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New Member

A Question About Loop Guard

I'm trying to better understand loop guard. From my understanding, loop guard helps to prevent forwarding loops in the case of unidirectional link failure.

Consider 3 switches, A, B, and C. A is root and B and C have root ports connected to A. The port connecting B to C is Designated and the port connecting C to B is non-designated or blocked.

According to STP, if switch C stops receiving BPDUs from switch B, it begins the transition to forwarding, why is this? Wouldn't this cause switch C to now have 2 forwarding paths? One to switch A (Root) and one to switch B, causing a forwarding loop. Why does traditional STP allow a non designated port that stops receiving BPDUs from a designated port to transition into a forwarding state if the root port is still alive?

I also understand that with Loop Guard enabled on the port connecting switch C to B, loop guard transitions the port to an inconsistent state on non-designated ports if it stops receiving BPDUs and reenables the port once it receives BPDUs.

3 REPLIES

Re: A Question About Loop Guard

Hi,

// According to STP, if switch C stops receiving BPDUs from switch B, it begins the transition to forwarding, why is this? Wouldn't this cause switch C to now have 2 forwarding paths? One to switch A (Root) and one to switch B, causing a forwarding loop. Why does traditional STP allow a non designated port that stops receiving BPDUs from a designated port to transition into a forwarding state if the root port is still alive? //

Because as per STP design designated port transmits BPDUs, and the       non-designated port receives BPDUs.

Since the non-designated no longer       receives BPDUsfrom designated port, the STP conceives that the topology is loop free. Eventually,       the blocking port from the alternate or backup port becomes designated and       moves to a forwarding state. This situation creates a loop.

The loop guard feature makes additional checks. If BPDUs are not       received on a non-designated port, and loop guard is enabled, that port is       moved into the STP loop-inconsistent blocking state, instead of the listening /       learning / forwarding state.

Hope it helps you.

Regards

Karuppu

New Member

Re: A Question About Loop Guard

Why does STP rely on a non-designated port to stop receiving BPDUs before it puts it in a forwarding state if non-designated ports are SUPPOSED to receive BPDUs? Aren't non-designated ports supposed to receive BPDUs? If it stopped receiving BPDUs on it's alternate link, why would it try and transition it into a forwarding state if the root port is still alive? This would cause a forwarding loop.

"Since the non-designated no longer       receives  BPDUsfrom designated port, the STP  conceives that the topology is loop free." thus, tries to put it in a forwarding state, and causing a loop.

Re: A Question About Loop Guard

Hi,

//If it stopped receiving BPDUs on it's alternate link, why would it try and transition it into a forwarding state if the root port is still alive? //

If the port is not recieved BPDUs obviously the port will go to forwarding mode,even if you have a root bridge.

Take a Example that , you have connected a host with L2 switch,by any how the host cannot generate the BPDUs,then this scenario the port has to be in forwarding mode.

//Why does STP rely on a non-designated port to stop receiving BPDUs before it puts it in a forwarding state if non-designated ports are SUPPOSED to receive BPDUs? Aren't non-designated ports supposed to receive BPDUs? //

The non-designated port has to recieve the BPDUs from the designated port else how STP will calculate ??.

If the port is not recieved the BPDUs then STP might thinking that some hosts are connected to that port and it is going to forwarding state.But actually it is not.so that the loop is occuring in the network

regards

karuppu

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