Cisco Catalyst 4500 Series Software Configuration Manual page 748

Chapter 24 Configuring Flex Links and MAC Address-Table Move Update
MAC Address-Table Move Update
If port 1 shuts down, port 2 starts forwarding traffic. If there is no traffic from PC to the server after
failover to port 2, switch C does not learn MAC address of the PC on port 4. As a result, switch C keeps
forwarding traffic from the server to the PC out of port 3. There is traffic loss from server to PC because
port 1 is down. This problem is alleviated by sending out a dummy multicast packet with source MAC
address of the PC over port 2. Switch C learns the PC MAC address on port 4 and start forwarding traffic
from server to the PC out of port 4. One dummy multicast packet is sent out for every MAC address.
which is the default Flex Links behavior. The MAC address-table move update (MMU) feature may be
enabled to further expedite downstream convergence. MMUs are special packets that carry multiple
MAC addresses. Switch A is configured to transmit these packets and switches B, C, and D are
configured to receive such packets. If MMU transmit is enabled on Switch A, MAC move updates are
transmitted before dummy multicast packets over port 2. Switch D processes and floods MMUs over to
Switch C. Switch C processes these packets, and moves the MAC addresses contained within the packets
from port 3 to port 4. Because one packet carries multiple MAC addresses, downstream convergence is
faster.
Figure 24-3 MAC Address-Table Move Update Example
Server
Switch C
Port 3 Port 4
Switch B Switch D
Port 1 Port 2
Switch A
PC
Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
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