Spanning Tree Demo From Cisco

Hi all,

I found what I thought was a good demo of Spanning Tree from Cisco, however I'm confused by the last few slides.

http://www.ciscosystems.com/image/gif/paws/10556/spanning_tree1.swf

Slide 19 shows Bridge B no longer receiving BPDUs from Bridge C, Bridge B then places the port between itself and Bridge C into a forwarding state. This is the link that BPDUs had stopped being received on. Why would it do that? Why would it place a port that is connected to a failed link into a forwarding state?

I've been studying since 9am this morning, maybe my brain is fried.

Matt

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Comments

GT-Rob

Well without any kind of guard on, if a port stops receiving BPDUs, its going to believe there is no longer a switch there, and puts it into forwarding.

Say you took one of your trunk links and plugged it into a computer for example.

If it really was a failed link, the interface would goto down/down anyway, so it doesn't matter what the STP state is.

kryolla

In order for the port to remain in block mode it has to receive superior BPDUs if it stops receiving them it will go through the normal STP process. The max age is how long it will store the BPDU so after it expires it no longer has a copy of the BPDU and will transition the port to designated and forward traffic. The physical link is still up but maybe due to congestion it doesn't send BPDUs.

HTH