Cisco Catalyst 2960 Series, Configuration Manual

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The following figure displays a VLAN Flex Links load-balancing configuration.
Figure 40: VLAN Flex Links Load-Balancing Configuration Example
Multicast Fast Convergence with Flex Links Failover
Multicast fast convergence reduces the multicast traffic convergence time after a Flex Links failure. Multicast
fast convergence is implemented by a combination of learning the backup link as an mrouter port, generating
IGMP reports, and leaking IGMP reports.
Related Topics
Configuring Multicast Fast Convergence with Flex Links Failover: Examples, on page 389
Learning the Other Flex Links 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 Links ports receiving queries. Flex Links 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 Links port.
The other Flex Links port is then learned as the mrouter port. After changeover, multicast traffic then flows
through the other Flex Links port. To achieve faster convergence of traffic, both Flex Links ports are learned
as mrouter ports whenever either Flex Links port is learned as the mrouter port. Both Flex Links ports are
always part of multicast groups.
Although both Flex Links ports are part of the groups in normal operation mode, all traffic on the backup port
is blocked. 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
Links 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.
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
376
Information About Flex Links and MAC Address-Table Move Update