speed auto-detect

Sometimes I see speed-mistach and retransmissions on routers.

If two devices connected to one another are set to auto-detect speed/duplex mode and you hard-code one device, should that make the connection better?

For example, let's say the router is connected to switch and they both auto-negotiate 100 MB Full Duplex. If you hard-code the router 100 MB Full Duplex, what will happen?

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Comments

dtlokee

The other sie wil auto-negotiate 10 half

This stems from the fact that if one side is hard coded, it won't respond to the side that is trying to autonegotiate.

binarysoul

dtlokee wrote:

The other sie wil auto-negotiate 10 half

This stems from the fact that if one side is hard coded, it won't respond to the side that is trying to autonegotiate.

I could have never thought of this! So what you're saying is that because the hard-coded device doesn't respond, the device on the other hand will simply assume its partner is 10 half.

Any reason why the hard-coded device won't respond?

dtlokee

There is a reason I'm sure, I am not sure what it is. Gigabit Ethernet works OK because it will respond to the negotiation, but FE won't.

Netstudent

binarysoul wrote:

dtlokee wrote:

The other sie wil auto-negotiate 10 half

This stems from the fact that if one side is hard coded, it won't respond to the side that is trying to autonegotiate.

I could have never thought of this! So what you're saying is that because the hard-coded device doesn't respond, the device on the other hand will simply assume its partner is 10 half.

Any reason why the hard-coded device won't respond?

Yes this is explained in detail in the first couple cahpters of the new BCMSN official study guide. It basically states that this occurs because the auto-negotiating side does not hear any auto-negotiating parameters, and it is fastethernet's behavior to default to half-duplex in the absense of auto-negotiation. Maybe it's backward compatibility issues with its predecessor, 10mb.

if you really want the nitty griddy, read IEEE 802.3u

binarysoul

Thanks folks. A follow up question.

If two devices are set to auto-negotiate, how often they check one another to verify their speed or duplex mode hasn't changed? Is it every few seconds/minutes?

And I found a good doc on auto negotiation

http://www.sun.com/blueprints/0704/817-7526.pdf

On page 6 it says: "The notion of “autonegotiation is unreliable” can
no longer be substantiated."

So, this document says Don't disable auto-negotiate. Now, network folks I work with, some with CCIEs sometimes hard-code and they're smart folks! I'm now confused whether disabling auto negotiation is good or bad.

Netstudent

Why would it change? and that is a really good document by the way.

dtlokee

If someone has been in networking for awhile you may see them jump at hard coding the speed/duplex of a link and this comes from the time when auto-negotiation was very unreliable in the way it worked. You would have problems like link failures, the link wouldn't negotiate the correct speed and so on. The solution to fix this was to hard code the speed. I typically wouldn’t use this on end systems anymore, but it is still common between routers and switches. You should never hard code the speed and duplex on connections to portable devices such as laptops. That causes a whole mess of problems when people take their laptop home and it doesn’t work on their home network, or at a different branch office.

As long as the physical layer stays up via the keepalives, auto-negotiation should not occur again unless one of the hosts asks for it.