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TOPIC: bridge forwarding

bridge forwarding 13 years 2 weeks ago #11122

  • gundja
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i am bit confused about how a bridge forwards frames.let say there are 2 network segments: 1 and 2 which has been separated using a bridge.
on segment 1 there are 3 hosts:A,B and C.
on segment 2 there are 3 hosts:X,Y and Z.

now host A sends a frame to host X.upon receipt of the frame,what will the bridge do?will it forward it to all X,Y and Z i.e to all hosts on network segment 2 or will the bridge forward it to only host x?

i hope that someone can clear my doubts upon this subject.
thanks

Bridge 13 years 2 weeks ago #11129

Hello gundja
A bridge, in it's most basic form, has two interfaces. One goes to segment 1 and the other to segment 2. The bridge works at MAC level and deals only with MAC addresses. For each packet that arrives, the only decision it makes is "does this packet need to cross the bridge, or does it not". If the bridge's internal filter table shows that the destination MAC address in the packet is on the far side of the bridge, it will forward the packet. But if the destination MAC address is on the same side of the bridge, the bridge does not forward (discards) the packet. Once over the bridge, a forwarded packet is detected by all hosts on the segment but only the host with the correct MAC address will accept the packet and respond
Please feel free to come back with mnore questions if that doesn't clear it up for you

Re: bridge forwarding 13 years 2 weeks ago #11136

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so in this situation,the bridge will foward the frame to all hosts on segment 2.is it correct?

if it has been a switch,only host X would receive the frame.correct!

Bridge 13 years 2 weeks ago #11139

The short answer is yes. But remeber that a bridge is an older device that was connected between two segments, each with multiple hosts. A switch on the other hand is effectivley a multi-port bridge. It makes the same forward/not forward decision but each port of the switch usually goes to just one host. Remeber also that we are dealing not with IP addressing here but with MAC addresses. The normal mechanism for an Ethernet segment is that a frame placed onto the segment is recieved by all hosts but only the host with the correct MAC address will process it

Re: bridge forwarding 13 years 1 week ago #11236

Exactly.
Another way of looking at it is, Bridge/Switch will learn the MAC address of X on the second port (1st being on which ABC are connected).
It will simply send the frame on the wire. It is of no concern to the bridge/switch who is connected or even if "x" exists or not. It learned X on port 2 and so it will forward the frame for X on the wire on Port 2. Since y & z are also connected to the same wire, they are bound to receive the frame though it is destined only for X.
Once all hosts construct the Frame from the bits on the wire, they can make out if the frame recived is actually for them by looking into the desination address field of the frame. If yes, it is accepted else discarded.
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