Cisco Systems 3750E manual MAC Address-Table Move Update, 21-3

Chapter 21 Configuring Flex Links and the MAC Address-Table Move Update Feature

Understanding Flex Links and the MAC Address-Table Move Update

Figure 21-2 VLAN Flex Links Load Balancing Configuration Example

Uplink		Uplink
switch B		switch C
Forwarding		Not Forwarding
gi2/0/6		gi2/0/8
		201398
	Switch A

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 21-3, switch A is an access switch, and ports 1 and 2 on switch A are connected to uplink switches B and D through a Flex Link 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 21-3and 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,

Catalyst 3750-E and 3560-E Switch Software Configuration Guide

OL-9775-02 21-3