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Community Member

UDLD just on one side of a link

Hello,

If UDLD (normal -not aggressive-)is only configured on one switch A and not on the remote one (switch B),

will switch B counter/add UDLD packets received as runt (smaller than 64 octets packets) or as any other error ?

Will the switch B react regarding its hardware or software ? (a cat3750 switch will not react as a cat2950 switch as switch B) ?

Regards,

2 ACCEPTED SOLUTIONS

Accepted Solutions

Re: UDLD just on one side of a link

The situation would work like this:-

Switch B successfully receives traffic from Switch A on the interface. However, Switch A does not receive traffic from Switch B on the same interface. UDLD detects the problem and disables the interface.

Ideally you could have it on both sides.

HTH>

Re: UDLD just on one side of a link

Hi,

UDLD (or any other protocol!) will never create packets shorter than the minimum legal length for the media. (64 bytes for Ethernet). Also, UDLD won't do much if it's only on one end - but the multicasts it generates will simply be ignored by the other end.

HTH

Andrew.

12 REPLIES

Re: UDLD just on one side of a link

The situation would work like this:-

Switch B successfully receives traffic from Switch A on the interface. However, Switch A does not receive traffic from Switch B on the same interface. UDLD detects the problem and disables the interface.

Ideally you could have it on both sides.

HTH>

Community Member

Re: UDLD just on one side of a link

hello Andrew,

ideally, yes the feature should be configured on both sides.

In my situation, this is not (I have no access on switch A) and switch A has a single attachement to only switch B (there is no redondancy, no port blocking by STP).

before the ends customers (switch A and B) update there configuration to have the 2 of them having UDLD configured the same way, I'm ask to look for the switch B reacts.

I aggree, that it is easier to just configure UDLD on both side the same way. But I could get information about the way to handle such UDLD packets, this would be great (on the side not yet configured for UDLD).

Regards,

Re: UDLD just on one side of a link

Community Member

Re: UDLD just on one side of a link

hello Andrew,

Thank your very much for this link (configuration and why to use it), but I didn't find the answer I'm looking for.

Best regards,

Re: UDLD just on one side of a link

What are you looking for - that link has to some pretty cool stuff?

Community Member

Re: UDLD just on one side of a link

Hello Andrew,

I'm sorry I don't understand "pretty cool stuff".

The end customer see some runts errors on the switch B, and i'm looking for if those runts are or aren't the UDLD packets received. The link beetween A and B is provided by an Internet Operator who said that the link is ok, and I can't get a sniffer on any of those sites.

What I know is that UDLD is configured on swith A, not on switch B.

I know I have only a few information(s).

Anyway i'm trying with this forum, if someone knows. I can not open a TAC case just for this.

Anyway thank you very much for the time you spand trying to help me. I DO appreciate.

Best regards,

Re: UDLD just on one side of a link

My "pretty cool stuff" was the explanation of how it works - at a basic level

Runts are typically packets that are to small, I seem to vagely remember a runt is smaller than 64 bytes!

But I have read somewhere that a UDLD frame is 64 bytes....but might be smaller if the TLV is not used.

HTH>

Re: UDLD just on one side of a link

Yep - quick search in the RFC database, you should read the below - a more in depth working of Cisco's UDLD:-

http://tools.ietf.org/html/rfc5171

Community Member

Re: UDLD just on one side of a link

Hello Andrew,

yes, that a very (very very) nice link, I should read ( and re-read).

maybe some IOS do not use all of those TLV, and of course the length of each one (or some of them may not be always the same (like the device name one)). So regarding some TLV used by one IOS release, maybe I get a runt packet. well I think it will be very difficul to find more information.

Andrew, thank you very much for your help, I do appreciate.

Best regards,

Re: UDLD just on one side of a link

That is possible I suppose! Try searching the feature navigator based on IOS version, this may give some results:-

http://tools.cisco.com/ITDIT/CFN/jsp/index.jsp

np - glad to help.

Re: UDLD just on one side of a link

Hi,

UDLD (or any other protocol!) will never create packets shorter than the minimum legal length for the media. (64 bytes for Ethernet). Also, UDLD won't do much if it's only on one end - but the multicasts it generates will simply be ignored by the other end.

HTH

Andrew.

Community Member

Re: UDLD just on one side of a link

hello Andrew,

Thank you very much for this information.

best Regards,

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