Cisco 3750G - Catalyst Integrated Wireless LAN Controller Configuration Manual page 500

Understanding Flex Links and the MAC Address-Table Move Update
You configure Flex Links on one Layer 2 interface (the active link) by assigning another Layer 2
interface as the Flex Link or backup link. The Flex Link can be on the same switch or on another switch
in the stack. When one of the links is up and forwarding traffic, the other link is in standby mode, ready
to begin forwarding traffic if the other link shuts down. At any given time, only one of the interfaces is
in the linkup state and forwarding traffic. If the primary link shuts down, the standby link starts
forwarding traffic. When the active link comes back up, it goes into standby mode and does not forward
traffic. STP is disabled on Flex Link interfaces.
In Figure
configured as Flex Links, only one of the interfaces is forwarding traffic; the other is in standby mode.
If port 1 is the active link, it begins forwarding traffic between port 1 and switch B; the link between
port 2 (the backup link) and switch C is not forwarding traffic. If port 1 goes down, port 2 comes up and
starts forwarding traffic to switch C. When port 1 comes back up, it goes into standby mode and does
not forward traffic; port 2 continues forwarding traffic.
You can also choose to configure a preemption mechanism, specifying the preferred port for forwarding
traffic. For example, in the example in
preemption mode. In the scenario shown, when port 1 comes back up and has more bandwidth than port
2, port 1 begins forwarding traffic after 60 seconds. Port 2 becomes the standby port. You do this by
entering the interface configuration switchport backup interface preemption mode bandwidth and
switchport backup interface preemption delay commands.
Figure 21-1
Uplink
switch B
If a primary (forwarding) link goes down, a trap notifies the network management stations. If the standby
link goes down, a trap notifies the users.
Flex Links are supported only on Layer 2 ports and port channels, not on VLANs or on Layer 3 ports.
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
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.
Catalyst 3750 Switch Software Configuration Guide
21-2
Chapter 21
21-1, ports 1 and 2 on switch A are connected to uplink switches B and C. Because they are
Flex Links Configuration Example
Port 1
Switch A
21-2, switch A is an access switch, and ports 1 and 2 on switch A are connected to uplink
Configuring Flex Links and the MAC Address-Table Move Update Feature
Figure 21-1, you can configure the Flex Links pair with
Uplink
switch C
Port 2
OL-8550-02