Unidirectional: - refers to data either sent or received in one direction.
Consider the couple of cisco catalyst switches interconnected with fiber like we see here
The interesting things about fiber optic cable is we have typically 2 straight interconnected our devices because one device is going to transmit on one straight and it will receive on the other
For Example in switch Sw1. We have got these 2 orange lines representing the two fiber optics straight. They both plugging in same switch port. But there are straight of fiber in that connecter and, the top straight on SW1 is, being used for transmission, and that straight is being used for receiving on SW2 and vice versa. The transmission or the “TX straight” on Sw2 is, the receiver the “RX straight” on SW1. SW1 will send traffic over SW2 using its transmission straight, and the same thing in SW2 will used its transmission straight to send traffic over SW1.
However let’s imagine that one of the straight get damaged. There is phenomena in the industry known as “backhoe fade”
that’s where backhoe might come and damage fiber optic cable as doing some digging, or other things cause it to happen But the term often “backhoe fade” where fiber optic straight has been damaged by somebody digging, and they hit the cable, and this situation let’s imagine that “backhoe” is damaged the top straight, but not the bottom straight
With these switches have been anyway knowing that there interconnection was damaged. SW1 it can still transmit traffic, and it would not know if that traffic got dropped, and SW2 would not know that traffic got dropped. Because it never knew that it getting the traffic. SW2 it could send traffic, it being received by SW1.How these switches know something wrong
That’s why cisco proprietary feature called UDLD comes in. UDLD is layer 2 technology and its going to allow cisco switches enable for UDLD
To communicate one another at layer2. It gonna use well known mac address 01:00:0C:CC:CC:CC , different model of cisco devices use different interval for sending out UDLD messages But usually on order of about 15 seconds
When switch sends the UDLD message to far end device. It expect that UDLD message to be returned, if it not returned that can be indication to the switch “something went wrong”. We might have unidirectional link if we do have unidirectional link what the switch can do, it can put that port into “err-disable” state. Where the switch not gonna consider that port for forwarding traffic
In CCNA we could logically bundled together multiple physical link into “etherchannel”
Using UDLD with Etherchannel is that if we detect we have unidirectional link on one of those physical connection making up etherchannel bundle. The switch not going to put entire bundle into “err-disable” state. It only gonna put one physical port into “err-disable” state
Different mode of UDLD
Normal Mode: - if a switch detect but it believes to be unidirectional link one of its ports. It just going to mark that port has having an “undetermined” state and also generate syslog message.
Aggressive mode: - Recommended mode use for most of the time. If switch believes it has a unidirectional link on of its ports. It’s going to send UDLD message, every seconds for 8 Seconds, if it never sees those UDLD message returned to itself. It’s going to place that port into “err-disable”
Configuration and Verification
I have 3560 Series switch. It have mainly copper port and couple of gigabit fiber ports. In UDLD our focus on fiber, normally UDLD used with Fiber ports. It can be used with copper ports
Message Time:-How often in seconds we gonna sending out UDLD message
Ø Enable Mode: - Normal mode
Its only enable UDLD on fiber ports. The recommendation from Cisco is that we do turned on UDLD globally as supposed to dong an interface by interface bases
To Turn UDLD to copper Port
we think we resolved the issue. Fiber issue has been repaired. If we have to reset all over the port set have been put into ‘err-disabled”, state because of UDLD
This command same effect as we going into interface and doing “shutdown” followed by “no shutdown”