Mechanisms To Detect Unidirectional Links - Alcatel-Lucent OmniSwitch AOS Release 7 Manual

Configuring UDLD

Mechanisms to Detect Unidirectional Links

The UDLD protocol is implemented to correct certain assumptions made by other protocols, and to help
the Spanning Tree Protocol to function properly to avoid the creation of dangerous Layer 2 loops.
UDLD uses two basic mechanisms:
•
It advertises the identity of a port and learns about its neighbors. This information about the neighbors
is maintained in a cache table.
•
It sends continuous echo messages in certain circumstances that require fast notifications or fast
re-synchronization of the cached information.
Neighbor database maintenance
UDLD learns about other UDLD neighbors by periodically sending a Hello packet (also called an
advertisement or probe) on every active interface to inform each device about its neighbors.
When the switch receives a Hello message, the switch caches the information until the age time expires. If
the switch receives a new Hello message before the aging of an older cache entry, the switch replaces the
older entry with the new one.
Whenever an interface is disabled and UDLD is running, or UDLD is disabled on an interface, or the
switch is reset, UDLD clears all the existing cache entries for the interfaces that are affected by the
configuration change. UDLD sends a message to the neighbors to flush the part of their caches affected by
the status change. This UDLD message is intended to synchronize the caches.
Echo detection
UDLD depends on an echo-detection mechanism. UDLD restarts the detection window on its side of the
connection and sends echo messages in response to the request, whenever a UDLD device learns about a
new neighbor or receives a re-synchronization request from an out-of-sync neighbor. This behavior is the
same on all UDLD neighbors because the sender of the echoes expects to receive an echo as a response.
If the detection window ends and no valid response is received, the link is shut down, depending on the
UDLD mode. When UDLD is in normal mode, the link is considered to be undetermined and is not
shut down. When UDLD is in aggressive mode, the link is considered to be unidirectional, and the
interface is shut down.
In normal mode, if UDLD 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 re-synchronize with potentially out-of-sync
neighbors.
In aggressive mode, if UDLD is in the advertisement or in the detection phase and all the neighbors of a
port are aged out, UDLD restarts the link-up sequence to re-synchronize with potentially out-of-sync
neighbors. UDLD shuts down the port, after the continuous messages, if the link state is undetermined.
OmniSwitch AOS Release 7 Network Configuration Guide March 2011
UDLD Overview
page 2-5