Aztech ASX6600 Instructions for use

HPE FlexNetwork 5120 SI Switch Series
IP Multicast
Configuration Guide
Part number: 5998-8495
Software version: Release 1516
Document version: 6W100-20151125

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Contents
Multicast overview···························································································1
Introduction to multicast·····································································································································1
Information transmission techniques··········································································································1
Multicast features·······································································································································3
Common notations in multicast··················································································································4
Advantages and applications of multicast··································································································4
Multicast models ················································································································································4
Multicast architecture·········································································································································5
Multicast addresses ···································································································································5
Multicast protocols ·····································································································································9
Multicast packet forwarding mechanism··········································································································11
IGMP snooping configuration········································································12
IGMP snooping overview·································································································································12
Principle of IGMP snooping······················································································································12
Basic concepts in IGMP snooping ···········································································································13
How IGMP snooping works······················································································································14
IGMP snooping proxying··························································································································15
Protocols and standards ··························································································································16
IGMP snooping configuration task list··············································································································17
Configuring basic functions of IGMP snooping································································································18
Configuration prerequisites······················································································································18
Enabling IGMP snooping ·························································································································18
Configuring the version of IGMP snooping ······························································································18
Configuring static multicast MAC address entries····················································································19
Configuring IGMP snooping port functions ······································································································20
Configuration prerequisites······················································································································20
Configuring aging timers for dynamic ports······························································································20
Configuring static ports ····························································································································21
Configuring simulated joining···················································································································21
Configuring fast leave processing············································································································22
Disabling a port or a group of ports from changing into dynamic router ports ·········································23
Configuring IGMP snooping querier·················································································································23
Configuration prerequisites······················································································································23
Enabling IGMP snooping querier·············································································································24
Configuring IGMP queries and responses·······························································································24
Configuring source IP address of IGMP queries······················································································25
Configuring IGMP snooping proxying ··············································································································26
Configuration prerequisites······················································································································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··············································································································26
Configuration prerequisites······················································································································26
Configuring a multicast group filter···········································································································27
Configuring multicast source port filtering································································································27
Configuring the function of dropping unknown multicast data··································································28
Configuring IGMP report suppression······································································································28
Configuring maximum multicast groups that a port can join ····································································29
Configuring multicast group replacement·································································································29
Configuring 802.1p precedence for IGMP messages··············································································30
Displaying and maintaining IGMP snooping ····································································································31
IGMP snooping configuration examples ··········································································································31
Group policy and simulated joining configuration example······································································31
Static port configuration example·············································································································34
IGMP snooping querier configuration example························································································37
IGMP snooping proxying configuration example······················································································39
Troubleshooting IGMP snooping configuration································································································41

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Switch fails in layer 2 multicast forwarding·······························································································41
Configured multicast group policy fails to take effect···············································································42
Multicast VLAN configuration········································································43
Introduction to multicast VLAN·························································································································43
Multicast VLAN configuration task list··············································································································44
Configuring multicast VLAN·····························································································································44
Configuration prerequisites······················································································································45
Configuring user port attributes················································································································45
Configuring multicast VLAN ports············································································································45
Displaying and maintaining multicast VLAN ····································································································46
Multicast VLAN configuration examples ··········································································································46
MLD snooping configuration·········································································50
MLD snooping overview···································································································································50
Introduction to MLD snooping··················································································································50
Basic concepts in MLD snooping·············································································································51
How MLD snooping works ·······················································································································52
MLD snooping proxying ···························································································································53
Protocols and standards ··························································································································54
MLD snooping configuration task list ···············································································································54
Configuring basic functions of MLD snooping··································································································56
Configuration prerequisites······················································································································56
Enabling MLD snooping···························································································································56
Configuring the version of MLD snooping································································································56
Configuring IPv6 static multicast MAC address entries ···········································································57
Configuring MLD snooping port functions········································································································58
Configuration prerequisites······················································································································58
Configuring aging timers for dynamic ports······························································································58
Configuring static ports ····························································································································59
Configuring simulated joining···················································································································59
Configuring fast leave processing············································································································60
Disabling a port or a group of ports from changing into dynamic router ports ·········································61
Configuring MLD snooping querier ··················································································································61
Configuration prerequisites······················································································································61
Enabling MLD snooping querier···············································································································62
Configuring MLD queries and responses·································································································62
Configuring source IPv6 addresses of MLD queries················································································63
Configuring MLD snooping proxying················································································································64
Configuration prerequisites······················································································································64
Enabling MLD snooping proxying ············································································································64
Configuring a source IPv6 address for the MLD messages sent by the proxy ········································64
Configuring an MLD snooping policy ···············································································································65
Configuration prerequisites······················································································································65
Configuring an IPv6 multicast group filter ································································································65
Configuring IPv6 multicast source port filtering························································································66
Configuring dropping unknown IPv6 multicast data·················································································66
Configuring MLD report suppression ·······································································································67
Configuring maximum multicast groups that a port can join ····································································67
Configuring IPv6 multicast group replacement ························································································68
Configuring 802.1p precedence for MLD messages················································································68
Displaying and maintaining MLD snooping······································································································69
MLD snooping configuration examples············································································································70
IPv6 group policy and simulated joining configuration example·······························································70
Static port configuration example·············································································································72
MLD snooping querier configuration example··························································································75
MLD snooping proxying configuration example·······················································································77
Troubleshooting MLD snooping·······················································································································80
Switch fails in layer 2 multicast forwarding·······························································································80
Configured IPv6 multicast group policy fails to take effect·······································································80

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IPv6 multicast VLAN configuration································································82
Introduction to IPv6 multicast VLAN ················································································································82
IPv6 multicast VLAN configuration task list······································································································83
Configuring IPv6 multicast VLAN·····················································································································83
Configuration prerequisites······················································································································84
Configuring user port attributes················································································································84
Configuring IPv6 multicast VLAN ports····································································································84
Displaying and maintaining IPv6 multicast VLAN ····························································································85
IPv6 multicast VLAN configuration examples ··································································································85
Document conventions and icons·································································89
Conventions·····················································································································································89
Network topology icons····································································································································90
Support and other resources ········································································91
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1
Multicast overview
NOTE:
This document focuses on the IP multicast technology and device operations. Unless otherwise
stated, the term multicast in this document refers to IP multicast.
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.
The multicast technology enables a network operator to 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.
Information transmission techniques
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.
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

2
must send a 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, HostA and Host C also receive it. In addition to information security issues,
broadcasting to hosts that do not need the information 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 information is replicated only on nodes where the trees branch.

3
Figure 3 Multicast transmission
In Figure 3, Host B, Host D and Host E are receivers of the information. They 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:
•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 will not increase the load of the
source and will not remarkably add to the usage of network resources.
•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
Multicast transmission has the following 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 amulticast 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 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 with
the transmission of TV programs.

4
Table 1 Comparing TV 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.
Advantages and applications of multicast
Advantages of multicast
The multicast technique has the following advantages:
•Enhanced efficiency—Reduces the CPU 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.
Applications of multicast
The multicast technique has the following 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, such as stock quotes
•Any other point-to-multipoint applications for data distribution
Multicast models
Multicast models—any-source multicast (ASM), source-filtered multicast (SFM), and source-specific
multicast (SSM)—determines how the receivers treat the multicast sources.

5
ASM model
In the ASM model, any sender can send information to a multicast group as a multicast source, and
numbers of receivers can join a multicast group, which is identified by a group address, and can
obtain multicast information addressed to that multicast group. In this model, receivers do not
determine 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. In the SFM 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, not all multicast sources are valid because 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:
•Multicast addressing—Where should the multicast source transmit information to?
•Host registration—What receivers exist on the network?
•Multicast source discovery—Where is the multicast source that will provide data to the
receivers?
•Multicast routing—How should information be transmitted to the receivers?
IP multicast is an end-to-end service. The multicast architecture involves the following parts:
1. Addressing mechanism—A multicast source sends information to a group of receivers through
a multicast address.
2. Host registration—Receiver hosts can join and leave multicast groups dynamically. This
mechanism is the basis for group membership management.
3. 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.
4. 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.
Multicast addresses
Network-layer multicast addresses—namely, multicast IP addresses—enable 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.

6
IP multicast addresses
1. IPv4 multicast addresses
Internet Assigned Numbers Authority (IANA) assigned the Class D address space (224.0.0.0 to
239.255.255.255) for 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 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:
•The membership of a group is dynamic. Hosts can join or leave multicast groups at any time.
•"Glop" is a mechanism for assigning multicast addresses between different autonomous
systems (ASs). By filling an AS number into the middle two bytes of 233.0.0.0, you get 255
multicast addresses for thatAS. For more information, see RFC 2770.
Table 3 Some reserved multicast 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 Distance Vector Multicast Routing Protocol (DVMRP) routers
224.0.0.5 Open Shortest Path First (OSPF) routers
224.0.0.6 OSPF designated routers/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
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

7
Address Description
224.0.0.16 Designated Subnetwork Bandwidth Management (SBM)
224.0.0.17 All SBMs
224.0.0.18 Virtual Router Redundancy Protocol (VRRP)
2. IPv6 multicast addresses
Figure 4 IPv6 multicast format
The following describes the meanings of 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 Format of the Flags field
Table 4 Description on the bits of the Flags field
Bit Description
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.
•Scope—The Scope field contains four bits, which indicate that 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

8
Value Meaning
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
When a unicast IP packet is transmitted over Ethernet, the destination MAC address is the MAC
address of the receiver. When a multicast packet is transmitted over Ethernet, the destination
address is a multicast MAC address because the packet is directed to a group formed by a number
of receivers, rather than to one specific receiver.
1. 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 least-significant 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 device might receive
some multicast data destined for other IPv4 multicast groups. The upper layer must filter such
redundant data.
2. 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
NOTE:
•Generally, Layer 3 multicast refers to IP multicast working at the network layer. The related
multicast protocols are Layer 3 multicast protocols, which include IGMP/MLD, PIM/IPv6 PIM,
MSDP, and MBGP/IPv6 MBGP. Layer 2 multicast refers to IP multicast working at the data link
layer. The related multicast protocols are Layer 2 multicast protocols, which include IGMP
snooping/MLD snooping, and multicast VLAN/IPv6 multicast VLAN.
•IGMP snooping, IGMP, multicast VLAN, PIM, MSDP, and MBGP are for IPv4, MLD snooping,
MLD, IPv6 multicast VLAN, 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
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
1. Multicast group management protocols
FF1E 0000 0000 0000 0000 0000 F30E 0101
0101aF30E
48-bit MAC address 3333
32 bits
mapped
128-bit IPv6 address
……
16-bit MAC
address prefix

10
Typically, the internet group management protocol (IGMP) or multicast listener discovery protocol
(MLD) is used between hosts and Layer 3 multicast devices that directly connected to the hosts.
These protocols define the mechanism ofestablishing and maintaining group memberships between
hosts and Layer 3 multicast devices.
2. 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 anAS 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 the dense mode (often called "PIM-DM"),
and sparse mode (often called "PIM-SM").
•An inter-domain multicast routing protocol delivers multicast information between two ASs.
Mature solutions include Multicast Source Discovery Protocol (MSDP) and Multicast Border
Gateway Protocol (MBGP). MSDP propagates multicast source information among different
ASs, and MBGP is an extension of the Multi-protocol 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 positions of the multicast sources, 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 and multicast VLAN/IPv6
multicast VLAN.
Figure 9 Positions of Layer 2 multicast protocols
1. IGMP snooping/MLD snooping
IGMP snooping and MLD snooping are multicast constraining mechanisms that run on Layer 2
devices. They manage and control multicast groups by listening to 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.
2. Multicast VLAN/IPv6 multicast VLAN

11
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 needs to 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 the
multicast data 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 received on one incoming interface 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, 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
IGMP snooping configuration
This chapter includes these sections:
•IGMP snooping overview
•IGMP snooping configuration task list
•Displaying and maintaining IGMP snooping
•IGMP snooping configuration examples
•Troubleshooting IGMP snooping configuration
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.
Principle of IGMP snooping
By analyzing received IGMP messages, a Layer 2 switch that is running IGMPsnooping establishes
mappings between ports and multicast MAC addresses, and forwards multicast data based on these
mappings.
When IGMP snooping is not running on the switch, multicast packets are flooded to all devices at
Layer 2. When IGMP snooping is running on the switch, multicast packets for known multicast
groups are multicast to the receivers, rather than broadcast to all hosts, at Layer 2.
Figure 10 Before and after IGMP snooping is enabled on the Layer 2 device
IGMP snooping forwards multicast data to only the receivers that require the data at Layer 2. It brings
the following advantages:
•Reducing Layer 2 broadcast packets, saving network bandwidth
•Enhancing the security of multicast traffic

13
•Facilitating the implementation of per-host accounting
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—also called "multicast group members".
Figure 11 IGMP snooping related ports
IGMP snooping involves the following ports:
•Router port—A router port is a port on a Layer 2 switch that leads toward a Layer 3 multicast
device—DR or IGMP querier. In Figure 11, GigabitEthernet 1/0/1 of Switch A and
GigabitEthernet 1/0/1 of Switch B are router ports. Each switch registers all its local router ports
in its router port list.
•Member port—A member port is a port on a Layer 2 switch that leads toward multicast group
members. In Figure 11, GigabitEthernet 1/0/2 and GigabitEthernet 1/0/3 of Switch A and
GigabitEthernet 1/0/2 of Switch B are member ports. Each switch registers all the member ports
on the local device in its IGMP snooping forwarding table.
NOTE:
•Whenever mentioned in this document, a router port is a port on the switch that leads the switch
to a Layer 3 multicast device, rather than a port on a router.
•Unless otherwise specified, router/member ports mentioned in this document include static and
dynamic ports.
•An IGMP-snooping-enabled switch deems that all its ports on which IGMP general queries with
the source IP address other than 0.0.0.0 or PIM hello messages are received are dynamic route
r
ports.
Aging timers for dynamic ports in IGMP snooping and related messages and actions
Table 6 Aging timers for dynamic ports in IGMP snooping and related messages and actions
Timer Description
Message before
expiry Action after expiry
Dynamic router port For each dynamic router IGMP general query of The switch removes this

14
Timer Description
Message before
expiry Action after expiry
aging timer port, the switch sets a
timer initialized to the
dynamic router port
aging time.
which the source
address is not 0.0.0.0 or
PIM hello
port from its router port
list.
Dynamic member port
aging timer
When a port dynamically
joins a multicast group,
the switch sets a timer
for the port, which is
initialized to the dynamic
member port aging time.
IGMP membership
report
The switch removes this
port from the IGMP
snooping forwarding
table.
NOTE:
The port aging mechanism of IGMP snooping works only for dynamic ports; a static port will never
age out.
How IGMP snooping works
A switch that is running IGMP snooping performs different actions when it receives different IGMP
messages.
CAUTION:
The description about adding or deleting a port in this section is only for a dynamic port. Static ports
can be added or deleted only through the specific configurations described in "Configuring static
ports."
When receiving a general query
The IGMP querier periodically sends IGMP general queries to all hosts and routers—224.0.0.1—on
the local subnet to determine whether active multicast group members exist on the subnet.
Upon receiving an IGMP general query, the switch forwards it through all ports in the VLAN except
the receiving port and performs the following on the receiving port:
•If the receiving port is a dynamic router port that exists in its router port list, the switch resets the
aging timer for this dynamic router port.
•If the receiving port is not a dynamic router port that exists in its router port list, the switch adds
it into its router port list and sets an aging timer for this dynamic router port.
When receiving a membership report
A host sends an IGMP report to the IGMP querier in the following circumstances:
•If the host is already a member of a multicast group, the host responds with an IGMP report
after receiving an IGMP query.
•If the host wants to join a multicast group,the host sends an IGMPreport to the IGMP querier to
announce that it is interested in the multicast information addressed to that 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, and performs the following operation:
•If no entry in the forwarding table exists for the reported group, the switch creates an entry, adds
the port as a dynamic member port to the outgoing port list, and starts a member port aging
timer for that port.
•If an entry in the forwarding table exists for the reported group, but the port is not included in the
outgoing port list for that group, the switch adds the port as a dynamic member port to the
outgoing port list, and starts an aging timer for that port.

15
•If an entry in the forwarding table exists for the reported group and the port is included in the
outgoing port list, which means that this port is already a dynamic member port, the switch
resets the aging timer for that port.
NOTE:
A switch does not forward an IGMP report through a non-router port. This is because if the switch
forwards a report message through a member port, all the attached hosts listening to the reported
multicast address will suppress their own reports upon receiving this report according to the IGMP
report suppression mechanism on them, and this will prevent the switch from knowing 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, so
the switch cannot determine immediately that the host has left the multicast group. However, as the
host stops sending IGMP reports as soon as it leaves a multicast group, the switch deletes the
forwarding entry for the dynamic member port corresponding to the host from the forwarding table
when its aging timer expires.
When an IGMPv2 or IGMPv3 host leaves a multicast group, the host sends an IGMPleave message
to the multicast router.
When the switch receives an IGMP leave message on a dynamic member port, the switch first
checks whether an entry in the forwarding table exists for the group address in the message, and, if
one exists, whether the outgoing port list contains the port.
•If the entry in the forwarding table does not exist or if the outgoing port list does not contain the
port, the switch discards the IGMP leave message instead of forwarding it to any port.
•If the entry in the forwarding table exists and the outgoing port list contains the port, the switch
forwards the leave message to all router ports in the native VLAN. Because the switch cannot
determine whether any other hosts attached to the port are still monitoring that group address,
the switch does not immediately remove the port from the outgoing port list of the entry in the
forwarding table for that group. Instead, it resets the aging timer for the port.
After receiving the IGMP leave message from a host, the IGMP querier resolves the multicast group
address in the message and sends an IGMP group-specific query to that multicast group through the
port that received the leave message. After receiving the IGMP group-specific query, the switch
forwards the query through all its router ports in the VLAN and all member ports for that multicast
group, and performs the following to the port on which it received the IGMP leave message:
•If any IGMP report in response to the group-specific query is received on the port—suppose it is
a dynamic member port—before its aging timer expires, this means that a host attached to the
port is receiving or expecting to receive multicast data for that multicast group. The switch
resets the aging timer of the port.
•If no IGMP report in response to the group-specific query is received on the port before its aging
timer expires, this means that no hosts attached to the port are still monitoring that group
address. The switch removes the port from the outgoing port list of the entry in the forwarding
table for that multicast group when the aging timer expires.
IGMP snooping proxying
The IGMP snooping proxying function on an edge device reduces the number of IGMP reports and
leave messages sent to its upstream device. The device configured with IGMP snooping proxying is
called "IGMP snooping proxy." It is a host from the perspective of its upstream device.
NOTE:
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.