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Catalyst 3560 Switch Software Configuration Guide
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Chapter 20 Configuring Flex Links and the MAC Address-Table Move Update Feature
Understanding Flex Links and the MAC Address-Table Move Update
Figure 20-2 VLAN Flex Links Load Balancing Configuration Example
Flex Link Multicast Fast Convergence
Flex Link Multicast Fast Convergence reduces the multicast traffic convergence time after a Flex Link
failure. This is implemented by a combination of these solutions:
• Learning the Other Flex Link Port as the mrouter Port, page 20-3
• Generating IGMP Reports, page 20-3
• Leaking IGMP Reports, page 20-4
• Configuration Examples, page 20-4

Learning the Other Flex Link Port as the mrouter Port

In a typical multicast network, there is a querier for each VLAN. A switch deployed at the edge of a
network has one of its Flex Link ports receiving queries. Flex Link ports are also always forwarding at
any given time.
A port that receives queries is added as an mrouter port on the switch. An mrouter port is part of all the
multicast groups learned by the switch. After a changeover, queries are received by the other Flex Link
port. The other Flex Link port is then learned as the mrouter port. After changeover, multicast traffic
then flows through the other Flex Link port. To achieve faster convergence of traffic, both Flex Link
ports are learned as mrouter ports whenever either Flex Link port is learned as the mrouter port. Both
Flex Link ports are always part of multicast groups.
Though both Flex Link ports are part of the groups in normal operation mode, all traffic on the backup
port is blocked. So the normal multicast data flow is not affected by the addition of the backup port as
an mrouter port. When the changeover happens, the backup port is unblocked, allowing the traffic to
flow. In this case, the upstream multicast data flows as soon as the backup port is unblocked.

Generating IGMP Reports

When the backup link comes up after the changeover, the upstream new distribution switch does not start
forwarding multicast data, because the port on the upstream router, which is connected to the blocked
Flex Link port, is not part of any multicast group. The reports for the multicast groups were not
forwarded by the downstream switch because the backup link is blocked. The data does not flow on this
port, until it learns the multicast groups, which occurs only after it receives reports.
The reports are sent by hosts when a general query is received, and a general query is sent within 60
seconds in normal scenarios. When the backup link starts forwarding, to achieve faster convergence of
multicast data, the downstream switch immediately sends proxy reports for all the learned groups on this
port without waiting for a general query.
Switch A
Uplink
switch B
gi2/0/6
Forwarding
(1-50)
gi2/0/8
Forwarding
(51-100)
Uplink
switch C
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