Computer Network | Bridges (local Internetworking device)

Prerequisites – Network Devices, Types of switches
Local Internetworking is one which is within the same organization i.e. same building or same campus, then for the networking we may not require the full power of the router. We can do it with a data link layer device called bridge.

Bridges are a data link layer device and can connect to different networks as well as connect different networks of different types. Bridges from 802.x to 802.y where x & y may both be ethernet or one can be ethernet and other may be token ring, etc. It locally connects small LANs, whereas if LAN’s are big then bridges can no longer handle them. Bridge follows a protocol in IEEE format execute 802.1 which is a spanning tree of bridges.

In the above figure there are four LAN’s which are connected by two bridges. Bridge 1 has two ports, one connecting to LAN 1 other connecting to LAN 2, Bridge 2 has three connecting to LAN 2, LAN 3 and LAN 4. So, A can communicate with H through two bridges.

  • Bridges (Link Layer Device) –
    It stores and forwards Ethernet frames, i.e., it has to do with the MAC address rather than the IP address, they handle the hardware addresses. I also examines frame header and selectively forwards frames based on MAC destination address, such as in the given figure if Bridge 2 receives a packet then it will selectively decide whether to send it to LAN 3 or LAN 4.

    When frame is to be forwarded in a segment it uses CSMA/CD to access segment. These are transparent, i.e., hosts are unaware of presence of bridges, it appears to them as a single whole network. Bridges need not to be configured they are plug-and-play and self-learning devices, i.e. a bridge has a learning table, they learn which hosts can be reached through which interfaces. At the physical level the bridge boosts the signal strength like a repeater or completely regenerates the signal.

  • Ethernet Bridges –
    A bridge stores the hardware addresses observed from frames received by each interface and uses this information to learn which frames need to be forwarded by the bridge.

What if host is moved to another segment or new host is connected to a segment?
If new host is connected then learning process of bridges is going to be a continuous process. Suppose if we move from LAN 1 to LAN 2 some machine, i.e. MAC address moves from LAN 1 to LAN 2, which means table entries should leave after sometime so that the data remains fresh and relevant.

Bridges Spanning Tree:
For increased reliability, it is desirable to have redundant, alternative paths from source to destination, and with multiple paths, cycles result- bridges may multiply and forward frame forever without the frames being dropped. Solution is to organize bridges in a spanning tree by disabling subset of interfaces.

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