HPE FlexNetwork 5130 EI Series User manual

HPE FlexNetwork 5130 EI Switch Series
IP Multicast Configuration Guide
Part number:5998-5477s
Software version: Release 3111P02 and later
Document version: 6W101-20161010

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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 benefits and applications ············································································································4
Multicast models ················································································································································4
IP multicast architecture·····································································································································5
Multicast addresses ···································································································································5
Multicast protocols ·····································································································································8
Multicast packet forwarding mechanism··········································································································11
Configuring IGMP snooping··········································································12
Overview··························································································································································12
Basic IGMP snooping concepts···············································································································12
How IGMP snooping works······················································································································14
Protocols and standards ··························································································································15
IGMP snooping configuration task list··············································································································15
Configuring basic IGMP snooping features ·····································································································16
Enabling IGMP snooping ·························································································································16
Specifying an IGMP snooping version·····································································································17
Setting the maximum number of IGMP snooping forwarding entries·······················································17
Configuring IGMP snooping port features········································································································18
Setting aging timers for dynamic ports·····································································································18
Configuring static ports ····························································································································19
Configuring a port as a simulated member host ······················································································19
Enabling fast-leave processing················································································································20
Disabling a port from becoming a dynamic router port ············································································20
Configuring the IGMP snooping querier···········································································································21
Configuration prerequisites······················································································································21
Enabling the IGMP snooping querier·······································································································21
Configuring parameters for IGMP queries and responses·······································································22
Configuring parameters for IGMP messages···································································································22
Configuration prerequisites······················································································································23
Configuring source IP addresses for IGMP messages············································································23
Setting the 802.1p priority for IGMP messages ·······················································································24
Configuring IGMP snooping policies················································································································24
Configuring a multicast group policy········································································································24
Configuring multicast source port filtering································································································25
Enabling dropping unknown multicast data······························································································25
Enabling IGMP report suppression··········································································································26
Setting the maximum number of multicast groups on a port····································································26
Enabling the multicast group replacement feature···················································································26
Displaying and maintaining IGMP snooping ····································································································27
IGMP snooping configuration examples ··········································································································28
Group policy and simulated joining configuration example (for VLANs)··················································28
Static port configuration example·············································································································30
IGMP snooping querier configuration example························································································33
Troubleshooting IGMP snooping ·····················································································································36
Layer 2 multicast forwarding cannot function···························································································36
Multicast group policy does not work·······································································································36
Configuring PIM snooping·············································································37
Overview··························································································································································37
Configuring PIM snooping································································································································38
Displaying and maintaining PIM snooping·······································································································38
PIM snooping configuration example···············································································································39

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Troubleshooting PIM snooping ························································································································42
PIM snooping does not work on a Layer 2 device ···················································································42
Configuring multicast VLANs········································································43
Overview··························································································································································43
Multicast VLAN configuration task list··············································································································45
Configuring a sub-VLAN-based multicast VLAN······························································································45
Configuration prerequisites······················································································································45
Configuration guidelines···························································································································45
Configuration procedure···························································································································45
Configuring a port-based multicast VLAN········································································································46
Configuration prerequisites······················································································································46
Configuring user port attributes················································································································46
Assigning user ports to a multicast VLAN································································································46
Setting the maximum number of multicast VLAN forwarding entries·······························································47
Displaying and maintaining multicast VLANs···································································································47
Multicast VLAN configuration examples ··········································································································48
Sub-VLAN-based multicast VLAN configuration example ·······································································48
Port-based multicast VLAN configuration example··················································································50
Configuring MLD snooping ···········································································54
Overview··························································································································································54
Basic MLD snooping concepts·················································································································54
How MLD snooping works ·······················································································································56
Protocols and standards ··························································································································57
MLD snooping configuration task list ···············································································································57
Configuring basic MLD snooping features·······································································································58
Enabling MLD snooping···························································································································58
Specifying an MLD snooping version·······································································································58
Setting the maximum number of MLD snooping forwarding entries ························································59
Configuring MLD snooping port features ·········································································································60
Setting aging timers for dynamic ports·····································································································60
Configuring static ports ····························································································································60
Configuring a port as a simulated member host ······················································································61
Enabling fast-leave processing················································································································61
Disabling a port from becoming a dynamic router port ············································································62
Configuring the MLD snooping querier ············································································································63
Configuration prerequisites······················································································································63
Enabling the MLD snooping querier·········································································································63
Configuring parameters for MLD queries and responses ········································································63
Configuring parameters for MLD messages ····································································································64
Configuration prerequisites······················································································································64
Configuring source IPv6 addresses for MLD messages··········································································65
Setting the 802.1p priority for MLD messages·························································································66
Configuring MLD snooping policies ·················································································································66
Configuring an IPv6 multicast group policy······························································································66
Configuring IPv6 multicast source port filtering························································································67
Enabling dropping unknown IPv6 multicast data ·····················································································67
Enabling MLD report suppression············································································································68
Setting the maximum number of IPv6 multicast groups on a port····························································68
Enabling IPv6 multicast group replacement·····························································································69
Displaying and maintaining MLD snooping······································································································69
MLD snooping configuration examples············································································································70
IPv6 group policy and simulated joining configuration example (for VLANs)···········································70
Static port configuration example·············································································································72
MLD snooping querier configuration example··························································································75
Troubleshooting MLD snooping·······················································································································78
Layer 2 multicast forwarding cannot function···························································································78
IPv6 multicast group policy does not work·······························································································78
Configuring IPv6 PIM snooping ····································································79
Overview··························································································································································79

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Configuring IPv6 PIM snooping ·······················································································································80
Displaying and maintaining IPv6 PIM snooping·······························································································81
IPv6 PIM snooping configuration example·······································································································81
Troubleshooting IPv6 PIM snooping················································································································84
IPv6 PIM snooping does not work on a Layer 2 device···········································································84
Configuring IPv6 multicast VLANs································································85
Overview··························································································································································85
IPv6 multicast VLAN configuration task list······································································································87
Configuring a sub-VLAN-based IPv6 multicast VLAN ·····················································································87
Configuration prerequisites······················································································································87
Configuration guidelines···························································································································87
Configuration procedure···························································································································87
Configuring a port-based IPv6 multicast VLAN································································································88
Configuration prerequisites······················································································································88
Configuring user port attributes················································································································88
Assigning user ports to an IPv6 multicast VLAN······················································································88
Setting the maximum number of IPv6 multicast VLAN forwarding entries·······················································89
Displaying and maintaining IPv6 multicast VLANs ··························································································89
IPv6 multicast VLAN configuration examples ··································································································90
Sub-VLAN-based IPv6 multicast VLAN configuration example·······························································90
Port-based IPv6 multicast VLAN configuration example··········································································92
Document conventions and icons·································································96
Conventions·····················································································································································96
Network topology icons····································································································································97
Support and other resources ········································································98
Accessing Hewlett Packard Enterprise Support ······························································································98
Accessing updates···········································································································································98
Websites ··················································································································································99
Customer self repair·································································································································99
Remote support········································································································································99
Documentation feedback ·························································································································99
Index···········································································································101

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 bandwidth-critical and
time-critical information services. These services include live webcasting, Web TV, distance learning,
telemedicine, Web radio, and real-time video conferencing.
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, Host B, Host D, and Host E need the information.Aseparate transmission channel must
be established from the information source to each of these hosts.
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.
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
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, only Host B, Host D, and Host E need the information. If the information is broadcast to
the subnet, Host Aand 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 not as efficient as multicast for sending data to groups of hosts.
Multicast
Multicast provides point-to-multipoint data transmissions with the minimum network consumption.
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
In Figure 3, the multicast source sends only one copy of the information to a multicast group. Host B,
Host D, and Host E, which are information receivers, 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—Multicast data is replicated and distributed until it flows to the
farthest-possible node from the source. The increase of multicast receivers will not remarkably
increase the load of the source or the usage of network resources.
•Advantages over broadcast—Multicast data is sent only to the receivers that need it. This
saves network bandwidth use and enhances network security. In addition, multicast data is not
confined to the same subnet.
Multicast features
•A multicast group is a multicast receiver set identified by an IP multicast address. Hosts must
join a multicast group to become members of the multicast group before they receive the
multicast data addressed to that multicast group. Typically, a multicast source does not need to
join a multicast group.
•Amulticast source is an information sender. It can send data to multiple multicast groups at the
same time. Multiple multicast sources can send data to the same multicast group at the same
time.
•The group memberships are dynamic. Hosts can join or leave multicast groups at any time.
Multicast groups are not subject to geographic restrictions.
•Multicast routers or Layer 3 multicast devices are routers or Layer 3 switches that support Layer
3 multicast. They provide multicast routing and manage multicast group memberships on stub
subnets with attached group members. A multicast router itself can be a multicast group
member.
For a better understanding of the multicast concept, you can compare multicast transmission to the
transmission of TV programs.

4
Table 1 Comparing TV program transmission and multicast transmission
TV program 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 on the channel. The receiver starts to receive the multicast data sent by the source 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)—Rendezvous point tree (RPT), or a multicast packet that any multicast source sends to
multicast group G. The asterisk (*) represents any multicast source, and "G" represents a
specific multicast group.
•(S, G)—Shortest path tree (SPT), or a multicast packet that multicast source "S" sends to
multicast group "G." "S" represents a specific multicast source, and "G" represents a specific
multicast group.
Multicast benefits and applications
Multicast benefits
•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
•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
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 multicast sources can send information to a multicast group. Receivers can
join a multicast group and get multicast information addressed to that multicast group from any

5
multicast sources. In this model, receivers do not know the positions of the multicast sources in
advance.
SFM model
The SFM model is derived from the ASM model. To a multicast source, 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, but are filtered.
SSM model
The SSM model provides a transmission service that enables multicast receivers to specify the
multicast sources in which they are interested.
In the SSM model, receivers have already determined the locations of the multicast sources. This is
the main difference between the SSM model and theASM model. In addition, the SSM model uses a
different multicast address range than the ASM/SFM model. Dedicated multicast forwarding paths
are established between receivers and the specified multicast sources.
IP multicast architecture
IP multicast addresses the following issues:
•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 is the information transmitted to the receivers? (Multicast routing.)
IP multicast is an end-to-end service. The multicast architecture involves the following parts:
•Addressing mechanism—Amulticast source sends information to a group of receivers
through a multicast address.
•Host registration—Multicast receivers can join and leave multicast groups dynamically. This
mechanism is the basis for management of group memberships.
•Multicast routing—Amulticast 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—Asoftware system that supports multicast applications,such as video
conferencing, must be installed on multicast sources and multicast receivers. The TCP/IPstack
must support reception and transmission of multicast data.
Multicast addresses
IP multicast addresses
•IPv4 multicast addresses:
IANA assigns the Class D address block (224.0.0.0 to 239.255.255.255) to IPv4 multicast.
Table 2 Class D IP address blocks and description
Address block Description
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

6
Address block Description
group addresses. A packet destined for an address in this
block will not be forwarded beyond the local subnet regardless
of the 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. You can reuse them in domains administered by
different organizations without causing conflicts. For more
information, see RFC 2365.
NOTE:
"Glop" is a mechanism for assigning multicast addresses between different ASs. By filling an
A
S number into the middle two bytes of 233.0.0.0, you get 255 multicast addresses for that AS.
For more information, see RFC 2770.
Table 3 Common permanent multicast group addresses
Address Description
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 DVMRP routers.
224.0.0.5 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 RIPv2 routers.
224.0.0.11 Mobile agents.
224.0.0.12 DHCP server/relay agent.
224.0.0.13 All Protocol Independent Multicast (PIM) routers.
224.0.0.14 RSVP encapsulation.
224.0.0.15 All Core-Based Tree (CBT) routers.
224.0.0.16 Designated SBM.
224.0.0.17 All SBMs.
224.0.0.18 VRRP.
•IPv6 multicast addresses:

7
Figure 4 IPv6 multicast format
The following describes the fields of an IPv6 multicast address:
{0xFF—The most significant eight bits are 11111111.
{Flags—The Flags field contains four bits.
Figure 5 Flags field format
Table 4 Flags field description
Bit Description
0 Reserved, set to 0.
R
•When set to 0, this address is an IPv6 multicast
address without an embedded RP address.
•When set to 1, 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, this address is an IPv6 multicast
address not based on a unicast prefix.
•When set to 1, 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, this address is an IPv6 multicast
address permanently-assigned by IANA.
•When set to 1, this address is a transient or
dynamically assigned IPv6 multicast address.
{Scope—The Scope field contains four bits, which represent the scope of the IPv6
internetwork for which the multicast traffic is intended.
Table 5 Values of the Scope field
Value Meaning
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.

8
Value Meaning
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
•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 an IPv4 multicast
address.
Figure 6 IPv4-to-MAC address mapping
The most significant four bits of an IPv4 multicast address are fixed at 1110. In an IPv4-to-MAC
address mapping, five bits of the IPv4 multicast address are lost. As a result, 32 IPv4 multicast
addresses are mapped to the same IPv4 multicast MAC address. Therefore, a device might
receive unwanted multicast data at Layer 2 processing, which needs to be filtered by the upper
layer.
•IPv6 multicast MAC addresses:
As defined by IANA, the most significant 16 bits of an IPv6 multicast MAC address are 0x3333.
The least significant 32 bits are mapped from the least significant 32 bits of an IPv6 multicast
address. Therefore, the problem of duplicate IPv6-to-MAC address mapping also arises like
IPv4-to-MAC address mapping.
Figure 7 An example of IPv6-to-MAC address mapping
Multicast protocols
Multicast protocols include the following categories:
•Layer 3 and Layer 2 multicast protocols:

9
{Layer 3 multicast refers to IP multicast working at the network layer.
Layer 3 multicast protocols—IGMP, MLD, PIM, IPv6 PIM, MSDP, MBGP, and IPv6
MBGP.
{Layer 2 multicast refers to IP multicast working at the data link layer.
Layer 2 multicast protocols—IGMP snooping, MLD snooping, PIM snooping, IPv6 PIM
snooping, multicast VLAN, and IPv6 multicast VLAN.
•IPv4 and IPv6 multicast protocols:
{For IPv4 networks—IGMP snooping, PIM snooping, multicast VLAN, IGMP, PIM, MSDP,
and MBGP.
{For IPv6 networks—MLD snooping, IPv6 PIM snooping, IPv6 multicast VLAN, MLD, IPv6
PIM, and IPv6 MBGP.
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 Layer 2 multicast protocols, see
the related chapters.
Layer 3 multicast protocols
In Figure 8, 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:
Internet Group Management Protocol (IGMP) and Multicast Listener Discovery (MLD) protocol
are multicast group management protocols. Typically, they run between hosts and Layer 3
multicast devices that directly connect to the hosts to establish and maintain the multicast group
memberships.
•Multicast routing protocols:
A multicast routing protocol runs on Layer 3 multicast devices to establish and maintain
multicast routes and correctly and efficiently forward multicast packets. Multicast routes
constitute loop-free data transmission paths (also known as multicast distribution trees) from a
data source to multiple receivers.
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 anAS to deliver multicast data to receivers. Among a variety of
mature intra-domain multicast routing protocols, PIM is most widely used. Based on the

10
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 delivering multicast information
between two ASs. So far, mature solutions include Multicast Source Discovery Protocol
(MSDP) and MBGP. MSDP propagates multicast source information among different ASs.
MBGP is an extension of the 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
In Figure 9, 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 run on Layer 2 devices as multicast constraining
mechanisms to improve multicast forwarding efficiency. They generate Layer 2 multicast
forwarding tables by listening to IGMP or MLD messages exchanged between the hosts and
Layer 3 multicast devices. This effectively controls the flooding of multicast data in Layer 2
networks.
•PIM snooping and IPv6 PIM snooping:
PIM snooping and IPv6 PIM snooping run on Layer 2 devices. They work with IGMP snooping
or MLD snooping to analyze received PIM messages. Then, they add the ports that are
interested in specific multicast data to a PIM snooping routing entry or IPv6 PIM snooping
routing entry. In this way, multicast data can be forwarded to only the ports that are interested in
the data.
•Multicast VLAN and IPv6 multicast VLAN:
Multicast VLAN or IPv6 multicast VLAN runs on a Layer 2 device on a multicast network where
multicast receivers for the same group exist in different VLANs. With these protocols, 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 method avoids waste of network bandwidth and extra burden
on the Layer 3 device.

11
Multicast packet forwarding mechanism
In a multicast model, multicast receivers of a multicast group are usually located at different areas on
the network. They are identified by the same multicast group address. To deliver multicast packets to
these receivers, a multicast source encapsulates the multicast data in an IPpacket with the multicast
group address as the destination address. Multicast routers on the forwarding paths forward
multicast packets that an incoming interface receives through multiple outgoing interfaces.
Compared to a unicast model, a multicast model is more complex in the following aspects:
•To ensure multicast packet transmission in the network, different routing tables are used for
multicast forwarding. These routing tables include unicast routing tables, routing tables for
multicast (for example, the MBGP routing table), and static multicast routing tables.
•To process the same multicast information from different peers received on different interfaces,
the multicast device performs an RPF check on each multicast packet. The RPF check result
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
IGMP snooping runs on a Layer 2 device as a multicast constraining mechanism to improve
multicast forwarding efficiency. It creates Layer 2 multicast forwarding entries from IGMP packets
that are exchanged between the hosts and the router.
As shown in Figure 10, when IGMP snooping is not enabled, the Layer 2 device floods multicast
packets to all hosts. When IGMPsnooping is enabled, the Layer 2 device forwards multicast packets
of known multicast groups to only the receivers.
Figure 10 Multicast packet transmission without and with IGMP snooping
Basic IGMP snooping concepts
IGMP snooping related ports
As shown in Figure 11, IGMP snooping runs on Switch A and Switch B, and Host A and Host C are
receivers in 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, GigabitEthernet 1/0/1 of Switch A and
GigabitEthernet 1/0/1 of Switch B are the router ports.A Layer 2 device records all its router
ports in a 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 a Layer 2 interface.
•Member port—Multicast receiver-side port. In Figure 11, GigabitEthernet 1/0/2 and
GigabitEthernet 1/0/3 of Switch A and GigabitEthernet 1/0/2 of Switch B are the member ports.
A Layer 2 device records all its member ports in the 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:
When IGMP snooping is enabled, all ports that receive PIM hello messages or IGMP general
queries with the source addresses other than 0.0.0.0 are considered dynamic router ports.
Aging timers for dynamic ports in IGMP snooping
The following are aging timers for dynamic ports in IGMP snooping:
•Dynamic router port aging timer—The Layer 2 device starts this timer for a portthat receives
an IGMP general query with the source address other than 0.0.0.0 or a PIM hello message. If
the port does not receive either of these messages before the timer expires, the Layer 2 device
removes the port from its router port list.
•Dynamic member port aging timer—The Layer 2 device starts this timer for a port that
receives an IGMP report. If the port does not receive a report before the timer expires, the Layer
2 device removes the port from the IGMP snooping forwarding entries.
NOTE:
In IGMP snooping, only dynamic ports age out. Static ports never age out.
Router A Switch A
Switch B
GE1/0/1 GE1/0/2
GE1/0/3
GE1/0/1 GE1/0/2
Receiver
Receiver
Host A
Host B
Host C
Host D
Source
Multicast packets
Router port
Member port

14
How IGMP snooping works
The ports in this section are dynamic ports. For information about how to configure and remove static
ports, see "Configuring static ports."
IGMP messages types include general query, IGMP report, and leave message. An IGMP
snooping-enabled Layer 2 device performs differently depending on the message.
General query
The IGMP querier periodically sends IGMP general queries to all hosts and routers on the local
subnet to check for the existence of multicast group members.
After receiving an IGMPgeneral query, the Layer 2device forwards the query to all ports in the VLAN
except the receiving port. The Layer 2 device also performs one of the following actions:
•If the receiving port is a dynamic router port in the router port list, the Layer 2 device restarts the
aging timer for the port.
•If the receiving port does not exist in the router port list, the Layer 2 device adds the port to the
router port list. It also starts an aging timer for the port.
IGMP report
A host sends an IGMP report to the IGMP querier for the following purposes:
•Responds to queries if the host is a multicast group member.
•Applies for a multicast group membership.
After receiving an IGMP report from a host, the Layer 2 device forwards the report through all the
router ports in the VLAN. It also resolves the address of the reported multicast group, and looks up
the forwarding table:
•If no match is found, the Layer 2 device creates a forwarding entry with the receiving port as an
outgoing interface to the forwarding entry. It also marks the receiving port as a dynamic member
port and starts an aging timer for the port.
•If a match is found but the receiving port is not in the forwarding entry, the Layer 2 device adds
the port as an outgoing interface to the forwarding entry. It also marks the receiving port as a
dynamic member port and starts an aging timer for the port.
•If a match is found and the receiving port is in the forwarding entry, the Layer 2 device restarts
the aging timer for the port.
In an application with a multicast group policy configured on an IGMP snooping-enabled Layer 2
device, when a user requests a multicast program, the user's host initiates an IGMP report. After
receiving this report, the Layer 2 device resolves the multicast group address in the report and
performs ACL filtering on the report. If the report passesACL filtering, the Layer 2 device creates an
IGMP snooping forwarding entry with the receiving port as an outgoing interface. If the report does
not pass ACL filtering, the Layer 2 device drops this report, which means the receiver does not
successfully join the multicast group and cannot retrieve the program.
A Layer 2 device does not forward an IGMP report through a non-router port because of the IGMP
report suppression mechanism.
Leave message
An IGMPv1 host does not send any leave messages when it leaves a multicast group. The Layer 2
device cannot immediately update the status of the port that connects to the receiver host. In this
case, it does not remove the port from the outgoing interface list in the associated forwarding entry
until the aging timer for the multicast group on the port expires.
An IGMPv2 or IGMPv3 host sends an IGMP leave message when it leaves a multicast group.
When the Layer 2 device receives an IGMP leave message on a dynamic member port, the Layer 2
device first examines whether a forwarding entry matches the group address in the message.
•If no match is found, the Layer 2 device discards the IGMP leave message.

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•If a match is found but the receiving port is not in the forwarding entry, the Layer 2 device
discards the IGMP leave message.
•If a match is found and the receiving port is in the forwarding entry, the Layer 2 device forwards
the leave message to all router ports in the VLAN. The Layer 2 device does not immediately
remove the port from the forwarding entry for that group. Instead, it adjusts the aging timer for
the port.
After receiving the IGMP leave message, the IGMP querier resolves the multicast group address in
the message. Then, it 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 Layer 2 device forwards the query through all its
router ports in the VLAN and all member ports of the multicast group. Then, it waits for the
responding IGMP reports from the directly connected receivers. For the dynamic member port that
received the leave message, the Layer 2 device also performs one of the following actions:
•If the port receives an IGMP report before the aging timer expires, the Layer 2 device restarts
the aging timer for the port.
•If the port does not receive an IGMP report when the aging timer expires, the Layer 2 device
removes the port from the forwarding entry for the multicast group.
Protocols and standards
RFC 4541, Considerations for Internet Group Management Protocol (IGMP) and Multicast Listener
Discovery (MLD) Snooping Switches
IGMP snooping configuration task list
Tasks at a glance
Configuring basic IGMP snooping features:
•(Required.) Enabling IGMP snooping
•(Optional.) Specifying an IGMP snooping version
•(Optional.) Setting the maximum number of IGMP snooping forwarding entries
Configuring IGMP snooping port features:
•(Optional.) Setting aging timers for dynamic ports
•(Optional.) Configuring static ports
•(Optional.) Configuring a port as a simulated member host
•(Optional.) Enabling fast-leave processing
•(Optional.) Disabling a port from becoming a dynamic router port
Configuring the IGMP snooping querier:
•(Optional.) Enabling the IGMP snooping querier
•(Optional.) Configuring parameters for IGMP queries and responses
Configuring parameters for IGMP messages:
•(Optional.) Configuring source IP addresses for IGMP messages
•(Optional.) Setting the 802.1p priority for IGMP messages