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Cisco AVVID Network Infrastructure IP Multicast Design - page 23 / 98





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Chapter 2


IP Multicast in a Campus Network

Campus Deployment

Optimal bandwidth management can be achieved on IGMP snooping enabled switches by enabling the IGMP Fast-Leave processing. With Fast-Leave, upon receiving an “IGMP leave group” message, the switch immediately removes the interface from its Layer 2 forwarding table entry for that multicast group. Without leave processing, the multicast group will remain in the Layer 2 forwarding table until the default IGMP timers expire and the entry is flushed.

The following example shows how to configure IGMP Fast-Leave on a Catalyst switch running Native IOS:

switch(config)#ip igmp snooping vlan 1 immediate-leave

The following example shows how to configure IGMP Fast-Leave on a Catalyst switch running Catalyst OS:

CatOS> (enable)set igmp fastleave enable

Use Fast-Leave processing only on VLANs where only one host is connected to each Layer 2 LAN interface. Otherwise, some multicast traffic might be dropped inadvertently. For example, if multiple hosts are attached to a Wireless LAN Access Point that connects to a VLAN where Leave processing is enabled (as shown in Figure 2-1), then Fast-Leave processing should not be used.

Figure 2-1

When Not to Use Fast-Leave Processing

Fast leave





Both PCs receive the same stream. PC A "Leaves" PC B loses stream


Cisco Group Management Protocol (CGMP) is a Cisco-developed protocol that allows Catalyst switches to leverage IGMP information on Cisco routers to make Layer 2 forwarding decisions. CGMP must be configured on the multicast routers and the Layer 2 switches. With CGMP, IP multicast traffic is delivered only to the Catalyst switch ports that are attached to interested receivers. All ports that have not explicitly requested the traffic will not receive it unless these ports are connected to a multicast router. Multicast router ports must receive every IP multicast data packet.

The default behavior of CGMP is to not remove multicast entries until an event, such as a spanning tree topology change, occurs or the router sends a CGMP leave message. The following example shows how to enable the CGMP client (switch) to act on actual IGMP leave messages:

switch(config)#cgmp leave-processing


Due to a conflict with HSRP, CGMP Leave processing is disabled by default.If HSRP hellos pass through a CGMP enabled switch, then refer to CSCdr59007 before enabling CGMP leave-processing.

Table 2-1 lists the support for CGMP and IGMP snooping in Cisco switches.

Cisco AVVID Network Infrastructure IP Multicast Design


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