Cisco Systems 3560 IGMPv3 Host Signalling

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Catalyst 3560 Switch Software Configuration Guide

OL-8553-06

Chapter 44 Configuring IP Multicast Routing

Configuring IP Multicast Routing

• No MSDP source-active (SA) messages within the SSM range are accepted, generated, or

forwarded.

IGMPv3 Host Signalling

In IGMPv3, hosts signal membership to last hop routers of multicast groups. Hosts can signal group

membership with filtering capabilities with respect to sources. A host can either signal that it wants to

receive traffic from all sources sending to a group except for some specific sources (called exclude

mode), or that it wants to receive traffic only from some specific sources sending to the group (called

include mode).

IGMPv3 can operate with both ISM and SSM. In ISM, both exclude and include mode reports are

applicable. In SSM, only include mode reports are accepted by the last-hop router. Exclude mode reports

are ignored.

Configuration Guidelines

This section contains the guidelines for configuring SSM.

Existing applications in a network predating SSM do not work within the SSM range unless they are

modified to support (S, G) channel subscriptions. Therefore, enabling SSM in a network can cause

problems for existing applications if they use addresses within the designated SSM range.

Address management is still necessary to some degree when SSM is used with Layer 2 switching

mechanisms. Cisco Group Management Protocol (CGMP), IGMP snooping, or Router-Port Group

Management Protocol (RGMP) support only group-specific filtering, not (S, G) channel-specific

filtering. If different receivers in a switched network request different (S, G) channels sharing the same

group, they do not benefit from these existing mechanisms. Instead, both receivers receive all (S, G)

channel traffic and filter out the unwanted traffic on input. Because SSM can re-use the group addresses

in the SSM range for many independent applications, this situation can lead to decreased traffic filtering

in a switched network. For this reason, it is important to use random IP addresses from the SSM range

for an application to minimize the chance for re-use of a single address within the SSM range between

different applications. For example, an application service providing a set of television channels should,

even with SSM, use a different group for each television (S, G) channel. This setup guarantees that

multiple receivers to different channels within the same application service never experience traffic

aliasing in networks that include Layer 2 switches.

IGMPv3 uses new membership report messages that might not be correctly recognized by older IGMP

snooping switches.

For more information about switching issues related to IGMP (especially with CGMP), refer to the

“Understanding IGMP” section on page 44-3.