Configuring Transparent Bridging

Technology Overview

Figure 8 Bridge-Group Virtual Interface in the Router

Routed interface

Bridge group 1

E0

E1

 

E3

E2

10.0.0.1

BVI 1

10.0.0.2

Bridged

interfaces

S4757

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-addressvirtual 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

Cisco IOS Bridging and IBM Networking Configuration Guide

BC-26

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Cisco Systems BC-23 manual BC-26, Bridge-Group Virtual Interface in the Router

BC-23 specifications

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