[RSTP] Modifying Timers to avoid link flap related issues?

[martin_tsang]

Hi All + ftallet :-)

Assume the following scenario;

There are two links between two switches. The primary link fails, RSTP cuts over to the secondary link. Now, the primary comes up for 10 seconds, and goes down again.

Question:

1. Is there a feature that can overcome this problem? E.g. adjust some timer to say 60 seconds, such that the primary link has to be working for 60 seconds before reverting to it.

2. Better still is there a feature that sends 'hello' style packets to check if a link is up for 60 seconds before reverting to the primary link? (like tracking in a 1800 series router )

I really only need to know if such features exist, and if they do, what are they called so I can search for it and read about it.

Cheers

Martin

[martin_tsang]

no hints or even guesses here?

[arvindchari]

AFAIK no such option exists. What you are trying to accomplish might be possible using a routing protocol (using hold down timer?) but I dont remember seeing anything to this effect in 802.1w. If you have two links between two switches, one of them will be a designated port and one will be a backup port so if you dont get three hellos you will definitely have the backup port coming in as the designated port.

[Francois Tallet · Accepted solution]

Hi Martin,

I don't know of any such protocol. It would not be so complex to implement something that plays around with the STP cost of an interface based on its recent history though...

STP's memory does not go beyond max age. Uplinkfast is able to prevent an uplink that went down from becoming active immediately for some few additional seconds, but there nothing configurable there.

Finally, there is a solution called "flexlink", that basically keeps an interface blocked as a backup for a primary. If the primary fails, the backup interface goes forwarding. If the old primary comes back up, it becomes a backup interface...

That's all I can think of, which is not much you see;-)

F.