Section 2.1.b (i) UDLD
Unidirectional Link Detection is a Cisco Proprietary protocol used to detect issues with links that utilize separate cables (fiber).
UDLD can be enabled and becomes active when it detects a neighbor from the other side (both sides must be configured)
Thus UDLD can only successfully work when configured on both ends of a link.
UDLD protects us from:
1. Connecting cables to ourselves by accident
2. A link connected to a HUB or dumb switch (multiple UDLD peers on one link is not good)
3. Cut transmit or receive (in aggressive mode)
4. Misconnected fiber strands (connected to proper device but wrong ports)
If UDLD hits the hold time for a neighbor, before declaring the number dead the UDLD enabled device will shoot 8 UDLD messages in a row to the neighbor.
UDLD hellos are sent by default every 15 seconds.
UDLD is disabled by default.
There are 2 ways to enable UDLD, globally or per interface.
If we enable UDLD globally, it ONLY is enabled on fiber interfaces.
conf t
udld <enable | aggressive>
If we enable UDLD per interface, it can be also enabled on copper links (not much to gain though unless we are connected through dumb switches or media converters).
int g0/1
udld port <aggressive>
Note: Global enabling is NOT required for port enabling
We have two ways to reset an errdisable udld link:
udld reset
shut and no shut the port
Verify commands:
show udld neighbors
Show udld
Sources:
https://learningnetwork.cisco.com/docs/DOC-26839
Please note that although the below resource is good, he is not correct about aggressive mode. Both normal and aggressive mode protect against unidirectional links, the only difference with aggressive mode is that it also protects you from when you stop to receive UDLD messages. UDLD normal mode will still trigger if it detects misconnected ports, a hub (with multiple UDLD neighbors, plugging into yourself.
http://packetlife.net/blog/2011/mar/7/udld/
https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3560/software/release/12-1_19_ea1/configuration/guide/3560scg/swudld.html
2.1.b (i) UDLD