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I am reading the book on Data Communications and Networking by Behrouz A. Forouzan

Suppose a device A on LAN 1 wants to connect to the device B on LAN 2 . Initially the bridge table is empty . When A sends frames to a bridge it will forward this frame to LAN 2 . The table will be filled with the MAC address of device A and does not know the address of device B . What happens to the frame that was sent on LAN 2 ?

Suppose the table has all details now , If a device wants to connect to a device on the same LAN , then can the frames be sent through point to point connection (I think there is one) that is between them , instead of first going to the bridge and then looking at the table , and then sending it to the destination .

What are the things I am missing here? I am confused. Please help .

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    it seems you are confusing bridge and router. A bridge (most commonly a switch) connect two hosts in a single LAN. It doesn't connect two LAN together, this is the role of a router.
    – JFL
    Commented Aug 28, 2017 at 12:43
  • @JFL bridge connects two LANs of same protocols .
    – shikhar
    Commented Sep 8, 2017 at 9:48
  • Strictly speaking, according to the 802.1D-2004 specification you are right. However this is no more the way the word "bridge" is used today. If you use this kind of terminology about a modern network, this will cause confusion. According to this definition every single 'ethernet switch port-cable-host' triplet is a distinct LAN.
    – JFL
    Commented Sep 8, 2017 at 9:57

2 Answers 2

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When host A send an ethernet frame to host B, with both hosts connected to an ethernet switch (I.E. bridge)

  • If the MAC table of the switch is empty, the switch will record the MAC address of host A, and since it doesn't know on which port the host B is connected, it will flood the frame on all the other ports (I.E. it will sent a copy of the frame on all ports except the port on which it received the frame).

Then when host B respond, the switch will record B MAC address and send the frame of the port it knows A is connected to.

  • for subsequent frames the switch already knows where both hosts are connected, and so will put the frame only on the needed ports.

Note that host A, host B and the switch form a single LAN.

For a more detailed explanation you can read this page on Practical Networking.Net

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The operation of a bridge can be thought of as two simple functions: Forwarding and learning.

Forwarding rule: If the DESTINATION MAC is in the forwarding table, forward the frame on the specified port, except if the destination port is the same as the received port. Otherwise, forward the frame out all ports (flooding).

Learning rule: If the SOURCE MAC is not in the forwarding table (or the port is different), add the MAC and the port it was received on to the table.

To answer your specific questions,

  1. The frame is forwarded on LAN 2. If there is a device on the LAN with the correct MAC address, it receives the packet. Every other device on that LAAN ignores it.

  2. On a LAN, there is no point to point connection between hosts. When host A sends a frame to host B on the same LAN, the bridge looks in its table and sees that the destination port is the same as the received port. There is no need for bridging, so it ignores the frame.

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