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Cisco IE 3010 Switch Software Configuration Guide
OL-23145-01
Chapter 20 Configuring Flex Links and the MAC Address-Table Move Update Feature
Understanding Flex Links and the MAC Address-Table Move Update
Leaking IGMP Reports
To achieve multicast traffic convergence with minimal loss, a redundant data path must be set up before
the Flex Link active link goes down. This can be achieved by leaking only IGMP report pac kets on the
Flex Link backup link. These leaked IGMP report messages are processed by upstream distribution
routers, so multicast data traffic gets forwarded to the backup interface. Because all incoming traffic on
the backup interface is dropped at the ingress of the access switch, no duplicate multicast traffic is
received by the host. When the Flex Link active link fails, the access switch starts accepting traffic from
the backup link immediately. The only disadvantage of this scheme is that it co nsumes bandwidth on the
link between the distribution switches and on the backup link betwee n the distribution and access
switches. This feature is disabled by default and can be configured by using the switchport backup
interface interface-id multicast fast-convergence command.
When this feature has been enabled at changeover, the switch does not generate the pr oxy reports on the
backup port, which became the forwarding port.
Configuration Examples
These are configuration examples for learning the other Flex Link port as the mrouter port when Flex
Link is configured, with output for the show interfaces swit chport backup command:
Switch# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Switch(config)# interface FastEthernet0/1
Switch(config-if)# switchport mode trunk
Switch(config-if)# switchport backup interface FastEthernet0/2
Switch(config-if)# exit
Switch(config)# interface FastEthernet0/2
Switch(config-if)# switchport mode trunk
Switch(config-if)# end
Switch# show interfaces switchport backup detail
Switch Backup Interface Pairs:
Active Interface Backup Interface State
Preemption Mode : off
Multicast Fast Convergence : Off
Mac Address Move Update Vlan : auto
This output shows a querier for VLANs 1 and 401, with their quer ies reaching the switch through the
specified port:
Switch# show ip igmp snooping querier
Vlan IP Address IGMP Version Port
-------------------------------------------------------------
1 1.1.1.1 v2 Fa0/1
401 41.41.41.1 v2 Fa0/1
Here is output for the show ip igmp snooping mrouter command for VLANs 1 and 401:
Switch# show ip igmp snooping mrouter
Vlan ports
---- -----
1 Fa0/1(dynamic), Fa0/2(dynamic)
401 Fa0/1(dynamic), Fa0/2(dynamic)
Similarly, both Flex Link ports are part of learned groups. In this example, FastEthernet0/1 is a
receiver/host in VLAN 1, which is interested in two multicast groups:
Switch# show ip igmp snooping groups
Vlan Group Type Version Port List
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