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New Member

Spanning tree and udld

Hi All,

We are going to setup a new core network. Is it necessary to enable udld on the gigabit ports? Isn't spanning tree taking over when the gig port is going down.

We tried to simulate this, we enabled on 3 routers spanntree and connected each of them via utp. When i cutted the receiving or sending pair of the utp cable the port detected is as line down, protocol down and spanning tree took over and shutted down the port.

Did we missed something during testing?

Best regards


Cisco Employee

Re: Spanning tree and udld

What was the UDLD mode configured on the switches. Was it enabled on both the switches.

Please make sure that you configure the UDLD mode as Agressive mode. In aggressive mode, UDLD can also detect unidirectional links due to one-way traffic on fiber-optic and twisted-pair links and to misconnected ports on fiber-optic links.

HTH,Please rate if it does.

-amit singh

New Member

Re: Spanning tree and udld


I thought it was agressive mode, but now i'm doubting if we set it up on both sides.

What I noticed was that when we cutted the sending or receiving pair of the upt cable, the port was going line down protocol down. and i'm not sure if we tested it right, otherwise i don't see why we implement it.

It sounds nice in theory, but i can't replicate it in or testlab.

Cisco Employee

Spanning tree and udld

UDLD is not required in UTP Cable due to Fast Link Pulse (FLP)  signals that already track the interface status on wired interface. instead UDLD is more commonly run in Fiber Optice interfaces.