changing ethernet speed to slower

I'm trying to change the ethernet speed with the following command:
ethtool -s eth0 speed 100
When I do that command, the link goes down for 3 seconds and when it comes
back up the speed is still 1000. Either the command is not really setting
it right, or the negotiation overruled it somehow. Is there a way to force
it to run at 100 no matter what, even if renegotiated later, without having
to add a 100-only hub in the line?

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phil-news-(E-Mail Removed) wrote:
> I'm trying to change the ethernet speed with the following command:
> ethtool -s eth0 speed 100
> When I do that command, the link goes down for 3 seconds and when it comes
> back up the speed is still 1000. Either the command is not really setting
> it right, or the negotiation overruled it somehow. Is there a way to force
> it to run at 100 no matter what, even if renegotiated later, without having
> to add a 100-only hub in the line?

man ethtool
look for autoneg off

You'll probably need to do the same at the other end of the wire, too.

On Wed, 31 Dec 2008 10:45:59 -0600 Allen Kistler <(E-Mail Removed)> wrote:
| phil-news-(E-Mail Removed) wrote:
|> I'm trying to change the ethernet speed with the following command:
|> ethtool -s eth0 speed 100
|> When I do that command, the link goes down for 3 seconds and when it comes
|> back up the speed is still 1000. Either the command is not really setting
|> it right, or the negotiation overruled it somehow. Is there a way to force
|> it to run at 100 no matter what, even if renegotiated later, without having
|> to add a 100-only hub in the line?
|
| man ethtool
| look for autoneg off
|
| You'll probably need to do the same at the other end of the wire, too.

The "other end of the wire" is a switch.

When that ethernet is connected to a gigabit switch (Linksys SD2008) I get
the following error messages:

tesla/root/c0 /root 1# ethtool -s eth0 autoneg off
Cannot set new settings: Invalid argument
not setting autoneg
tesla/root/c0 /root 2#

However when I connect to a fast ethernet switch (Netgear FS108) it works:

tesla/root/c1 /root 1# ethtool -s eth0 autoneg off
tesla/root/c1 /root 2# ethtool -s eth0 autoneg on
tesla/root/c1 /root 3#

But, of course, when connected to the fast ethernet switch I don't have any
issues, since it can't negotiate faster than 100mbps.

The Linksys SD2008 does work at 100 since a couple other computers that only
have 100 are connected to it and work OK. It also works fine at 1000 for the
other two computers that have 1000 (AND are using Cat5 to do it).

So for now I've got the extra switch in place to force this computer to use
100. Its ethernet RX is getting lots of errors at 1000, but works perfectly
at 100. Ironically it works better (55% packet loss) with a Cat5 cable than
with a Cat6 cable (90% packet loss). Now at 100mbps packet loss is zero.
Since Cat6 didn't fix things, I'm guessing this is not a crosstalk issue.

Ideally, there should be a way to restrict the autonegotiation modes, instead
of requiring it be turned off. Each mode should have an index code, and for
each a flag indicating if that mode is administratively disabled.

--
|WARNING: Due to extreme spam, googlegroups.com is blocked. Due to ignorance |
| by the abuse department, bellsouth.net is blocked. If you post to |
| Usenet from these places, find another Usenet provider ASAP. |
| Phil Howard KA9WGN (email for humans: first name in lower case at ipal.net) |

phil-news-(E-Mail Removed) wrote:
> On Wed, 31 Dec 2008 10:45:59 -0600 Allen Kistler <(E-Mail Removed)> wrote:
> | phil-news-(E-Mail Removed) wrote:
> |> I'm trying to change the ethernet speed with the following command:
> |> ethtool -s eth0 speed 100
> |> When I do that command, the link goes down for 3 seconds and when it comes
> |> back up the speed is still 1000. Either the command is not really setting
> |> it right, or the negotiation overruled it somehow. Is there a way to force
> |> it to run at 100 no matter what, even if renegotiated later, without having
> |> to add a 100-only hub in the line?
> |
> | man ethtool
> | look for autoneg off
> |
> | You'll probably need to do the same at the other end of the wire, too.
>
> The "other end of the wire" is a switch.
>
> [snip]

It doesn't matter. You should be able to configure autonegotiation on a
switch, too. Make sure you nail full/half duplex down, too.

In article <(E-Mail Removed)>, <phil-news-(E-Mail Removed)> wrote:
> On Wed, 31 Dec 2008 10:45:59 -0600 Allen Kistler <(E-Mail Removed)> wrote:
> | phil-news-(E-Mail Removed) wrote:
> |> I'm trying to change the ethernet speed with the following command:
> |> ethtool -s eth0 speed 100
> |> When I do that command, the link goes down for 3 seconds and when it comes
> |> back up the speed is still 1000. Either the command is not really setting
> |> it right, or the negotiation overruled it somehow. Is there a way to force
> |> it to run at 100 no matter what, even if renegotiated later, without having
> |> to add a 100-only hub in the line?
> |
> | man ethtool
> | look for autoneg off
> |
> | You'll probably need to do the same at the other end of the wire, too.
>
> The "other end of the wire" is a switch.

IIRC GigE needs an 8-conductor cable, while 100BT only needs 4. So if
you swap cables, that should do it.

> The Linksys SD2008 does work at 100 since a couple other computers that only
> have 100 are connected to it and work OK. It also works fine at 1000 for the
> other two computers that have 1000 (AND are using Cat5 to do it).

Hm, maybe not. So when IS an 8-conductor cable required?

--
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Q: What kind of modem did Jimi Hendrix use?
A: A purple Hayes.