Cisco Systems IE 2000 FlexLinks Multicast Fast Convergence

24-3
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
Figure 24-2 VLAN FlexLinks Load Balancing Configuration Example
FlexLinks Multicast Fast Convergence
FlexLinks Multicast Fast Convergence reduces the multicast traffic convergence time after a FlexLinks
failure.
Learning the Other FlexLinks 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 FlexLinks ports receiving queries. FlexLinks 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 FlexLinks
port. The other FlexLinks port is then learned as the mrouter port. After the changeover, multicast traffic
flows through the other FlexLinks port. To achieve faster convergence of traffic, both FlexLinks ports
are learned as mrouter ports whenever either FlexLinks port is learned as the mrouter port. Both
FlexLinks ports are always part of multicast groups.
Though both FlexLinks 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
FlexLinks 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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