Rapid spanning-tree and Forward Delay?

[wintermute000]

(Move this to homework if you think its appropriate though its not technically homework so...)


I'm struggling to understand the official Cisco answer to this question. Can someone assist?

(paraphrasing) non-root SW1 is discarding on a trunk to SW2 in same MST region. SW1 stops receiving BPDUs from SW2. What two timers have an impact on how long it takes for SW1 to move to forwarding?


And the answers are the Hello timer on the root and the Forward Delay on the root.

Everything I can find indicates that RSTP comes up after 3x hello in case of loss of BPDU, and that forward delay only comes into play with 802.1D backward compatibility. The question is on MST but I am treating it as RSTP theory as I can' see how it matters - if both switches are in the same region it will operate like RSTP per instance?

This is in the official v5 CCIE OCG....

[that1guy15]

That looks correct to me. Its one of the frustratig things with MST/RST that I have noticed. Everyone documents the shit out of 802.1d but only gives 2 lines to MST and Rapid...

One of the best documents I have read on Spanning-Tree is here: http://blog.ine.com/wp-content/uploads/2010/04/understanding-stp-rstp-convergence.pdf

Also lab it up and see what you get.

[wintermute000]

Are you saying the forward delay comes into play or that it doesn't?

[that1guy15]

It should not be used. Timer based convergence is only used with traditional STP and backwards comparability like you mentioned.

Dig through that link and it should help clear things up. There is also this link from cisco that might help.

http://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/24062-146.html