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Questions about udld.


Hi there.

Now I've got some strange situation with udld.

there are  two cisco switches which is ws6500 and one samsung L2 switch.

The topology is triangle.


     \                      / (uplink : trunk mode, allowed vlan 80)

       \                   / (uplink : trunk mode, allowed vlan 80)

        samsung L2


just enable udld normal mode on two 6500 swithces.

and the link between samsung and cisco are in trunk mode. and only allowed vlan 80

When I change vlan 80 to 81 on samsung L2 switch, After that,

the link which is conneced to samsung switch on c6500 enters into errdisable mode.

Is it common issue when changing vlan? why the interface enters into err?

help me out !

3 Replies 3

Reza Sharifi
Hall of Fame
Hall of Fame


Do you have the same issue without UDLD configured?

UDLD works only between Cisco devices sine it is Cisco proprietary protocol.

more info:


Yes I know it.

I can see udld table between c6500s across samsumg L2 switche. like CDP.

I can see udld table between c6500s across samsumg L2 switche. like CDP.

The Cisco layer-2 control protocols VTP, DTP, CDP, PAgP and UDLD use all the same destination multicast MAC-address: 01-00-0C-CC-CC-CC. The frames are send untagged on trunks, non-Cisco switches usually treat them as "normal" layer-2 multicast.

So the question is, why did the Samsung switch stop to forward those frames in its native VLAN after you changed the VLAN.

Is the Samsung switch running a rapid spanning-tree mode? If it's non-rapid, maybe the configurattion change made the port to run through Learning and Listening states for 30 seconds (I assume those STP states are port-based, not per-VLAN on Samsung switches)?

The UDLD timers on the 6500 are default (15s)?



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