Cisco Catalyst Multilayer Switching – Fallback Bridging

By default, Multilayer switching does not work for all routed protocols. For example, routed protocols such as Novell’s Internetwork Packet Exchange (IPX) and Apple’s AppleTalk are routed only in software, not in hardware.

In addition to this, there are some protocols, such as DECnet, that are simply not routable. These protocols, however, may be bridged between different VLANs and routed interfaces using fallback bridging, which allows the switch to forward traffic that cannot be routed.

Fallback bridging allows two or more VLANs or routed ports to forward non-routable traffic between them by assigning them to the same bridge group. Each bridge group that is configured on the switch functions in the same manner as a unique bridge. Bridge Protocol Data Units (BPDUs) are exchanged only within a unique bridge group, not between bridge groups. Figure 7-9 below illustrates a LAN that is using fallback bridging to support non-routable protocols:

Fig. 7-9.  Understanding Fallback Bridging

The diagram illustrated in Figure 7-9 shows two hosts connected to the CAT6K-MLS-Switch Multilayer switch. This switch is the default gateway for both VLANs, which allows interVLAN communication (routing) between the two subnets. In addition to this, the SVIs on the switch are also configured as part of the same bridge group. This allows non-routable traffic to be bridged between the two different subnets in a manner that is transparent to end hosts.

By default, fallback bridging is disabled. This feature is enabled by assigning two or more switch interfaces to a bridge group. Once the interfaces have been assigned to a bridge group, the interfaces are able to bridge all non-routed traffic between them and other member interfaces. This process happens regardless of the IP subnet the interfaces are configured on.

Fallback Bridging Configuration Guidelines

The following are guidelines and restrictions that you should be aware of when implementing fallback bridging on a Catalyst switch:

• Up to a maximum of thirty two (32) bridge groups can be configured on the switch
• An interface (an SVI or routed port) can be a member of only one bridge group
• Use a different bridge group for each separately bridged network connected to the switch
• Do not configure fallback bridging on a switch configured with private VLANs
• When enabled, all protocols are bridged, except for the following:
  • IP Version 4
  • IP Version 6
  • Address Resolution Protocol (ARP)
  • Reverse ARP (RARP)
  • Frame Relay ARP
• Shared STP packets are fallback bridged

Configuring Fallback Bridging

The following sequence of steps should be taken when configuring fallback bridging:

• Configure one or more bridge groups using the bridge [1-255] bridge-group protocol vlan-bridge global configuration command. Keep in mind that although any number between 1 and 255 is allowed, the switch only supports up to 32 unique groups.

• Assign either a Layer 3 (routed) switch port or SVI to the configured bridge group using the bridge-group [number] interface configuration command. It is important to know that this command is not supported on Layer 2 ports.

The following output illustrates the configuration of fallback bridging on a switch:

Verifying Fallback Bridging Configuration

The show bridge [number] [group] [verbose]  command is used to view information, such as learned MAC addresses, pertaining to the bridge group. The following output shows the show bridge command:

In the output above, the show bridge command prints the bridge group number (1), the addresses learned within the bridge group, the interfaces on which these addresses are learned, their age, and both transmitted and received packet statistics.