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Segmenting LANs using Bridges

Started by VelMurugan, Nov 04, 2008, 06:50 PM

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VelMurugan

Segmenting LANs using Bridges

Bridges work by regenerating signals. Bridges read the MAC address of a data frame to determine whether the destination computer resides on the local segment or a different segment. Based on this determination, the bridge does one of the following:

    * If the destination exists on the local segment, the frame is not forwarded out to all network segments to which it is connected.
    * If the destination does not exist on the local segment, the frame is forwarded out to all network segments to which it is attached.

When segmenting LANs using bridges, broadcast storms can occur due to the broadcast packets propagating throughout the network. This is typically caused by bridges forwarding frames out of all attached segments. Processing of frames can also lead to a significant increase in latency.

Bridges are responsible for the following functions:

    * Checking the MAC addresses of frames when frames are received.
    * Using the destination MAC address, bridges determine when frames should be filtered and forwarded.
    * Bridges utilize the Spanning-Tree Protocol (STP) to create a loop-free environment with the other bridges.

With reference to Ethernet LAN segmentation, the main characteristics of using bridges are summarized here:

    * Bridges forward broadcasts and multicast frames.
    * Bridges are not dependent on Layer 3 protocols.
    * Bridges use Layer 2 headers and logic to perform switching of packets.
    * Bridges utilize the store-and-forward logic which results in more latency than a single LAN segment. The whole frame has to be received before it is forwarded.
    * Bridges typically perform more functions before a packet is forwarded. This in turn results in latency.

The benefits of segmenting LANs using bridges are listed here:

    * Distance limitations are no longer an issue because each segment can be created with the maximum distance for the specific Ethernet type.
    * Each segment runs at 10 or 100 Mbps, which in turn increases total bandwidth.
    * For frames that has a source and destination on the same LAN segment, traffic can be contained in the specific LAN segment.
    * Because frames are filtered, collisions are decreased.