42-3
Catalyst 3750 Switch Software Configuration Guide
OL-8550-02
Chapter 42 Configuring Fallback Bridging
Configuring Fallback Bridging
Figure 42-1 Fallback Bridging Network Example
Fallback Bridging and Switch Stacks
When the stack master fails, a stack member becomes the new stack master by using the election process
described in Chapter 5, “Managing Switch Stacks.” The new stack master creates new VLAN-bridge
spanning-tree instance, which temporarily puts the spanning-tree ports used for fallback bridging into a
nonforwarding state. A momentary traffic disruption occurs until the spanning-tree states transition to
the forwarding state. All MAC addresses must be relearned in the bridge group.
Note If a stack master running the IP services image fails and if the newly elected stack master is running the
IP base image (formerly known as the standard multilayer image [SMI]), the switch stack loses its
fallback bridging capability.
If stacks merge or if a switch is added to the stack, any new VLANs that are part of a bridge group and
become active are included in the VLAN-bridge STP.
When a stack member fails, the addresses learned from this member are deleted from the bridge group
MAC address table.
For more information about switch stacks, see Chapter 5, “Managing Switch Stacks.”
Configuring Fallback Bridging
These sections contain this configuration information:
• Default Fallback Bridging Configuration, page 42-4
• Fallback Bridging Configuration Guidelines, page 42-4
• Creating a Bridge Group, page 42-4 (required)
• Adjusting Spanning-Tree Parameters, page 42-6 (optional)
101240
Host A
Host C
SVI 1172.20.128.1 172.20.129.1
Layer 3 switch
Routed port
172.20.130.1
SVI 2
VLAN 20
Host B
VLAN 30
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