Cisco Systems BC-23 Filtering Transparently Bridged Packets

Configuring Transparent Bridging

Transparent and SRT Bridging Configuration Task List

BC-45

Cisco IOS Bridging and IBM Networking Configuration Guide

Normally, the system forwards any frames for stations that it has learned about dynamically. By

disabling this activity, the bridge will only forward frames whose address have been statically configured

into the forwarding cache. To prevent or allow forwarding of dynamically determined stations, use one

of the following command in global configuration mode:

A packet with a RIF, indicated by a source address with the multicast bit turned on, is not usually

forwarded. However, you can configure bridging support to allow the forwarding of frames that would

otherwise be discarded because they have a RIF. Although you can forward these frames, the bridge table

will not be updated to include the source addresses of these frames.

To forward frames with multicast addresses, use the following command in global configuration mode:

A bridge forwards, floods, or drops packets based on the bridge table. The bridge table maintains both

static entries and dynamic entries. Static entries are entered by the network manager or by the bridge

itself. Dynamic entries are entered by the bridge learning process. A dynamic entry is automatically

removed after a specified length of time, known as aging time, from the time the entry was created or

last updated.

If hosts on a bridged network are likely to move, decrease the aging-time to enable the bridge to adapt

to the change quickly. If hosts do not transmit continuously, increase the aging time to record the

dynamic entries for a longer time and thus reduce the possibility of flooding when the hosts transmit

again.

To set the aging time, use the following command in global configuration mode:

Filtering Transparently Bridged Packets

A bridge examines frames and transmits them through the internetwork according to the destination

address; a bridge will not forward a frame back to its originating network segment. The bridge software

allows you to configure specific administrative filters that filter frames based upon information other

than paths to their destinations. You can perform administrative filtering by performing one of the tasks

in the following sections:

Command Purpose

no bridge bridge-group acquire Filters out all frames except those whose addresses have been

statically configured into the forwarding cache.

bridge bridge-group acquire Removes the ability to filter out all frames except those whose

addresses have been statically configured into the forwarding cache.

Command Purpose

bridge bridge-group multicast-source Allows the forwarding of frames with multicast source addresses.

Command Purpose

bridge-group bridge-group aging-time seconds Sets the bridge table aging time.