ethernet oddity

[Sandy Harris]

I have two machines, recently installed Xubuntu on both.
One Ethernet works and the other does not.

 Both use static IPs, 172.31.255.[12], they talk to the
 same router, and switching cables or router ports does
 not help. Xubuntu on both is 7.10 Gutsy.

 I used 1386 on both even though one is AMD, because
 one will be an xterm to the other and it seemed easier
 to just have one architecture. The AMD is working, and
 the Intel-based machine not, though, so architecture is
 not the problem.

Working machine has onboard 10/100 network that
uses Sundance driver.

One with problems is Core 2 duo on a Gigabyte board,
model #GA-G31M-S2L. Gigabyte docs say the onboard
gigabit ethernet uses Realtek R8111B chip, and lspci
says 8111/8168B, but the driver that is loading is R8169.
Could that be the problem?

 pinging its own address, 172...2 works, but not the
 router at .254 or other machine at .1. Router does
 respond to pings from the other machine.

 route -n gives results that look correct:
 Destination      Gateway              Mask                 Flags       Iface
 172.31.255.0   0.0.0.0                 255.255.255.0   U            eth0
 0.0.0.0             172.31.255.254    0.0.0.0              UG          eth0

 (except there's another line for 169.254.0.0; how do I get rid of it?)

 ifconfig eth0 looks OK too
 inet addr: 172.31.255.2   Bcast: 172.31.255.255  Mask: 255.255.255.0

 ethtool gives some results that look odd:
    Supported ports: [FIBRE]
    Supported link modes: 1000baseT/FULL
    Supports auto-negotiation: yes
    Speed: 1000 Mbps
    Auto-negotiation: on
    Link detected: yes

 Since the router is 10/100, this appears to be the wrong speed.
 "ethtool -s eth0 speed 100" gives no error messages, but does
 not solve the problem.

 I tried "modprobe -r r8169" and then "modprobe r8169" to
 re-insert it. After that, lsmod says it is there, but both ethtool
 and ifconfig give "no such device"messages for eth0.

 Not sure what to try next or how to diagnose the problem.

 --
 Sandy Harris,
 Nanjing, China