UDLD

Unidirectional Link Detection Protocol (UDLD)

 

 

 

 

 

UDLD is a Layer 2 (L2) protocol that works with the Layer 1 (L1) mechanisms to determine the physical status of a link. At Layer 1, auto-negotiation takes care of physical signaling and fault detection. UDLD performs tasks that auto-negotiation cannot perform, such as detecting the identities of neighbors and shutting down misconnected ports.
 

 UDLD works by exchanging protocol packets between the neighboring devices. In order for UDLD to work, both devices on the link must support UDLD and have it enabled on respective ports

It could be cause by several factors.

It got several possible reasons why it happens: ..For example somebody was digging and hit the fiber optic cable.


Each switch port configured for UDLD sends UDLD protocol packets that contain the port's own device/port ID, and the neighbor's device/port IDs seen by UDLD on that port. Neighboring ports should see their own device/port ID (echo) in the packets received from the other side.

If the port does not see its own device/port ID in the incoming UDLD packets for a specific duration of time, the link is considered unidirectional.

 If the UDLD  message doesn't return, that's an indication that something is wrong and can put that port into error disable state

If that would be a single connection , it wont put the entire bundle into error disable state.It's going to put that one physical port into disable state

UDLD Modes of Operation

UDLD can operate in two modes: normal and aggressive.

 1st arrow   We moved to configure terminal (conf t)
2nd arrow  We see the different udld states
3rd arrow : How often in seconds we need to send UDLD messages

When we do aggressive  , they will only enable fiber ports. (first  arrow)


We can also  configure it on interface mode instead of global configuration, in this case Fa0/1
conf t
int fa0/1
udld port ?
udld port aggressive  
end

Now we just enable  interface fa0/1!
we will use the command #show run

We can see the udld on fa0/1

We can see that using the command #show udld   as well

If we think the damage has been resolved , than the fiber optics has been repair we can reset all the ports:
You can issue the command #udld reset


That's how we can reset ports that has been error disable  because of UDLD

Conclussion: 

Now we know :

• how to configure udld on  global mode, 
• on interface mode, 
• how we can verify UDLD   and
•  how we can reset UDLD

This exercise is done! if you understood and like it , please share it..