[c-nsp] UDLD enabling port prematurely?

[Painting, Stuart]

Seeing STP in forwarding state isn't necessarily a cause for concern.
STP will start running when the port comes up (e.g. after the
errdisable recovery timeout) but will wait for an incoming BPDU before
entering forwarding state.

UDLD frames are only sent every 15 seconds by default, so may lag
behind STP status in normal running.

This is assuming that you haven't set STP portfast on that port.
Portfast would cause the port to transition to forwarding state
without waiting for an incoming BPDU, which would indeed cause
undesired traffic flows.

To be honest I'm not all that keen on UDLD (it can lead to
interesting failure modes if an STP-participating switch doesn't
also run UDLD) but if GigE autonegotiate isn't good enough for your
purposes you may be forced to use UDLD.
 

-----Original Message-----
From: cisco-nsp [mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of Victor Sudakov
Sent: 17 July 2014 09:15
To: cisco-nsp at puck.nether.net
Subject: [c-nsp] UDLD enabling port prematurely?

Colleagues,

I have several pairs of Catalyst 3560E switches connected via third
party MUXes (Avara ENT100). When the actual physical medium goes down,
the MUXes do not shutdown their Ethernet interfaces (i.e. they have no
"link poisoning"). So I need some sort of point-to-point L2 link fault
management between the switches.

Is UDLD suitable for this purpose? I have experimented a bit with
"udld port aggressive" and have found out the following strange
thing.

When the physical link goes down, UDLD detects this condition and
shuts the switch interface down. However, after several minutes, the
interface goes up again with "%PM-4-ERR_RECOVER: Attempting to recover
from udld err-disable state on Gi0/17". The interface is up even
though "Current bidirectional state: Unknown", and seems to be in the
STP forwarding state.

This does not look good. If a broadcast frame arrives to this port
before a BPDU does, there will be a storm and a lot of MAC flapping.
Is there a way to keep the port from forwarding traffic until the UDLD
state is Bidirectional ?

Thanks a lot for any input.

-- 
Victor Sudakov,  VAS4-RIPE, VAS47-RIPN
sip:sudakov at sibptus.tomsk.ru
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