2600 --> 3550 "duplex mismatch"

[chipk]

Ok, so I have a remote office that was previously hooked up like so:

2600 --> 2916 --> 3550-24

The interfaces between the 2600 and 2916 were set to 10/Full, because the 2600 only has a "slow" ethernet port. This setup was having some problems because all traffic is going through this 2916. So, I swapped out the cables, so now it looks like this:

2600 --> 3550-24 --> 2916

Everything is up, and if I look at the interfaces, I see that the link between the 3550 and 2600 negotiating to 10/Full as it's supposed to. The interfaces are also all clear of errors.

However, I keep getting duplex mismatches on both the 3550 and the 2600 telling me that the 2600 is not set to full duplex (it IS in the config), while the 3550 is set to full duplex (which it indeed is).

Weird. Anybody seen anything like this before? Is it just because the hardware is so old in the 2600 that they're not reading each other correctly?

Chip

 

[burtsbees]

I've always heard to manually set the duplex to match on devices if this error occurs. I have not ever heard of any rhyme or reason as to why the devices may behave like this, and I have heard some do behave like this. I suspect unresolved/unpatched bugs in the IOS.

Burt

 

[burtsbees]

Correction...not necesarily errors, but it pops up with that message. I always hoped that one could do this...
router(config)#shut up
lol

Burt

 

[vipergg]

We have seen on some old 2600s where even though the 10/full duplex command was there it does not work and is actually a 10/half duplex interface . Make sure both sides are hardcoded on both sides to 10/full . A 3550 interface will be auto by default so if the 2600 is a 10/full you then have a duplex mismatch because the 3550 will default to 10/half because the other end is not also set to auto , hardcode the 3550 to 10/full and see what happens.

 

[chipk]

Yup, I tried every possible combination of auto and hardcode that I could. I'm thinking it's what Burt first said, just a weird bug that was never fixed. Like I said, no errors, and the 3550 side "show interface" is showing that it's negotiating to 10/Full.

 

[plshlpme]

in general i think auto/auto is the enemy with cisco.
it may not be the case these days but i still try hard to never leave any of them to auto.

 

[jneiberger]

I recommend the exact opposite: *ALWAYS* use auto on both sides unless you have a very good reason for not doing so. For example, some Sun hardware from a few years ago simply would not autonegotiate properly. Other than that, auto is your friend.

Are you actually seeing late collisions on one side of this link?

Is the error message a CDP error? If so, ignore it.

 

[chipk]

It is a CDP error, which is why I'm not really worried about it. There are no Interface errors on either side. I'm with you on the Auto/Auto thing. I use it whenever possible and only hardcode if there is a problem.

 

[jneiberger]

Cool. If you don't have any actual interface errors then you don't have a duplex mismatch. It's just being misreported by CDP. Happens all the time.

 

[CiscoRC]

Yeah, when I test in a network environment, it does that to me too, but not all the time. I test many 2600 family Routers that generates a Duplex mismatch, there's no reason to be alarmed about it. You could always do the Auto Negotiate features...but not necessary. It's all CDP generate discovery in your network...

***************
RCorrigan
Network+, (working on CCNA)