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Catalyst 3750 SwitchSoftware Configuration Guide
OL-8550-09
Chapter21 Configuring Flex Links and the MAC Address-Table Move Update Feature
Understanding Flex Links and the MAC Address-Table MoveUpdate
VLAN Flex Link Load Balancing and Support
VLAN Flex Link load-balancing allows you to configure a Flex Link pair so that both ports
simultaneously forward the traffic for some mutually exclusive VLANs. For example, if Flex Link ports
are configured for 1-100 VLANs, the traffic of the first 50 VLANs can be forwarded on one port and the
rest on the other port. If one of the ports fail, the other active port forwards all the traffic. When the failed
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.
Figure21-2 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, page21-3
•Generating IGMP Reports, page21-4
•Leaking IGMP Reports, page21-4
•Configuration Examples, page21-4

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.
Switch A
Uplink
switch B
gi2/0/6
Forwarding
(1-50)
gi2/0/8
Forwarding
(51-100)
Uplink
switch C
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