Cisco Systems 3750E manual Configuring Udld, 29-3

Chapter 29 Configuring UDLD

Configuring UDLD

•Event-driven detection and echoing

UDLD relies on echoing as its detection mechanism. Whenever a UDLD device learns about a new neighbor or receives a resynchronization request from an out-of-sync neighbor, it restarts the detection window on its side of the connection and sends echo messages in reply. Because this behavior is the same on all UDLD neighbors, the sender of the echoes expects to receive an echo in reply.

If the detection window ends and no valid reply message is received, the link might shut down, depending on the UDLD mode. When UDLD is in normal mode, the link might be considered undetermined and might not be shut down. When UDLD is in aggressive mode, the link is considered unidirectional, and the port is disabled.

If UDLD in normal mode is in the advertisement or in the detection phase and all the neighbor cache entries are aged out, UDLD restarts the link-up sequence to resynchronize with any potentially out-of-sync neighbors.

If you enable aggressive mode when all the neighbors of a port have aged out either in the advertisement or in the detection phase, UDLD restarts the link-up sequence to resynchronize with any potentially out-of-sync neighbor. UDLD shuts down the port if, after the fast train of messages, the link state is still undetermined.

Figure 29-1shows an example of a unidirectional link condition.

Figure 29-1 UDLD Detection of a Unidirectional Link

Switch A

TX

RX

Switch B successfully receives traffic from Switch A on this port.

TX

RX

Switch B

However, Switch A does not receive traffic from Switch B on the same port. If UDLD is in aggressive mode, it detects the problem and disables the port. If UDLD is in normal mode, the logical link is considered undetermined, and UDLD does not disable the interface.

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Configuring UDLD

These sections contain this configuration information:

•Default UDLD Configuration, page 29-4

•Configuration Guidelines, page 29-4

•Enabling UDLD Globally, page 29-5

Catalyst 3750-E and 3560-E Switch Software Configuration Guide

OL-9775-02 29-3