[c-nsp] 65th VLAN

[A.L.M.Buxey at lboro.ac.uk]

Hi,

> The day before the migration, a human mistake had led to the configuration
> of a 65th VLAN on a switch (2950T-24) that supports only 64 STP instances
> (although it can support 256 VLANs). Nothing happened then, so nobody
> noticed; but when the migration started the day after, as cables were
> disconnected and reconnected between switches, Hell was unleashed. 
> 
> Did anyone ever experience a very bad outage due to more VLANs than
> available STP instances? What is the behaviour of a switch (2950/3524) when
> it has exhausted its STP instances ? Does the pattern of 10+20 seconds ring
> a bell to somebody?

are you running PVST? are/were all 65 VLANs in use through that switch. from
observed habaviour, nornally what happens is that the switch will, arbitrarily
disable spanning tree on a VLAN to get its value back to 64 - and if that VLAN
has a secondary spanning  tree route, that unleashes the network hounds of hell.

alan