Cisco Systems BC-23

Configuring Transparent Bridging

Technology Overview

BC-26

Cisco IOS Bridging and IBM Networking Configuration Guide

Figure 8 Bridge-Group Virtual Interface in the Router

The bridge-group virtual interface is a normal routed interface that does not support bridging, but does

represent its corresponding bridge group to the routed interface. It has all the network layer attributes

(such as a network layer address and filters) that apply to the corresponding bridge group. The interface

number assigned to this virtual interface corresponds to the bridge group that this virtual interface

represents. This number is the link between the virtual interface and the bridge group.

When you enable routing for a given protocol on the bridge-group virtual interface, packets coming from

a routed interface, but destined for a host in a bridged domain, are routed to the bridge-group virtual

interface and are forwarded to the corresponding bridged interface. All traffic routed to the bridge-group

virtual interface is forwarded to the corresponding bridge group as bridged traffic. All routable traffic

received on a bridged interface is routed to other routed interfaces as if it is coming directly from the

bridge-group virtual interface.

To receive routable packets arriving on a bridged interface but destined for a routed interface or to

receive routed packets, the bridge-group virtual interface must also have the appropriate addresses. MAC

addresses and network addresses are assigned to the bridge-group virtual interface as follows:

•The bridge-group virtual interface “borrows” the MAC address of one of the bridged interfaces in

the bridge group associated with the bridge-group virtual interface.

•To route and bridge a given protocol in the same bridge group, you must configure the network layer

attributes of the protocol on the bridge-group virtual interface. No protocol attributes should be

configured on the bridged interfaces, and no bridging attributes can be configured on the

bridge-group virtual interface.

Note When a bridged domain contains learning devices (such as switches or bridges) that can learn the

MAC address of a bridge-group virtual interface, the virtual interface must be configured with its

own MAC address—separate from the MAC addresses of the bridged interfaces in the bridge group

that are associated with the virtual interface. The MAC address is configured by using the

mac-address virtual interface command.

Because there can be only one bridge-group virtual interface representing a bridge group, and the bridge

group can be made up of different media types configured for several different encapsulation methods,

you may need to configure the bridge-group virtual interface with the particular encapsulation methods

required to switch packets correctly.

For example, the bridge-group virtual interface has default data link and network layer encapsulations

that are the same as those available on Ethernet interfaces, but you can configure the bridge-group virtual

interface with encapsulations that are not supported on an Ethernet interface. In some cases, the default

encapsulations provide appropriate results; in other cases they do not. For example, with default

encapsulation, Advanced Research Projects Agency (ARPA) packets from the bridge-group virtual

interface are translated to Subnetwork Access Protocol (SNAP) when bridging IP to a Token Ring- or

Bridged

interfaces

Bridge group 1

Routed

interface

S4757

BVI 1

10.0.0.2

10.0.0.1