Forcing Port Speed that is faster than host

[aarodav1]

I've been studying to take the CCNA by the end of the year, and I've come across a seemingly trivial situation that I can't find the answer to.


I understand auto negotiation and how it will pick the fastest speed that both devices support along with half/full duplex. I also understand that if auto negotiation is disabled on one device, it may be possible for the other device to sense the speed and the pick the default duplex setting based on the speed.


The case that I'm wondering about is if you set the speed to something like 1000mbps on the port and then plug in a computer that only has a 100mbps NIC. Will the link just not come up? From what I understand, hard coding in the speed means that is the only speed it will operate at and it won't negotiate anything lower. Also, if you hard code in the duplex along with the speed, auto negotiation is disabled completely. So even if the other host is able to sense the speed, it will still see 1000mbps, which is doesn't support.

My common sense is telling me the link just won't come up. Am I right?

 

[imagoon]

No it won't. The encoding and timing on 1000mbps vs 100mpbs wouldn't all communication at all if the ends are forced to a certain speed. Obviously 100mbps can't autonegotiate to 1000mbps anyway.

 

[Mushkins]

I've been studying to take the CCNA by the end of the year, and I've come across a seemingly trivial situation that I can't find the answer to.


I understand auto negotiation and how it will pick the fastest speed that both devices support along with half/full duplex. I also understand that if auto negotiation is disabled on one device, it may be possible for the other device to sense the speed and the pick the default duplex setting based on the speed.


The case that I'm wondering about is if you set the speed to something like 1000mbps on the port and then plug in a computer that only has a 100mbps NIC. Will the link just not come up? From what I understand, hard coding in the speed means that is the only speed it will operate at and it won't negotiate anything lower. Also, if you hard code in the duplex along with the speed, auto negotiation is disabled completely. So even if the other host is able to sense the speed, it will still see 1000mbps, which is doesn't support.

My common sense is telling me the link just won't come up. Am I right?

Correct, or it may say it's up from one end when really its not. It assumes the person configuring it knows what they're doing If one his hardcoded to a supported speed and the other is set to autonegotiate, it "usually" works right and autonegotiates to the hardcoded speed, but i've run into certain things that still dont like to work right with the mismatch even if the connection is perfectly fine unless you set them both to auto/auto or explicitly assign both. Welcome to the world of Cisco!