Cisco IE 4000, Software Configuration Manual

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Configuring FlexLinks and the MAC Address-Table Move Update
Information About Configuring the FlexLinks and the MAC Address-Table Move Update
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.
Leaking IGMP Reports
To achieve multicast traffic convergence with minimal loss, a redundant data path must be set up before the FlexLinks
active link goes down. This can be achieved by leaking only IGMP report packets on the FlexLinks backup link. These
leaked IGMP report messages are processed by upstream distribution routers, so multicast data traffic gets forwarded
to the backup interface. Because all incoming traffic on the backup interface is dropped at the ingress of the access
switch, no duplicate multicast traffic is received by the host. When the FlexLinks active link fails, the access switch starts
accepting traffic from the backup link immediately. The only disadvantage of this scheme is that it consumes bandwidth
on the link between the distribution switches and on the backup link between the distribution and access switches. This
feature is disabled by default and can be configured by using the
switchport backup interface interface-id multicast
fast-convergence command.
When this feature has been enabled at changeover, the switch does not generate the proxy reports on the backup port,
which became the forwarding port.
MAC Address-Table Move Update
The MAC address-table move update feature allows the switch to provide rapid bidirectional convergence when a
primary (forwarding) link goes down and the standby link begins forwarding traffic.
In Figure 59 on page 380 , switch A is an access switch, and ports 1 and 2 on switch A are connected to uplink switches
B and D through a FlexLinks pair. Port 1 is forwarding traffic, and port 2 is in the backup state. Traffic from the PC to the
server is forwarded from port 1 to port 3. The MAC address of the PC has been learned on port 3 of switch C. Traffic
from the server to the PC is forwarded from port 3 to port 1.
If the MAC address-table move update feature is not configured and port 1 goes down, port 2 starts forwarding traffic.
However, for a short time, switch C keeps forwarding traffic from the server to the PC through port 3, and the PC does
not get the traffic because port 1 is down. If switch C removes the MAC address of the PC on port 3 and relearns it on
port 4, traffic can then be forwarded from the server to the PC through port 2.
If the MAC address-table move update feature is configured and enabled on the switches in Figure 59 on page 380 and
port 1 goes down, port 2 starts forwarding traffic from the PC to the server. The switch sends a MAC address-table move
update packet from port 2. Switch C gets this packet on port 4 and immediately learns the MAC address of the PC on
port 4, which reduces the reconvergence time.
You can configure the access switch, switch A, to send MAC address-table move update messages. You can also
configure the uplink switches B, C, and D to get and process the MAC address-table move update messages. When
switch C gets a MAC address-table move update message from switch A, switch C learns the MAC address of the PC
on port 4. Switch C updates the MAC address table, including the forwarding table entry for the PC.
Switch A does not need to wait for the MAC address-table update. The switch detects a failure on port 1 and immediately
starts forwarding server traffic from port 2, the new forwarding port. This change occurs in 100 milliseconds (ms). The
PC is directly connected to switch A, and the connection status does not change. Switch A does not need to update the
PC entry in the MAC address table.