17-4
Cisco ME 3800X and 3600X Switch Software Configuration Guide
OL-23400-01
Chapter 17 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 packets 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 consumes bandwidth on the
link between the distribution switches and on the backup link between 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 proxy reports on the
backup port, which became the forwarding port.
Configuration Examples
This configuration example shows learning the other Flex Link port as the mrouter port when Flex Link
is configured on GigabitEthernet 0/11 and GigabitEthernet 0/12. The example shows the output for the
show interfaces switchport backup command:
Switch#
configure terminal
Switch(config)#
interface gigabitEthernet0/11
Switch(config-if)#
switchport trunk encapsulation dot1q
Switch(config-if)#
switchport mode trunk
Switch(config-if)#
switchport backup interface Gi0/12
Switch(config-if)#
exit
Switch(config)#
interface GigabitEthernet0/12
Switch(config-if)#
switchport trunk encapsulation dot1q
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
GigabitEthernet0/11 GigabitEthernet0/12 Active Up/Backup Standby
Preemption Mode : off
Multicast Fast Convergence : Off
Bandwidth : 100000 Kbit (Gi0/11), 100000 Kbit (Gi0/12)
Mac Address Move Update Vlan : auto
This output shows a querier for VLANs 1 and 401, with their queries reaching the switch through
GigabitEthernet 0/11:
Switch#
show ip igmp snooping querier
Vlan
IP Address
IGMP Version
Port
-------------------------------------------------------------
1 1.1.1.1
v2
Gi0/11
401 41.41.41.1 v2
Gi0/11
