Mismatched Encapsulations

If you enter a "sho cdp nei" command and you notice that one of the directly connected serial links is not showing up in the output. Being that CDP operates at layer 2, could a mismatched encapsulation cause this as well?

Example: 1 end is PPP and the other is HDLC.

Find more posts tagged with

SAVE $250 on Your CISCO Boot Camp — Use Code "EXAM26"

Exclusively for TechExams members for Infosec Boot Camps starting before April 30, 2026

View CISCO Boot Camps

Comments

Forsaken_GA

are you sure cdp is actually running on the remote end?

DerekAustin26

just pretend it is... I'm only asking if an encapsulation mismatch could cause this problem? Could it?

mikej412

DerekAustin26 wrote: »

I'm only asking if an encapsulation mismatch could cause this problem?

Yes it's possible. But things like this make more sense if you actually lab them up and figure them out for yourself.

tha_dub

Well I'll throw a guess at this.

I'm thinking it would leave the interface in an up down state which means no traffic and no cdp info.

Forsaken_GA

DerekAustin26 wrote: »

just pretend it is... I'm only asking if an encapsulation mismatch could cause this problem? Could it?

Well if this is theoretical, I'd say sure, it could. I've never actually done it, so I don't know for sure. I'd imagine that the line protocol wouldn't come up, which means you're not exchanging data.

And let me tell you right now, I never take anything on faith. I've run into far too many situations where when presented with a problem, I automatically start thinking along more complicated lines when the solution is more like 'plug it in' or 'push the button'.

DerekAustin26

Okay, after setting up a lab. If the encapsulation is the same.. Your int statis is up/up and cdp nei will work fine. everything will be perfect.

Now if the encapsulation is a mismatch. Your int will be up/down - Indicating a layer 2 protocol problem due to the encapsulation mismatch, hence the sho cdp nei command will not display the remote interface.

Selfmade

if you are using PPP and you are using HDLC, there is no authentication required, just that HDLC needs to be enabled at both ends, and it's the default PPP encapsulation, you just have to make sure it's set like that if you're troubleshooting.

If you use CHAP to authenticate, then you have to ensure that your Hostnames, usernames, and passwords are configured correctly and that CHAP is enabled at both ends of the link for you to get an up/up when you're looking at the show ip int brief. If you configure it at one end, it will say that it is down, but once you configure the other end correctly, it will go up.

It's not really hard to understand at all once you understand the basic concepts of PPP and the encapsulation and authenticaton mechanisms. The rest is just labbing it and getting used to implementing it. Show CDP neighbors works on any Cisco device that the device you are on is DIRECTLY connected to.

That's all that CDP does, working at layer 2. It gives you the IP address and some other information about the neighboring devices and that's handy if you're in the dark on what the ip address at the other end is and what features it has.

There are some Packet Tracers in chapter 8 of CCNA 4 that place you in just that situation, you're in the dark and you're forced to use CDP to discover neighboring devices and provide a diagram of the network and document it.

That's one of the best possible exercises out there.

You know NOTHING about the topology before you do it, you just have to get in and start discovering devices and then you troubleshoot problems untill you have discovered the whole network and have every problem resolved.

anyway, sorry for getting off track, but I thought that might provide a good reference to how useful CDP is if you use it right.