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udld errors on port channel interfaces (physical links)

Unanswered Question

Hi,

 

Here's the problem we are having:

Network 1 is network with 2 cisco 4510 core switches in hsrp mode.

Network 2 is network with HP 5800 core switches (2) stacked together.

 

Goal is to link these networks with port channels from both cisco's to HP stack.

 

However, when second port channel is connected, individual links are shut because of udld errors.  The port channels stay up (with one physical link)

When using just one port channel, then both links stay up.

 

So it does not seem to be a hardware error.

 

Anyone an idea what could be the cause ?

 

Thanks.

 

Koen

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Steve Fuller Mon, 03/24/2014 - 05:20
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Hi Koen,

As far as I'm aware, while both Cisco and HP support UDLD, the UDLD implementations are not compatible. What I've seen in the past when connecting Cisco to HP switches is that the UDLD frames from one Cisco switch are simply passed straight through to the second Cisco switch.

In the non port-channel configuration I was using this meant that Cisco Switch 1 Port 1 and Cisco Switch 2 Port 1 were neighbours from a Cisco UDLD perspective.

   --------- S1 P1   HP P1 ---- HP P2   S2 P1 --------
  |Cisco S1 |-------------| HP |-------------|Cisco S2|
   ---------               ----               --------

This worked because the only path the UDLD packets from Switch1 Port1 could take was to Switch2 Port1, and vice versa.

Now if we take your setup where you have port-channels:

   ------ CS1P1     HP1P1 ------ HP1P3   CS2P1 ------
  |      |---------------|      |-------------|      |
  |  CS1 |               |  HP1 |             |Cisco2|
  |      |---------------|      |-------------|      |
  ------- CS1P2     HP1P2 ------ HP1P4   CS2P2 ------

 
When a UDLD frame is sent on Cisco Switch 1 Port 1, it reaches HP1 Port 1, but the question is which ports is the frame now forwarded on? As Cisco UDLD uses a destination MAC with the Individual/Group bit set my assumption would be that the HP switch would forward the UDLD frame on both Port 3 and port 4. The second Cisco switch will object to receiving the same UDLD frame on two ports and so disable the ports.

I think this may be why your setup works when you only have a single link in each port-channel.

Even in a simple, sinlge link setup you should still be careful as UDLD can still break connectivity. For example, taking the first ASCII art scenario, as both S1 and S2 see each other as UDLD neighbours, when the link between Cisco S1 and the HP switch goes down, UDLD frames are no longer received at Cisco switch 2. The second Cisco switch, believing the link connected to Switch 2 Port 1 had become uni-directional, disables the link. Because of this when we run Cisco and HP switches together we disable UDLD on the Cisco switches.

As you're using link aggregation I'd recommend that you enable LACP between the Cisco and HP switches. This would give similar functionality to UDLD, although with default LACP rate of 30-seconds the detection would take too long so you may need to look at using the LACP Short timer i.e., 1-second.

Regards

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