Understanding Fallback Bridging - Cisco Catalyst 3550 series Software Configuration Manual

Configuring Fallback Bridging
This chapter describes how to configure fallback bridging (VLAN bridging) on your Catalyst 3550
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, you must have the enhanced multilayer software (EMI) image installed on your
switch.
For complete syntax and usage information for the commands used in this chapter, refer to the Cisco IOS
Note
Bridging and IBM Networking Command Reference for Release 12.1.
This chapter consists of these sections:
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Understanding Fallback Bridging

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.
Fallback bridging does not allow the spanning trees from the VLANs being bridged to collapse; each
VLAN has its own spanning-tree instance and a separate spanning tree, called the VLAN-bridge
spanning tree, which runs on top of the bridge group to prevent loops.
A VLAN bridge domain is represented with switch virtual interface (SVI). 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 interface. For
more information about SVIs and routed ports, see
A bridge group is an internal organization of network interfaces on a switch. Bridge groups cannot be
used to identify traffic switched within the bridge group outside the switch on which they are defined.
Bridge groups on the same switch function as distinct bridges; that is, bridged traffic and bridge protocol
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Understanding Fallback Bridging, page 36-1
Configuring Fallback Bridging, page 36-3
Monitoring and Maintaining Fallback Bridging, page 36-12
C H A P T E R
Chapter 10, "Configuring Interface Characteristics."
Catalyst 3550 Multilayer Switch Software Configuration Guide
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36-1