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Simulating UniDirectional Link Failure

d.hodgson
Level 1
Level 1

Hi folks,

I may be losing the plot but can I simulate unidrectional link failures using fibre to help my CCNP studies? I thought it would be a simple matter of pulling out just the rx on one of my switches but that brings the int down. Is this how it's supposed to work?

I want to do this to look further into udld.

thanks

Dave

5 Replies 5

Nagendra Kumar Nainar
Cisco Employee
Cisco Employee

You can use PACL to simulate unidirectional link failure. UDLD uses 0100.0ccc.cccc as destination MAC address.

So configure a PACL denying this mac address and apply the same to the interface on which you need to simulate the link failure.

HTH,

Nagendra

Thanks Nagendra,

I'll look up PACL.

Now what about the fibre failure I simulated, is expecting that to be a unidirectional link failure a wrong assumption?

ta

Dave

That is a brilliant idea, I wanted to reproduce a unidirectional failure of a fiber in a lab CML or GNS3.
Until I stumbled upon this page from a proficient Cisco employee :
https://cisco.goffinet.org/ccna/securite-lan/cisco-port-acl-pacls-vlan-acl-vacls/

I translate in English the main points :
- PACL only filter incoming traffic
- PACLs CANNOT filter L2 protocol frames like : VTP, CDP, DTP, PAgP, UDLD and STP
- PACLs are supported only on Hardware

With all that it doesn't seem possible to simiulate in a simulated Lab
if so it is simpler to build a HW network and then unplug 1 direction only fiber
in front of a RSTP blobked port by RSTP.


h.parsons
Level 3
Level 3

It is my understanding that your assumption is right. One thing to note is that both switches will have to establish a bidirectional state once before UDLD will shutdown an interface. Try connecting them and do a "show udld neighbors"

shaktik
Cisco Employee
Cisco Employee

I know its quite late for the answer, but just for other people visiting here.. I was just thinking that attaching the Rx to a higher voltage source should work out since it should complete the loop

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