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Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
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Chapter 18 Configuring Flex Links and the MAC Address-Table Move Update Feature
Understanding Flex Links and the MAC Address-Table Move Update
port comes back up, it resumes forwarding traffic in the preferred vlans. This way, apart from providing
the redundancy, this Flex Link pair can be used for load balancing. Also, Flex Link VLAN
load-balancing does not impose any restrictions on uplink switches.
Figure 18-2 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 18-3
• Generating IGMP Reports, page 18-3
• Leaking IGMP Reports, page 18-4

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 fa ster con v er gence of traf f ic, both Fl ex Li nk 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.
Switch A
Uplink
switch B
gi2/0/6
Forwarding
gi2/0/8
Not Forwarding
Uplink
switch C
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