UDLD port

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Oct 21st, 2008
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What is the advantage of configuring "UDLD port" on gigabit interfaces on a Cat 6500 switch?

The interfaces are part of a GEC and are also trunked.

What is the advantage of having the command on or off.

Do we really need it. ?

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Jon Marshall Tue, 10/21/2008 - 13:22
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As far as i understand you would use this if you wanted to apply a different UDLD setting to a specific interface than the global UDLD setting. By default UDLD is enabled on fibre interfaces so more often than not you would use the UDLD port command to turn it off on a particular interface. If for some reason you have globally disabled UDLD then you can turn it on for a particular interface with the UDLD port command.


qwertys21 Wed, 10/22/2008 - 00:23
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These are copper interfaces not fibre. So is it necessary to include the command "udld port" on the interfaces?

Could this be giving me DNS issues?

When I do a Show udld I get the below:

Port enable administrative configuration setting: Enabled

Port enable operational state: Enabled

Current bidirectional state: Unknown

Current operational state: Advertisement

Message interval: 7

Time out interval: 5

No neighbor cache information stored

Istvan_Rabai Wed, 10/22/2008 - 00:38
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Hi Ade,

On copper interfaces it is less critical to enable UDLD as they give a more reliable connection.

However you can enable UDLD on copper interfaces as well.

This will give you the advantage I described in my previous post.

The output of the show udld command shows that you enabled udld on this interface but either you did not enable it on the counterpart interface or there is no connection to it.



qwertys21 Wed, 10/22/2008 - 03:02
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When you say counterpart interface, where is this. These connection are to NAS servers. Do they need to be enabled on the NAS servers as well.

What is the impact if I remove them?


Istvan_Rabai Wed, 10/22/2008 - 05:31
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Hi Ade,

The counterpart interface is the interface on the other side of the link.

As I mentioned UDLD is a complement to the spanning-tree protocol.

If your NAS server is part of the spanning-tree then it can make sense to configure UDLD on it as well, if it supports this feature.

In other cases you can remove UDLD config from the switch port that connects to the NAS server.

You will not loose anything in that case.



Istvan_Rabai Tue, 10/21/2008 - 22:02
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Hi Ade,

The purpose of using UDLD is to complement the spanning-tree protocol.


Let's say a Gig port is in the STP blocking state and everything works fine.

This means that this port is sending and receiving STP BPDUs and the periodic receival of BPDUs keeps it in the blocking state so no loop is formed.

Now let's suppose the receiving side fiber is damaged.

The switch will not receive any more BPDUs and it will suppose that no other switches are connected to this segment.

As a result, after the max age timer and 2 times the forward delay timer the port will go into forwarding state.

A loop has formed and this part of the network collapses.

With UDLD, if the same situation happens, the switch will detect the damage of the receive side fiber and it will err-disable the port.

(Supposed the port is configured with the "udld port aggressive" command - aggressive mode udld.)

No loops will be formed and the production network will be able to continue to work.




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