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New Member

UDLD

Hello everyone. Can some one help me out in understanding UDLD feature on Cisco switches.tried googling it but could not understand.
Thanks in advance.

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17 REPLIES
New Member

Re: UDLD

is there something specific about udld you do not understand? you might want to be a bit more explicit in your question otherwise the feedback you will get is links to Cisco reading material.

Hall of Fame Super Gold

UDLD

Here's a simple, yet effective, rule-of-thumb about UDLD:  DO NOT use UDLD when you are configuring copper ports.

(UDLD works very well on fibre optic ports.)

New Member

Re:UDLD

Udld importance while configuring trunks. Udld port aggressive feature


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Hall of Fame Super Gold

UDLD

Udld importance while configuring trunks. Udld port aggressive feature

Sorry, I don't understand you.  Is that a statement or is that a question?

New Member

Re:UDLD

Its my question...


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Hall of Fame Super Gold

UDLD

Udld importance while configuring trunks. Udld port aggressive feature

802.1Q or ISL trunks have NOTHING to do with UDLD.

New Member

Re: UDLD

Hi Leo, which mode
Do you use, normal or aggressive

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Hall of Fame Super Gold

UDLD

Do you use, normal or aggressive

Aggressive.

New Member

Re: UDLD

Does normal mode make port go into error-disable mode, Leo.

Sent from Cisco Technical Support iPhone App

Re: UDLD

Does normal mode make port go into error-disable mode, Leo.

Both modes can result in error-disable, but the mechanisms are different:

"In normal mode, a protocol determination at the end of the detection   process is always based on information received in UDLD messages:   whether it's the information about the exchange of proper neighbor   identifications or the information about the absence of such proper   identifications.  Hence, albeit bound by a timer, normal mode   determinations are always based on gleaned information, and as such   are "event-based".  If no such information can be obtained (e.g.,   because of a bidirectional loss of connectivity), UDLD follows a   conservative approach based on the considerations in Section 3 and   deems a port to be in "undetermined" state.  In other words, normal   mode will shut down a port only if it can explicitly determine that   the associated link is faulty for an extended period of time."

"In contrast, in aggressive mode, UDLD will also shut down a port if   it loses bidirectional connectivity with the neighbor for the same   extended period of time mentioned above and subsequently fails   repeated last-resort attempts to re-establish communication with the   other end of the link.  This mode of operation assumes that loss of   communication with the neighbor is a meaningful network event in   itself and is a symptom of a serious connectivity problem.  Because   this type of detection can be event-less, and lack of information   cannot always be associated to an actual malfunction of the link,   this mode is optional and is recommended only in certain scenarios   (typically only on point-to-point links where no communication   failure between two neighbors is admissible)."

Hope that helps

Rolf

Hall of Fame Super Gold

UDLD

Does normal mode make port go into error-disable mode

All modes (aggressive and FAST UDLD) can put the port into error-disable if the hello packets are not being heard.

New Member

Re: UDLD

why not normal mode?

I mean is it necessary to use aggressive mode , is aggressive mode useful in real world? Or some disadvantage with aggressive mode.

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Hall of Fame Super Gold

UDLD

why not normal mode?

I've never heard of "normal mode" in UDLD.

I mean is it necessary to use aggressive mode , is aggressive mode useful in real world?

I've got 10 Gbps fibre uplinks.  I want to make sure that the very first sign that there's an issue of the link, I want all traffic to stop.  If I don't turn on UDLD, I would potentially have a network issue if the fibre link goes up/down at regular intervals per minute.  If one end does not hear the UDLD hello packet, then it means there could be a potential link issue and the switch will disable the port in question.

Cisco Employee

Re: UDLD

Hi Leo,

I've never heard of "normal mode" in UDLD. 

Check these links:

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a008009477b.shtml#udldmodes

http://tools.ietf.org/html/rfc5171#section-5.4

It has to be said, though, that the normal mode was always somewhat of a mystery to me. There are conflicting descriptions being circulated about it. The first document I've linked states:

In normal mode, if the link state of the       port was determined to be bi-directional and the UDLD information times out, no       action is taken by UDLD. The port state for UDLD is marked as undetermined. The port behaves according to its       STP state.

However, the RFC 5171 states:

   In other words, normal
   mode will shut down a port only if it can explicitly determine that
   the associated link is faulty for an extended period of time.

What is the "extended period of time" and how the "explicit determination" is accomplished is not described in any document I have been able to dig out so far.

Best regards,

Peter

Hall of Fame Super Gold

UDLD

It has to be said, though, that the normal mode was always somewhat of a mystery to me. There are conflicting descriptions being circulated about it. 

No wonder I've never heard of "normal mode".  LOL

Unfortunately for me, I've never had the chance to try out Fast UDLD.  But it's very beneficial when you're dealing with 10 Gbps and up. 

Re: UDLD

Hello Peter,

I had some problems too with those conflicting descriptions:

https://supportforums.cisco.com/message/3713963#3713963

Meanwhile I found some additional information in BRKRST-2333 - Network Failure Detection:

https://www.ciscolive365.com/connect/sessionDetail.ww?SESSION_ID=6015&tclass=popup

I have to say that's still not totally clear for me (especially the formulas for the detection time) but as far as I understand it, faulty link detection and time period refer to the UDLD Hellos.

If

1) Hellos are send on a local link and

2) Hellos are received on that link from a neighbor and

3) the own port-ID is not seen in the received Hello

the link is considered as faulty and will be error disabled.

Best regards

Rolf

New Member

UDLD

Here is another link, which said normal and aggressive mode can both lead to error disable mode.

http://www.cisco.com/en/US/docs/switches/datacenter/sw/design/vpc_design/vpc_best_practices_design_guide.pdf

and the recommand is normal mode.

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