CHAPTER
26-1
Catalyst 3550 Multilayer Switch Software Configuration Guide
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26
Configuring Fallback Bridging
This chapter describes how to configure fallback bridging on y our sw it ch. With fall back br idging , you
can forward non-IP protocols that the multilayer 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. All Catalyst 3550 Gigabit Ethernet switches ship with the E MI i nsta lled. Cata lyst 3550 Fa st
Ethernet switches can be shipped with either the standard multilayer software image (SMI) or EMI
pre-installed. You can order the Enhanced Multilayer Software Image Upgrade kit to upgrade
Catalyst 3550 Fast Ethernet switches from the SMI to the EMI.
Note For complete syntax and usage information for the commands used in this chapter, refer to the Cisco
IOS Bridging and IBM Networking Command Reference for Release 12.1.
This chapter consists of these sections:
•Understanding Fallback Bridging, page 26-1
•Configuring Fallback Bridging, page 26-3
•Monitoring and Maintaining the Network, page 26-12

Understanding Fallback Bridging

With fallback bridging, the switch bridges together two or more VL ANs or ro ut ed port s, e ssen tial ly
connecting multiple VLANs within one bridge domain. Fallback bridging forwards traffic that the
multilayer 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 Protocol (STP) instance and a separate spanning tree, called the
VLAN-bridge spanning tree, which runs on top of the bridge g rou p to pr even t loop s.
A VLAN bridge domain is represented using the switch virtual interface (SVI). A se t of SVIs and routed
ports (which do not have any VLANs associated with th em ) can be conf igur ed to for m a brid ge grou p.
Recall that an SVI represents a VLAN of switch ports as one interface to the routing or bridging function
in the system. Only one SVI can be associated with a VLAN, and it is only necessary to configure an
SVI for a VLAN when you want to route between VLANs, to fa llb ack- bridg e no nrou tab le pro to col s
between VLANs, or to provide IP host connectivity to the switch. A routed port is a physical port that