.
IGMP general query of
which the source address
is not 0.0.0.0 or PIM
hello.
The switch removes this
port from its router port
list.

14
Timer Descri
p
tion Messa
g
e before ex
p
ir
y
Action after ex
p
ir
y
Dynamic member port
aging timer
When a port dynamically
joins a multicast group,
the switch starts an aging
timer for the port. When
the timer expires, the
dynamic member port
ages out.
IGMP membership
report.
The switch removes this
port from the IGMP
snooping forwarding
table.
NOTE:
In IGMP snooping, only dynamic ports age out. Static ports never age out.
How IGMP snooping works
In this section, the involved ports are dynamic ports. For information about how to configure and remove
static ports, see "Configuring static ports."
A switch that runs IGMP snooping performs different actions when it receives different IGMP messages.
When receiving a general query
The IGMP querier periodically sends IGMP general queries to all hosts and routers identified by the
address 224.0.0.1 on the local subnet to determine whether any active multicast group members exist on
the subnet.
After receiving an IGMP general query, the switch forwards it to all ports in the VLAN, except the port
that received the query. The switch also performs one of the following actions:
•If the receiving port is a dynamic router port in the router port list, restarts the aging timer for the
port.
•If the receiving port is not in its router port list, adds it into its router port list as a dynamic router port
and starts an aging timer for the port.
When receiving a membership report
A host sends an IGMP report to the IGMP querier for the following purposes:
•If the host has been a member of a multicast group, responds to the query with an IGMP report.
•Applies for joining a multicast group.
After receiving an IGMP report, the switch forwards it through all the router ports in the VLAN, resolves
the address of the reported multicast group. The switch also performs one of the following actions:
•If no forwarding entry matches the group address, creates a forwarding entry for the group, adds
the receiving port as a dynamic member port to the forwarding entry, and starts an aging timer for
the port.
•If a forwarding entry matches the group address, but the receiving port is not in the forwarding
entry for the group, adds the port as a dynamic member port to the forwarding entry and starts an
aging timer for the port.
•If a forwarding entry matches the group address and the receiving port is in the forwarding entry
for the group, restarts the aging timer for the port.
A switch does not forward an IGMP report through a non-router port. If the switch forwards a report
message through a member port, the IGMP report suppression mechanism causes all the attached hosts

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that are monitoring the reported multicast address suppress their own reports. This makes the switch
unable to know whether the reported multicast group still has active members attached to that port.
When receiving a leave message
When an IGMPv1 host leaves a multicast group, the host does not send an IGMP leave message, and the
switch cannot know immediately that the host has left the multicast group. However, because the host
stops sending IGMP reports as soon as it leaves the multicast group, the switch removes the port that
connects to the host from the forwarding entry for the multicast group when the aging timer for the port
expires.
When an IGMPv2 or IGMPv3 host leaves a multicast group, the host sends an IGMP leave message to
the multicast router.
When the switch receives an IGMP leave message on a dynamic member port, the switch first examines
whether a forwarding entry matches the group address in the message, and, if a match is found, whether
the forwarding entry for the group contains the dynamic member port.
•If no forwarding entry matches the group address, or if the forwarding entry does not contain the
port, the switch directly discards the IGMP leave message.
•If a forwarding entry matches the group address and the forwarding entry contains the port, the
switch forwards the leave message to all router ports in the VLAN. Because the switch does not
know whether any other hosts attached to the port are still listening to that group address, the switch
does not immediately remove the port from the forwarding entry for that group. Instead, it restarts
the aging timer for the port.
After receiving the IGMP leave message, the IGMP querier resolves the multicast group address in the
message and sends an IGMP group-specific query to the multicast group through the port that received
the leave message. After receiving the IGMP group-specific query, the switch forwards it through all its
router ports in the VLAN and all member ports of the multicast group. The switch also performs the
following judgment for the port that received the IGMP leave message:
•If the port (assuming that it is a dynamic member port) receives an IGMP report in response to the
group-specific query before its aging timer expires, it indicates that some host attached to the port
is receiving or expecting to receive multicast data for the multicast group. The switch restarts the
aging timer for the port.
•If the port receives no IGMP report in response to the group-specific query before its aging timer
expires, it indicates that no hosts attached to the port are still listening to that group address. The
switch removes the port from the forwarding entry for the multicast group when the aging timer
expires.
IGMP snooping proxying
You can configure the IGMP snooping proxying function on an edge device to reduce the number of
IGMP reports and leave messages sent to its upstream device. The device configured with IGMP
snooping proxying is called an IGMP snooping proxy. It is a host from the perspective of its upstream
device.
Even though an IGMP snooping proxy is a host from the perspective of its upstream device, the IGMP
membership report suppression mechanism for hosts does not take effect on it.