Question from Official Cisco ICND2 cert guide - STP

masterk

Hello all

Quick one for you, anyone who has this book could you just flick to page 75? Near the top it describes the Listening and Learning spanning tree port states.

My question is, the description for the Listening state, and about how it makes the old MAC entries time out to stop temporary routing loops (as old MAC entries cause routing loops), can anyone put that in the context of the diagram on the page before?

I am struggling to see how that would cause a problem, or be relevant?

Thanks

CodeBlox

Dont have the book with me, but the "Listening" state prevents the Mac-address-table from being updated. I think they were getting at the fact that if you add a host to one port on a switch, then remove it and put it in another port on another switch, both ports would reference the same MAC. MAC needs to timeout but the odd thing is (at least in packet tracer) that the mac address is immediately removed from the mac table, I thought it'd timeout.

masterk

Hi thanks for the reply.


In regard to the bit about packet tracer its probably normal, as with switches as soon as a cable is pulled any MAC entries for that port are immediately flushed, as the port is down. Same for STP with the max age timer, if the port goes down physically before the max age timer runs out the timer stops (as the switch knows it wont get any more hello's).

I agree with that you are saying that mac aging is needed, if not only to stop traffic going into a black hole for 5 minutes for the normal timer. But what i couldnt see is how MAC table entries on the wrong port could cause a loop..........