UDLD error on uplink port of empty switch

[anselsantosa]

I have 5 switches (4xMS22P and 1xMS120-24) plugged directly into an MX-100. I'm getting a UDLD alert (Unidirectional link (outbound fault)) on the uplink port of the MS120-24. It's configured with all the same settings as the uplink ports on the MS22Ps and there is currently nothing else on that switch. The uplink port is the only one with anything plugged into it.

The neighbor ID in the error references one of the sibling switches attached to the same MX-100. The switch in question is the RSTP root. Is that the issue? Should one of the other switches be the RSTP root? It's not clear to be how to pick because all 5 switches are connected directly to the same router. I picked this one because it was the least busy and the newest hardware.

[PhilipDAth · Accepted solution]

I wouldn't use UDLD to an MX100.  It does not support UDLD, and you are meant to be have UDLD compatible neighbours on each side of the link.

[anselsantosa]

I didn't realize the MX100 didn't support UDLD. That makes sense then. Should I turn off UDLD on all ports (e.g. to client devices?) UDLD is currently enable for all ports, client and uplink, on all switches but this is the only port that's alerting. That still confuses me. Also confused why the alert says the switch's neighbor is one of the other switches plugged into the same router even though they are not directly connected.

[PhilipDAth]

Using UDLD to the MX100 is particularly problematic, because it will probably repeat those UDLD packets out other ports, making switches think they are directly connected to each other when they are not.

Your other ports really are point to point links (switch to client, and switch to switch), so they should not have the same issue.

[anselsantosa]

That makes sense. I went to disable UDLD on all my uplink ports to the MX100 but it looks like UDLD can be set only to Alert or Enforce, not Disabled?

[PhilipDAth]

I haven't played around with UDLD a lot, but I think you are right.  I think it can only be set to Alert.

[Lukard]

Is this still true? I ask because there does not seems to be an option to disable on the MS uplink port to the MX100.

[GIdenJoe]

I find it strange you can even enable this feature on non-fiber ports.

Normally UDLD is a feature only used between switchlinks to avoid unidirection communication causing spanning-tree to erroneously put a discarding port into designated forwarding due to no longer receiving bpdu's.

This error state usually only exists if one fiber of the pair no longer transmits due to optics error or cable error.

So if you want to enable this feature, only do it on trunks that connect two switches, since only switches support the protocol.  Never enable it between switches and hosts/AP's/Firewalls unless they specifically support the feature and have it enabled.

You should also start with alert mode ( = normal mode ) before you do aggressive mode which actually blocks a link in case of a unidirectional link.

[anselsantosa]

Not only can it be turned on, it can't be turned off.  I have them all set to alert mode because that's the closest I'm allowed to get to off...