Fallback Bridging

This section describes how to configure fallback bridging (VLAN bridging) on your 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.

Fallback Bridging Network Diagram

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.

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 data units (BPDUs) are not exchanged between different bridge groups on a switch. An interface can be a member of only one bridge group. Use a bridge group for each separately bridged (topologically distinct) network connected to the switch.

Routed Port Configuration

Switch(config)# bridge 10 protocol vlan-bridge Switch(config)# interface gigabitethernet0/1

Switch(config-if)# no switchport

Switch(config-if)# ip address 172.20.130.1 2 55.255.255.0

Switch(config-if)# no shutdown Switch(config-if)# bridge-group 10

Switched Virtual Interface Configuration

Switch(config)# bridge 10 protocol vlan-bridge

Switch(config)# vlan 2 Switch(config-vlan)# exit Switch(config)# interface vlan2

Switch(config-if)# ip address 172.20.128.1 2 55.255.255.0

Switch(config-if)# exit

© 2002, CiscoSystems, Inc. All rights reserved.

Cisco CCIE Prep v1.0—Module 5-6!

To configure fallback bridging for a set of SVIs or routed ports, these interfaces must be assigned to bridge groups. All interfaces in the same group belong to the same bridge domain. Each SVI or routed port can be assigned to only one bridge group. A maximum of 31 bridge groups can be configured on the switch.

Note The protected port feature is not compatible with fallback bridging. When fallback bridging is enabled, it is possible for packets to be forwarded from one protected port on a switch to another protected port on the same switch if the ports are in different VLANs.

Use the steps outlined in the following table to create a bridge group and assign an interface to it:

Table 5-49: Bridge Group

Command

Purpose

bridge bridge-group protocol vlan-bridge

Assign a bridge group number, and specify the VLAN-bridge spanning-tree protocol to run in the bridge group. The ibm and dec keywords are not supported.

For bridge-group, specify the bridge group number. The range is 1 to 255. You can create up to 31 bridge groups.

Frames are bridged only among interfaces in the same group..

interface interface-id

Enter interface configuration mode, and specify the interface on which you want to assign the bridge group.

The specified interface must be one of these:

■ A routed port: a physical port that you have configured as a Layer 3 port by entering the no switchport interface configuration command.

■ An SVI: a VLAN interface that you created by using the interface vlan vlan-id global configuration command.

These ports must have IP addresses assigned to them.

bridge-group bridge-group

Assign the interface to the bridge group created in Step 2.

By default, the interface is not assigned to any bridge group. An interface can be assigned to only one bridge group.

To remove a bridge group, use the no bridge bridge-group global configuration command. To remove an interface from a bridge group and to remove the bridge group, use the no bridge-group bridge-group interface configuration command.

To remove a bridge group, use the no bridge bridge-group global configuration command. To remove an interface from a bridge group and to remove the bridge group, use the no bridge-group bridge-group interface configuration command.

+3 -1

Post a comment