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Cisco IE 2000 Switch Software Configuration Guide
OL-25866-01
Chapter 24 Configuring FlexLinks and the MAC Address-Table Move Update
Information About Configuring the FlexLinks 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 FlexLinks active link goes down. This can be achieved by leaking only IGMP report packets on the
FlexLinks 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 FlexLinks 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.
MAC Address-Table Move Update
The MAC address-table move update feature allows the switch to provide rapid bidirectional
convergence when a primary (forwarding) link goes down and the standby link begins forwarding traffic.
In Figure 24-3, switch A is an access switch, and ports 1 and 2 on switch A are connected to uplink
switches B and D through a FlexLinks pair. Port 1 is forwarding traffic, and port 2 is in the backup state.
Traffic from the PC to the server is forwarded from port 1 to port 3. The MAC address of the PC has
been learned on port 3 of switch C. Traffic from the server to the PC is forwarded from port 3 to port 1.
If the MAC address-table move update feature is not configured and port 1 goes down, port 2 starts
forwarding traffic. However, for a short time, switch C keeps forwarding traffic from the server to the
PC through port 3, and the PC does not get the traffic because port 1 is down. If switch C removes the
MAC address of the PC on port 3 and relearns it on port 4, traffic can then be forwarded from the server
to the PC through port 2.
If the MAC address-table move update feature is configured and enabled on the switches in Figure 24-3
and port 1 goes down, port 2 starts forwarding traffic from the PC to the server. The switch sends a MAC
address-table move update packet from port 2. Switch C gets this pa cket on port 4 and immediately
learns the MAC address of the PC on port 4, which reduces the reconvergence time.
You can configure the access switch, switch A, to send MAC address-table move update messages. You
can also configure the uplink switches B, C, and D to get and process the MAC address-table move
update messages. When switch C gets a MAC address-table move update message from switch A,
switch C learns the MAC address of the PC on port 4. Switch C updates the MAC address table,
including the forwarding table entry for the PC.
Switch A does not need to wait for the MAC address-table update. The switch detects a failure on port 1
and immediately starts forwarding server traffic from port 2, t he new forwarding port. This change
occurs in 100 milliseconds (ms). The PC is directly connected to switch A, and the connection status
does not change. Switch A does not need to update the PC entry in the MAC address table.