UDLD and Block port STP

[Rodrigo Baza]

Hi Everyone, i have a doubt.

If a have a stp block port and this port i have udld configured, i still recibed udld hellos on stp block port?

Cheers

[jmperlewitz]

Since UDLD is a layer 2 protocol, it should not receive hellos over the STP blocked segment.  It will this go into blocking which is already the state that the segment is in due to STP.  

[Peter Paluch · Accepted solution]

Hello,

Since UDLD is a layer 2 protocol, it should not receive hellos over the STP blocked segment.

I respectfully disagree - but I admit that this topic is more convoluted than it might appear.

If STP blocks a port, it blocks it for ordinary data traffic. However, selected control-plane protocols, such as STP itself, CDP, LLDP, DTP, VTP, PAgP, LACP, and UDLD, continue being allowed even on a port in STP Blocking/Discarding state - these are allowed to be sent and received.

If this was not true, then STP would in fact interfere and disrupt the proper operation of these protocols whenever it blocks a port - it would even interfere with itself; recall that for a port to stay in Blocking/Discarding state, it must keep receiving and processing incoming BPDUs that are better than its own BPDUs. If a Blocking/Discarding port stopped receiving all Layer2 protocols altogether, it would necessarily age out the superior BPDU, and eventually become Forwarding.

Feel welcome to ask further!

Best regards,
Peter

[mbustani]

Hi Peter,

Do you know, by the way, why the UDLD frames can continue be exchanged through these ports even one of them is in alter/blk status? Is there any implicit policy in the control-plane that assure this?

Thank you in advance!

[Giuseppe Larosa]

Hello mbustani,

as explained in Peter's post STP does not block other L2 control protocols on ports even if the STP state is blocking or alternate/blocking.

There is no need for a policy to allow UDLD frames to flow between two ports.

User traffic is not allowed.

UDLD checks the L2 bidirectional connectivity between two ports and validates the l2 link.

In other words it is a wanted behaviour and not a defect.

Hope to help

Giuseppe

[mbustani]

Hi Giuseppe,

I got it! I believe this granted by the control-plane in somehow. But do you know how this is granted, assured by the control-plane? Thats why I thought in some EXISTENT implicit policy (or default policy) included in the control-plane by default. Can we, maybe, see this implicit command/rule?

Am going into deep on this? I was just wondering about this how it works...just for curiosity.

Thanks!