I took two Cat3560G with IOS 12.2(25)SEE3 and did some tests.
Now I have several questions concerning UDLD, it would be fine if somebody could help me.
1. If I use the "normal" mode udld recognizes an unidirectional link but the switchport still changes from blocking to forwarding when it stops receiving bpdus. So there is still a stp loop created, is this correct? If yes, what is the gain of using the normal mode?
Only using agressive mode deactivated the switchport after 8s trying to reestablish the udld connection after the timeout.
2. When I used errordisable recovery in agressive mode, after 30s the port went up again. udld again tried 8s to reestablish the udld connection but timed out because the link was still unidirectional. After the timeout the port went up and in stp forwarding mode which formed a stp loop, is this behaviour correct?
3. I use RSTP on the two switches. If there is a blocking port which stops receiving bpdus because of a unidirectional link failure, it takes ca. 36s until this port changes to forwarding state (6s "maxage" + 15s "listening" + 15s learning). With default timers of udld (interval 15s, timeout 45s) it takes 53s (45s + 8s agressive mode) to deactivate the port. So for the duration of ca. 17s there would be a stp loop. Does anybody know if there is a recommendation which udld message interval to use with rstp?
4. How do udld packets be sent?
Does any switch send the packets independently or is every sent packet an answer to a received packet? Must the message intervals of the both switches be equal? Do both switches need to use the same mode (normal/aggressive)?
5. The output of the "show udld <interface>"-command looked like
Port enable administrative configuration setting: Enabled / in aggressive mode
Port enable operational state: Enabled / in aggressive mode
Current bidirectional state: Bidirectional
Current operational state: Advertisement - Single neighbor detected
Message interval: 7
Time out interval: 5
Expiration time: 25
Device ID: 1
Current neighbor state: Bidirectional
Device name: FOC1118Z2PX
Port ID: Gi0/1
Neighbor echo 1 device: FOC1118Z2N8
Neighbor echo 1 port: Gi0/1
Message interval: 10
Time out interval: 5
CDP Device name: switch-1
Does anybody know what the "Time out interval: 5" is standig for?
If you need further information, please give me a short feedback.
1.When UDLD was released initially,its main aim was to tell that the network is facing problem because of the unidirectional link.
But in normal mode since there is nothing done to stop this the Aggrisive mode came into picture,which shut down the port.
2.Here i am not sure but what i will say is UDLD was developed to detect the unidirectional link and with aggressive we set the time so that the port autorecover,there is always that risk and that is why you should take care when you set this feature.
3.Well i am not sure here because ideally the total time should be 20+15+15 (maxage+lis+learn)since in ideal situation the max age time should always be greter then the listening and learning.
I am not sure if i get the question right.
4.The packet are send on per port basis,which could be both the reply as well as checking the link.Well the message interval
as far as i think it will not make any difference,since the UDLD message are send independently of the other switch,the switch which send the UDLD echo
request wait for the echo reply for that time.Again mode will also not make any difference since it is also port based.
what you say is what I expected but my practical tests showed a different result:
I tested that on two 3560G with IOS Version 12.2(25)SEE3(c3560-ipservices-mz.122-25.SEE3.bin) which were connected to each other with two links:
- One normal fiber link
- One normal copper cable (capable to simulate a unidirectional link)
The copper port correctly went to stp blocking on one switch to prevent forming a loop.
After connecting the switches the copper link went up and udld packets were exchanged and an udld neighborship formed. Then I changed the link to unidirectional. Both switched detected that because of missing incoming udld packets and consequently errdisabled the port -> errdisabling = shutdown = no link.
After that I just reenabled the two ports via shut / no shut. The link went up, the udld status of both ports to "undetermined" and the stp status to forwarding -> which formed now a loop!
In some cisco documents concerning udld I read that udld only works after establishing a correct udld neighborship. In the above case after the ports were errdisabled and the link went down, from the switches point of view the link was considered disconnected. And after reenabling the ports they tried to establish a new udld neighborship which was not possible and so udld did not start working because there "never" hat been an established udld neighborship since the link went up.
I hope I described the problem in an understandable way.
Hi everyone, I would like to thank you in advance for any help you can provide a newcomer like myself!
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