Fallback Bridging

November 29th, 2007 neteng

Today, I want to take a closer look at one of the more overlooked topics when it comes to switch configuration: Fallback Bridging. In a world of IP-based applications, few of us rarely come into contact with legacy applications and protocols that may require the use of bridging. But being that a CCIE is considered an Internetworking Expert, it’s something candidates will need to know for the lab exam. And if you ever work in a large enterprise, you may run into this when dealing with old mainframes and ancient equipment.

So let’s go ahead and see what this fallback bridging, aka VLAN bridging, is all about when configured on the Catalyst 3550/3560 platforms. Its main use is to allow machines that speak non-routed or non-supported protocols (SNA, DECNet, AppleTalk, etc.) to communicate across VLANs and routed ports. The idea is to put Switched VLAN Interfaces (SVI) and routed ports (no switchport) for which you want to bridge traffic, into a common bridge group. On the 3550, you’re limited to 31 simultaneous bridge groups (32 on the 3560) and an interface can be a member of only a single bridge group. Another important point to remember is that each VLAN has its own spanning-tree instance. It runs on top of the bridge group to prevent loops.

As always, things are best explained through an example:

If we were asked to configure the switch in the diagram, here’s what we’d do:

CCIE-SW2#conf t
Enter configuration commands, one per line. End with CNTL/Z.
CCIE-SW2(config)#bridge 1 protocol vlan-bridge
CCIE-SW2(config)#int vlan1
CCIE-SW2(config-if)#bridge-group 1
CCIE-SW2(config)#int vlan2
CCIE-SW2(config-if)#bridge-group 1
CCIE-SW2(config)#int fa0/1
CCIE-SW2(config-if)#no switchport
CCIE-SW2(config-if)#bridge-group 1

Now let’s verify that our config took:

CCIE-SW2#sh bridge 1 group

Bridge Group 1 is running the VLAN Bridge compatible Spanning Tree protocol

Port 28 (Vlan1) of bridge group 1 is forwarding
Port 30 (Vlan2) of bridge group 1 is forwarding
Port 31 (FastEthernet0/1) of bridge group 1 is up

That’s all there is to it. Now the switch will build a bridging table and bridge all packets between those interfaces except the following: IPv4 & IPv6, ARP, RARP, LOOPBACK, and Frame Relay ARP.

For more information:

Cisco Documentation: Configuring Fallback Bridging

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