Just a little tip from something that I was working on earlier (DTP)...

pamccabe

Working on my home network earlier and ran across some unknown protocol drops on one of my internal interfaces. This interface is connected to a switch. Did the usual google searches and found that my switch was running CDP while it was disabled on my router. I disabled that, reset the counters, and still found random protocol drops. This had me stumped. Someone randomly asked if it was due to DTP. At first, I didn't think so because I set the switch interface to 'switchport mode trunk'. However, after digging around, I found this...



Configuration Command on One Side
Short Name

Meaning
To Trunk, Other Side Must Be




switchport mode trunk ;
switchport nonegotiate

Nonegotiate
Always trunks on this end; does not send DTP messages (good when other switch is a non-Cisco switch)
On



switchport mode access;
switchport nonegotiate

Access (with nonegotiate)
Never trunks; does not send DTP messages
(Never trunks)



switchport mode trunk

Trunk
Always trunks on this end; sends DTP to help other side choose to trunk
On, desirable, auto



switchport mode dynamic desirable

Desirable
Sends DTP messages, and trunks if negotiation succeeds
On, desirable, auto



switchport mode dynamic auto

Auto
Replies to DTP messages, and trunks if negotiation succeeds
On, desirable



switchport mode access

Access
Never trunks; sends DTP to help other side reach same conclusion
(Never trunks)




Turns out, even with the trunk command, the interface will still send DTP messages. I issued the nonegotiate command and now my counters are clear.

I just wanted to post this because it was a learning experience and some of you might get something from it. Not only that, I love learning new things. Even as a CCNA, I am constantly humbled by the things I don't know, or maybe have forgotten. I know this is a topic among the CCNA exams so thought I'd throw it out there.

DCD

You will also find that information in the CCNA Security and CCNP course.

WilyOne

pam: thanks for the insightful post, but one question: What "unknown protocol drops" and how did you see them? Are you saying DTP prevented other protocols from working?

TheNewITGuy

The interface stats will show 'unknown protocol drops' so basically the switch was sending DTP messages to the router port and since it has no idea what to do with them (unknown protocol) it dropped it.

WilyOne

Ah OK, so it was the router calling DTP frames unknown protocol. Thanks.

But this begs another question: if the trunk is established why doesn't the router know what these frames are?

networker050184

Routers do not use DTP on routed ports.

WilyOne

Yep, definitely learned something in this thread. Thanks all!

So the best practice for trunks between switches and routers is to disable DTP negotiation:
System Requirements to Implement Trunking - Cisco Systems