[c-nsp] RSTP Learning State

[PW]

Hi Everyone,

It would be great if someone can help me out here...

I've got a situation here with 4 switches (a mix of 2960/3550), all running
RSTP (rapid-pvst). The setup:

Switch C --------- Switch B ======= Switch A
 |                     |
 |                     |
Switch D --------------|

Basically a triangle setup BCD, and a dual link between AB.

Switch A is the root, and when the primary link between AB fails, the root
port of B goes down, transitions to ALT link almost instanteously.

However, the spanning tree recalculation that occurs in the triangle BCD
baffles me... I see links between Switch B, C and D transitions into the
Learning state for 15 seconds!
I thought RSTP should be sub-second failover for P2P links, so I'm not sure
why this 15 seconds is coming up...

The spanning tree state under normal operation:

Switch A:
Fa0/1           Desg FWD 36        128.1   P2p  (Secondary to B)
Fa0/2           Desg FWD 4         128.2   P2p  (Primary to B)

Switch B:
Fa0/1           Desg FWD 5000      128.1   P2p  (to D)
Fa0/2           Altn BLK 200       128.2   P2p  (Secondary to A)
Fa0/3           Root FWD 5         128.3   P2p  (Primary to A)
Fa0/4           Desg FWD 19        128.4   P2p  (to C)

Switch C:
Fa0/1           Altn BLK 200       128.1   P2p  (to D)
Fa0/2           Root FWD 10        128.2   P2p  (to B)

Switch D:
Fa0/1           Desg FWD 400       128.1   P2p  (to C)
Fa0/2           Root FWD 10        128.2   P2p  (to B)

There is nothing fancy about the RSTP configuration but setting port costs
to reflect the desired primary path.
Ports are all trunk ports, without "spanning-tree portfast trunk"
configured.

My questions are:
 - Why are the ports in Learning state for 15 secs?
 - Is it possible to reduce/eliminate this 15 secs?

Any help will be much appreciated!

Thanks.

PW