CHAPTER
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Catalyst 3750-X and 3560-X Switch Software Configuration Guide
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Configuring Fallback Bridging
This chapter describes how to configure fallback bridging (VLAN bridging) on the Catalyst 3750-X
or 3560-X switch. With fallback bridging, you can forward non-IP packets that the switch does not route
between VLAN bridge domains and routed ports.
To use this feature, the switch or stack master must be running the IP services feature set. Unless
otherwise noted, the term switch refers to a Catalyst 3750-X or 3560-X standalone switch and to a
Catalyst 3750-X switch stack.
Note For complete syntax and usage information for the commands used in this chapter, see the Cisco IOS
Bridging and IBM Networking Command Reference, Volume 1 of 2, Release 12.4.
• Understanding Fallback Bridging, page 1-1
• Configuring Fallback Bridging, page 1-3
• Monitoring and Maintaining Fallback Bridging, page 1-10
Understanding Fallback Bridging
• Fallback Bridging Overview, page 1-1
• Fallback Bridging and Switch Stacks, page 1-3
Fallback Bridging Overview
With fallback bridging, the switch bridges together two or more VLANs or routed ports, essentially
connecting multiple VLANs within one bridge domain. Fallback bridging forwards traffic that the switch
does not route and forwards traffic belonging to a nonroutable protocol such as DECnet.
A VLAN bridge domain is represented with switch virtual interfaces (SVIs). A set of SVIs and routed
ports (which do not have any VLANs associated with them) can be configured (grouped together) to
form a bridge group. Recall that an SVI represents a VLAN of switch ports as one interface to the routing
or bridging function in the system. You associate only one SVI with a VLAN, and you configure an SVI
for a VLAN only when you want to route between VLANs, to fallback-bridge nonroutable protocols
between VLANs, or to provide IP host connectivity to the switch. A routed port is a physical port that
acts like a port on a router, but it is not connected to a router. A routed port is not associated with a
particular VLAN, does not support VLAN subinterfaces, but behaves like a normal routed port. For more
information about SVIs and routed ports, see Chapter 1, “Configuring Interface Characteristics.”
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