STP LOOP

[sivam siva]

HI

i saw the following lines in HSRP document( https://www.cisco.com/c/en/us/support/docs/ip/hot-standby-router-protocol-hsrp/10583-62.html )"Be sure to check the CAM aging time in order to determine how quickly the entries are aged. If the time equals the configured value for STP forward delay, which is 15 seconds by default, there is a strong possibility that there is an STP loop in the network" 

can anyone explain this ? how loop happen while reduce the cam aging time ?

Thanks in advance.

[Georg Pauwen]

Hello,

I think what they mean is this:

If the forward delay timer, which is the time interval that is spent in the listening and learning state, is equal to the CAM aging time, the time after which the CAM entry disappears if unused, is the same, by the time STP moves to the forwarding state, the CAM entry will have timed out, and there is no entry in the CAM table where to forward traffic to.

So, if the CAM entry disappears after 15 seconds, and STP has to wait for the exact amount of time before actually moving traffic to that CAM entry, by the time it wants to start moving the traffic, the CAM entry is not there anymore. Therefore, the CAM aging time must always be higher than the STP forward delay timer.

A simple analogy (I cannot come up with something more intelligent) would be: you write down on a piece of paper where you live, so I can get there. But you use magic ink that disappears after 15 seconds. For security reasons, I am only allowed to read your piece of paper after 15 seconds. By the time I am allowed to read it, the information will be gone...

Does that make sense ?

[sivam siva]

I can understood that if the cam table aging and stp state forwarding happen simultaneously, there will be no cam entries in tha cam table to forward the traffic, then as per switching process switch will flood that traffic,what is the point of possible loop here? I'm little bit confused, can you please tell me one example so that I can understand more better. Thanks for your reply.

[Peter Paluch]

Hi Sivam, hi Georg,

I've checked the document, and it seems to say in a very unfortunate way that if the current aging time of MAC address table entries is set to STP forward_delay seconds then you had a very recent STP topology change event. You surely recall that with the legacy 802.1D STP, when a topology change is detected, the aging time of MAC address table entries will be shortened to forward_delay seconds, 15s by default. The period of the validity of this shortened aging time is the max_age + forward_delay seconds (35s by default).

RSTP and MSTP do not handle a topology change event in the same way - instead, they immediately flush all MAC address table entries except the ones on the port where the topology change event was learned on. With these protocols, you would not see a shortened MAC address table aging time.

The "STP loop" term is a misnomer in itself. You can have a switching loop, but an "STP loop" as a term has no substance.

My $0.02... :)

Best regards,
Peter

[sivam siva]

Thank you very much peter