Myth / network question

Dave Johnson dave-gnhlug-list at davej.org
Sun Oct 18 16:11:50 EDT 2009


bruce.labitt at autoliv.com writes:
> Trying to diagnose an ongoing myth / network issue at home.  Have two 
> HDHomeRun tuners connected to a 1 Gb switch and then to eth1.  After some 
> time, (weeks, I think) it looks like eth1 port turns off.  The link light 
> to eth1 on the switch is off.  Last time this happened, I physically 
> checked the back of the computer and saw the eth1 link light was off on 
> the eth1 jack. 
> 
> Which logs can I search to possibly find the problem.  At this point I 
> don't know if this is a hardward / software / OS problem.  I did replace 
> the Gbit ethernet card with one of the same type a while back.  I suppose 
> it is possible to be hardware.  Maybe the card(s) are junk.  What should I 
> look for?  Running ubuntu Jaunty.

Well first, I'll assume you have already checked that the cable is in
good shape (isn't crimped, broken, or otherwise bad) and that it's
CAT 5e (not CAT 5).


Check to see if the driver logged a link down in the kernel message
(dmesg or via syslog), also if it's reporting lots of spurious up and
downs it could indicate a bad connection:

[13727.735716] eth0: Link is Down

Since you didn't mention the brand/model of the NIC (or the switch),
I'll have to assume the driver supports ethtool ioctls (most modern
ones will).

Since the switch is showing link down then it's unlikely to be an OS
issue (such as a stuck netif queue, interrupt disable, or other driver
problem).

If you can catch the issue occuring, check the negotiated speed/duplex
and link detected items:

$ ethtool eth0
Settings for eth0:
        Supported ports: [ MII ]
        Supported link modes:   10baseT/Half 10baseT/Full 
                                100baseT/Half 100baseT/Full 
                                1000baseT/Full 
        Supports auto-negotiation: Yes
        Advertised link modes:  10baseT/Half 10baseT/Full 
                                100baseT/Half 100baseT/Full 
                                1000baseT/Full 
        Advertised auto-negotiation: Yes
        Speed: 1000Mb/s                       <<<<<<<<<<<
        Duplex: Full                          <<<<<<<<<<<
        Port: MII
        PHYAD: 1
        Transceiver: external
        Auto-negotiation: on
        Supports Wake-on: g
        Wake-on: d
        Link detected: yes                    <<<<<<<<<<<
$ 

Most drivers support restarting autonegotiation which may resume
operation if either side gets stuck for some reason.

$ ethtool --negotiate eth0

You can also get whatver counters the MAC supports (each MAC is different)

$ ethtool --statistics eth0

You can also ifdown/up the interface which will close/open the network
driver.  For most drivers, this will reinitilize the MAC/PHY on the NIC.

If the switch is managed, then any data from it could also prove useful.


-- 
Dave


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