Hi Bill,
The UDLD is sadly underdocumented in several sources, and even the informational RFC does not contain all the necessary information to properly understand UDLD modes.
The truth is, according to U.S. Patent 6765877, that there are two ways in UDLD of detecting an unidirectional link: an explicit and an implicit method. To understand the explicit method, it is important to keep in mind that each switch indicates its own [Device ID, Port ID] tuple as the sender information in an UDLD message, and echoes back the [Device ID, Port ID] tuples received from its neighbors in an echo portion of the message. In other words, each UDLD message carries a device and port identity of its sender, and contains an echo list of all neighbors and their ports whose UDLD messages have been received on the port that sends out this UDLD message itself.
An unidirectional link is detected explicitly if one or more of the following events occur during the neighbor detection and linkup phase:
- UDLD messages received from a neighbor do not contain the tuple that matches the receiving switch and port in their echo portion. This would indicate a split fiber or a truly unidirectional link.
- UDLD messages received from a neighbor contain the same sender as the receiving switch and its port. This would indicate a self-looped port.
- UDLD message received from a neighbor contain a different list of entries in the echo portion (i.e. a different set of neighbors) than the set of neighbors heard by this switch on this port. This would indicate a shared segment with a partial impaired connectivity between connected switches.
Whenever any of these three events is observed during an UDLD neighborship buildup with a neighboring switch, the link is declared unidirectional and subsequently disabled. Once again, this is called explicit unidirectional link detection. The setting of aggressive or normal mode has no effect on explicit detection.
An implicit unidirectional link detection refers to the event when UDLD messages arriving from the neighbor device suddenly cease to be received, without the port actually going down. This is where the aggressive or normal UDLD modes make a difference. In the normal mode, you simply age out the neighbor and do nothing, hoping that if there was a physical unidirectional fault on the link, the physical and link-layer mechanism would have brought the link down anyway. In aggressive mode, you assume that the physical layer has no capability of detecting and dealing with the unidirectional link condition itself, and when UDLD messages stop arriving, you err-disable the port just to be safe.
So the difference between the normal and aggressive modes relates to the difference in handling an implicit uni-directional link event. If an uni-directional link is detected explicitly, the port will always be err-disabled, regardless of the normal/aggressive mode.
The related U.S. Patent is a good, albeit tiresome, reading on this, especially the Figure 7 with the decision diagram; check out this thread:
https://supportforums.cisco.com/thread/2179927
Best regards,
Peter
