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Bridges (local Internetworking device)

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  • Difficulty Level : Basic
  • Last Updated : 25 Aug, 2021
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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 a 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 a token ring, etc. It locally connects small LANs, whereas if LANs 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 LANs that 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 examine the frame header and selectively forward 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 a frame is to be forwarded in a segment it uses CSMA/CD to access the segment. These are transparent, i.e., hosts are unaware of the presence of bridges, it appears to them as a single whole network. Bridges need not 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 the host is moved to another segment or a new host is connected to a segment? 
If a new host is connected then the 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 some time 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. The solution is to organize bridges in a spanning tree by disabling a subset of interfaces.

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