Bridges are capable of filtering frames based on any Layer 2 fields. For example, a bridge can be programmed to reject, not forward, all frames sourced from a particular network. Because link layer information often includes a reference to an upper-layer protocol, bridges can usually filter on this parameter. Furthermore, filters can be helpful in dealing with unnecessary broadcast and multicast packets.

Once the bridge has built the local address table, it is ready to operate. When it receives a frame, it examines the destination address. If the frame address is local, the bridge ignores it. If the frame is addressed for another LAN segment, the bridge copies the frame onto the second segment.

• Ignoring a frame is called filtering.
• Copying the frame is called forwarding.

Basic filtering keeps local frames local and sends remote frames to another LAN segment.

Filtering on specific source and destination addresses performs the following actions:

• Stopping one station from sending frames outside of its local LAN segment
• Stopping all "outside" frames destined for a particular station, thereby restricting the other stations with which it can communicate

Both types of filtering provide some control over internetwork traffic and can offer improved security.

Most Ethernet bridges can filter broadcast and multicast frames. Occasionally, a device will malfunction and continually send out broadcast frames, which are continuously copied around the network. A broadcast storm, as it is called, can bring network performance to zero. If a bridge can filter broadcast frames, a broadcast storm has less opportunity to act.

Today, bridges are also able to filter according to the network-layer protocol. This blurs the demarcation between bridges and routers. A router operates on the network layer using a routing protocol to direct traffic around the network. A bridge that implements advanced filtering techniques is usually called a brouter. Brouters filter by looking at network layer information but they do not use a routing protocol.

Web Links LAN Switching and VLANs http://www.cisco.com/univercd/cc/ td/doc/cisintwk/ito_doc/lanswtch.htm