Bridging Loops

Unanswered Question
Jun 11th, 2008

Hello please explain how bridging loops occur because I cannot understand this; http://www.cisco.com/en/US/docs/internetworking/technology/handbook/Transparent-Bridging.html

Figure 23-2

If host A sends a frame to host B than host B will receive 2 frames and that I understand it. What I do not understand is why both bridges will think that both Host A and Host B are on the same network ?

I have this problem too.
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Jon Marshall Wed, 06/11/2008 - 12:26

Andrea

Host A sends a packet to Host B. Both bridges receive the packet so they both record Host A as being reachable out of the interface connected to Net2.

Both bridges now forward on the packet to Net1. So yes host B sees two packets. But bridge A will receive the packet that bridge B forwarded on to Net1. And bridge B will receive the packet that bridge A forwarded onto Net 1.

So in effect there is now a loop because bridge A & B are seeing the same packet on 2 diifferent ports connected to 2 separate segements.

Broacasts in this setup are far worse because a switch will always forward broadcasts. So

Host A sends broadcast packet. Both bridge A & B forward it onto Net1. But as above they both receive the broadcast from each other on Net1 interfaces so they must then forward it back onto Net2 and so on. The broadcast will keep getting forwarding by both bridges onto both segments until you either break the loop or more likely your network grinds to a halt.

Does this make sense ?

Jon

aconticisco Wed, 06/11/2008 - 12:39

That makes much more sense however once Host B replies to the frame will not both Bridges learn that Host B is on network 1

Jon Marshall Wed, 06/11/2008 - 13:16

Andrea

Yes they will. But they also believe host A is on Net1 as well. This can lead to incorrect forwarding of frames and inconsistent bridge tables. So you can get intermittent connectivity in this scenario which can be difficult to troubleshoot.

As i said in previous post the real danger lies in broadcast packets which will just continually loop round the network.

Jon

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