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What's UDLD? Unidirectional Link Detection

Unidirectional?It means sending packects more than two ways!

Summary:

It is a layer two protocol (Data Link Layer)

Goals and Purposes

-Eliminate Loops

-Detects broken unidirectional link e.g. transmitted packets do not arrive at the receiver, or the fibers are connected to different ports

Functions

-Complements to STP

-Only exchange packets between neighboring device

-UDLD Timer

-When the port does not see the packet from neighboring device for specific duration of time, the link is considered unidirectional

Broken

-Once it's Unidirectional, the repective port is disabled, and Message will print on the consolde: "UDLD-3-DISABLE: unidirectional link detected on port 1/2. Port Disabled"

UDLD Packets

For each device and for each port, a UDLD packet is sent to the port it links to:

-Contains sender ID(Device and Port)

-Expected Receiver ID(Device and port)

-Each port then checks the UDLD packet it received for it's own device and port ID

Modes

Normal

Aggressive

-Adds two more functions detection:

1. Link is up on both side, but on one side of the port not sending and receiving packet

2. Link is up on one side but down on the other, Up on local, Down on Remote side.

Commands

These commands detail the UDLD configuration on Catalyst switches that run CatOS. UDLD needs to first be enabled globally (default is disabled) with this command:

Vega> (enable) set udld enable UDLD enabled globally

Issue this command: to verify whether the UDLD is enabled

Vega> (enable) show udld UDLD: enabled Message Interval: 15 seconds

UDLD also needs to be enabled on necessary ports with this command:

Vega> (enable) set udld enable 1/2 UDLD enabled on port 1/2

Issue the show udld port command to verify whether UDLD is enabled or disabled on the port and what the link state is:

Vega> (enable) show udld port UDLD : enabled Message Interval : 15 seconds Port Admin Status Aggressive Mode Link State -------- ------------ --------------- ---------------- 1/1 enabled disabled undetermined 1/2 enabled disabled bidirectional

Aggressive UDLD is enabled on a per-port basis with the set udld aggressive-mode enable <module/port> command:

Vega> (enable) set udld aggressive-mode enable 1/2 Aggressive UDLD enabled on port 1/2. Vega> (enable) show udld port 1/2 UDLD : enabled Message Interval : 15 seconds Port Admin Status Aggressive Mode Link State -------- ------------ --------------- ---------------- 1/2 enabled enabled undetermined

Issue this command to change the message interval:

Vega> (enable) set udld interval 10

#UDLD