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UDLD QUERY

Hi Ive setup a 3 switch Triange as follows:

Switch A - Vtp server root 4096
Port 1 connected to Switch B port 1
Port 12 connected to Switch C port 12
Port 11 connected to Switch C port 11

Switch B - Vtp server secondary root 8192
Port 1 connected to Switch A port 1
Port 10 connected to Switch C port 10

Switch C - Access switch 32768
Port 10 connected to Switch B port 10
Port 11 connected to Switch A port 11
Port 12 connected to Switch A port 12


Uplinkfast has been added on Switch C

Which ports do I enable 'udld', although I think they specifically go on: Switch A ports 1 & 11 & 12, Switch B port 1 & Switch C port 12 - ?
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Sr.Net.Eng
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Top Expert 2011
Commented:
UDLD should be enabled on all the ports connecting each switch to one another.

Author

Commented:
Thoughts so perfect... If UDLD was added correctly on some ports and maybe not on some others then Im assuming 'udld' would not function properly and the 'sh spanning-tree' would just show 'errordisable' state/inconsistency'  ?
SouljaSr.Net.Eng
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Top Expert 2011
Commented:
Yes it has to be enable and supported on both ends of the link. Spanning tree would either block the port first if in rapid, or udld will if in aggressive mode.

Author

Commented:
Ok... Ive selected 'Aggressive as it tries to re-establish upto 8 attempts, instead of Normal mode', although not sure in what scenario I should choose..?

Qns1.  Is there any form of software or something out their that can cause me to simulate this as in my mind if 1 cable is connecting 2 switches and one end has a faulty pin cable inside the rj45 connection then Im assuming 'udld' switches from blocking to forwarding - ?

Qns2. If it does switch to forwarding does the traffic follow a different direction or use another pin cable inside the fault rj45, as cant get my head around that as also thinking about full duplex.. ?

Once those are answered above I think that is it for 'UDLD'..:)
SouljaSr.Net.Eng
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Top Expert 2011
Commented:
Qns1.  Is there any form of software or something out their that can cause me to simulate this as in my mind if 1 cable is connecting 2 switches and one end has a faulty pin cable inside the rj45 connection then Im assuming 'udld' switches from blocking to forwarding - ?

I don't think GNS3 or packet tracer has UDLD. You will have to go with real switches.
If UDLD detects an issue and shuts the port down, and that is the only cable connecting the two switches then you will lose communication between them. Why would it switch to forwarding. It would go from forwarding to disabled. If you have a redundant cable plugged between them then the other will become active through spanning tree.

Author

Commented:
I have real switches..GNS3 - Oh ok did not know that..!

Qns1. Ok as per my main thread my 3 switch triangle with 2 VTP servers and 1 access switch.  Ive also added 'uplinkfast' on Switch C for port 11 so if the main traffic route is faulty then 'uplinkfast' will start, sending traffic in the 'opposite' direction..!?

Qns2. Are you saying that for eg between both Vtp Servers A & B, I could also add a secondary cable between them both so 'udld' would 'disable one cable and switch to the secondary cable keeping traffic flow in same direction - ?

Qns3. I suppose both 'Qns 1 & 2' can be used at same time or maybe some equipment may or may not have both features so they are just to different ways of keeping backup, but I would if 'udld' could work with 'HSRP' - ?

I think these last 3 questions above should do it...:)
SouljaSr.Net.Eng
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Top Expert 2011
Commented:
Qns1. Ok as per my main thread my 3 switch triangle with 2 VTP servers and 1 access switch.  Ive also added 'uplinkfast' on Switch C for port 11 so if the main traffic route is faulty then 'uplinkfast' will start, sending traffic in the 'opposite' direction..!?

Forget about uplinkfast and use rapid spanning tree. Uplink fast just improves covergence time for standard STP. Rapid it the mode you want to use. Yes, if one uplink fails, the other will become active.


Qns2. Are you saying that for eg between both Vtp Servers A & B, I could also add a secondary cable between them both so 'udld' would 'disable one cable and switch to the secondary cable keeping traffic flow in same direction - ?


Yes, if you have two cables spanning tree will have one blocked and to the other forwarding. If the forwarding connection fails the blocked connection becomes active, but in most real world situation, those two cables are placed into an etherchannel and spanning tree treats them as on connection, and even if one fails the connection stays up.

Qns3. I suppose both 'Qns 1 & 2' can be used at same time or maybe some equipment may or may not have both features so they are just to different ways of keeping backup, but I would if 'udld' could work with 'HSRP' - ?

Spanning tree is the mechanism that allows your switches to fail over to a redundant link, not UDLD. UDLD, just detects faulty cabling, thus shutting the port down. It's spanning tree that notices the port fail and then enables the redundant link. HSRP is for Layer 3 redundancy, not layer 2, but you can use both at the same time.

Author

Commented:
These STP modes are confusing.....!!!! Ive already set both 'udld & uplinkfast' on all my switches - 'spanning-tree mode pvst anyway..!!

Qns1. My switches by default has - 'spanning-tree mode pvst', which I think is the 'standard STP', so presumably I should change to 'spanning-tree mode rapid-pvst' - ?

Qns2. I understand that if I use 'UDLD' then I dont really need 'uplinkfast', as traffic can stay in same flow direction..Im assuming it is faster convergence than 'uplinkfast' - ?
SouljaSr.Net.Eng
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Top Expert 2011
Commented:
Qns1. My switches by default has - 'spanning-tree mode pvst', which I think is the 'standard STP', so presumably I should change to 'spanning-tree mode rapid-pvst' - ?

Yes, change to rapid.

Qns2. I understand that if I use 'UDLD' then I dont really need 'uplinkfast', as traffic can stay in same flow direction..Im assuming it is faster convergence than 'uplinkfast' - ?

If you use Rapid, you don't need uplink fast, backbonefast, etc. UDLD, has nothing to do with uplink fast or spanning tree. It works hand in hand with spanning tree, it's not a substitution.



On another note, start new questions instead of continuously stacking them in the same thread.

Author

Commented:
Apologies..!! I think thats it now for this.. appreciated.. Im gonna read some more on this now as I think I get it..!!thanks for being patient..:)
SouljaSr.Net.Eng
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Top Expert 2011

Commented:
No problem.

Author

Commented:
Hi There was one question I asked earlier but you never replied to that one at: 37064232, as below:

Qns1. Ok... Ive selected 'Aggressive as it tries to re-establish upto 8 attempts, instead of Normal mode', although not sure in what scenario I should choose..?
SouljaSr.Net.Eng
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Top Expert 2011
Commented:
Qns1. Ok... Ive selected 'Aggressive as it tries to re-establish upto 8 attempts, instead of Normal mode', although not sure in what scenario I should choose..?

The difference is if you want UDLD to take action on a detected defective cable (aggressive) or just notify you (normal).

Author

Commented:
Hi, is that it..okay!!:)
Don JohnstonInstructor
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Top Expert 2015
Commented:
Minor point: UDLD isn't really a spanning-tree feature. It's more of a mechanism designed to identify a unidirectional link.

Author

Commented:
Hi don, yes I can appreciate that.!!

Author

Commented:
Although the question was answered first time with Soulja, I feel the other responses to some of my extra questioning around STP was pertinent and gave more breadth to the answer that assisted in proper understanding..Expert Don also gave clarity with the response..All appreciated!!
SouljaSr.Net.Eng
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Top Expert 2011

Commented:
Could this be any more clear?

UDLD, has nothing to do with uplink fast or spanning tree. It works hand in hand with spanning tree, it's not a substitution.

Author

Commented:
Hi Soulja, apologies if Ive annoyed you some and yes I can see your comments at:37065230  as assumed that if another Expert comments on same thread that they would get some partial points...In this case how do I re-allocate who points to you?
SouljaSr.Net.Eng
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Top Expert 2011

Commented:
No Biggie, just wanted to point out that I had already stated that UDLD had nothing to do with spanning tree.

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