to prevent stp loops in our cisco network I am evaluating udld.
I already read the two documents "Understanding and Configuring UDLD" (http://cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a008009477b.shtml) and "Configuring UDLD" (http://cisco.com/en/US/customer/products/hw/switches/ps708/products_configuration_guide_chapter09186a0080160a5d.html).
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.
Many thanks in advance,