Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member

Unknown Protocol Drops

I have an 1841 connected up to a 2960 switch and then into a cable connection that we're pushing IPSec over. I am noticing that when I do a show int on the port that is connecting into the switch I am getting several "unknown protocol drops" on it. I was wondering if someone might know a debug command on the router to see what this protocol is???

Hall of Fame Super Silver

Re: Unknown Protocol Drops

Hello Dennis,

in your case switch STP BPDUs and other L2 protocol messages like DTP can be seen as unknown protocols on the router port.

Another example we have seen in our network is that some switches count as unknown protocol drops IS-IS hello packets sent by distribution nodes.

You should be fine.

To check if they are related to STP clear the interface counters and then see the rate of increment if it is one every 2 seconds or N every 2 seconds (if the port is a trunk carrying N vlans) you should have a confirmation they are just STP BPDUs

Hope to help


New Member

Re: Unknown Protocol Drops

Thanks Giuseppe. Yeah it's once or twice every few seconds.