SMC Networks SMC8126PL2-F Layer 2 IGMP (Snooping and Query)

Multicast Filtering

Layer 2 IGMP (Snooping and Query)

IGMP Snooping and Query – If multicast routing is not supported on other switches

in your network, you can use IGMP Snooping and Query (3-210) to monitor IGMP

service requests passing between multicast clients and servers, and dynamically

configure the switch ports which need to forward multicast traffic.

When using IGMPv3 snooping, service requests from IGMP Version 1, 2 or 3 hosts

are all forwarded to the upstream router as IGMPv3 reports. The primary

enhancement provided by IGMPv3 snooping is in keeping track of information about

the specific multicast sources which downstream IGMPv3 hosts have requested or

refused. The switch maintains information about both multicast groups and

channels, where a group indicates a multicast flow for which the hosts have not

requested a specific source (the only option for IGMPv1 and v2 hosts unless

statically configured on the switch), and a channel indicates a flow for which the

hosts have requested service from a specific source.

Only IGMPv3 hosts can request service from a specific multicast source. When

downstream hosts request service from a specific source for a multicast service,

these sources are all placed in the Include list, and traffic is forwarded to the hosts

from each of these sources. IGMPv3 hosts may also request that service be

forwarded from all sources except for those specified. In this case, traffic is filtered

from sources in the Exclude list, and forwarded from all other available sources.

Notes: 1. When the switch is configured to use IGMPv3 snooping, the snooping

version may be downgraded to version 2 or version 1, depending on the

version of the IGMP query packets detected on each VLAN.

2. IGMP snooping will not function unless a multicast router port is enabled on

the switch. This can be accomplished in one of two ways. A static router port

can be manually configured (see “Specifying Static Interfaces for a Multicast

Router” on 3-215). Using this method, the router port is never timed out, and

will continue to function until explicitly removed. The other method relies on

the switch to dynamically create multicast routing ports whenever multicast

routing protocol packets or IGMP query packets are detected on a port.

3. A maximum of up to 255 multicast entries can be maintained for IGMP

snooping, and 255 entries for Multicast Routing, when both of these features

are enabled. If the table’s capacity is exceeded, the IGMPv3 snooping will

not support multicast source filtering, but will forward multicast traffic from all

relevant sources to the requesting hosts.

Static IGMP Router Interface – If IGMP snooping cannot locate the IGMP querier,

you can manually designate a known IGMP querier (i.e., a multicast router/switch)

connected over the network to an interface on your switch (3-215). This interface will

then join all the current multicast groups supported by the attached router/switch to

ensure that multicast traffic is passed to all appropriate interfaces within the switch.

Static IGMP Host Interface – For multicast applications that you need to control

more carefully, you can manually assign a multicast service to specific interfaces on

the switch (3-217).