Lost connectivity - Aix

This is a discussion on Lost connectivity - Aix ; I have an AIX box (5300-04-CSP) that, a coupe of days ago, without any warnings lost connectivity to the outside world. Everything was ok within its subnet; all tcp/ip services were just fine. It has one NIC, "lsattr -El inet0" ...

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Thread: Lost connectivity

  1. Lost connectivity

    I have an AIX box (5300-04-CSP) that, a coupe of days ago, without any
    warnings lost connectivity to the outside world. Everything was ok
    within its subnet; all tcp/ip services were just fine. It has one NIC,
    "lsattr -El inet0" and "netstat -rn" looked ok (as compared to other
    units' on the same subnet). The Default Gateway _pinged ok_, except that
    nothing went thru it, either ways.

    I was at a remote location (connecting thru another box on the subnet)
    so I did not want to diddle with tcp/ip settings, instead I rebooted
    the machine. That helped! The reboot must have cleaned out or flushed
    something that was blocking the default route... (and no, it's not
    a Windows box! ;-)

    Does anyone have any idea what has gone wrong, what the reboot might
    have fixed, and what to watch for while the network is ok or when this
    happens again?

    Thanks a lot!

  2. Re: Lost connectivity

    I've seen something like this a few times, mostly on servers with
    HACMP (redundant NICs in failover config).
    For reason I can only guess, AIX routing table redirected outbound
    traffic to 127.0.0.1. Only connection within the same subnet worked
    OK.

    Once I disabled PMTU discovery and manually corrected routing table,
    outbound connections started working fine.
    Apparently, PMTU discovery dynamically updates AIX routing table with
    optimized paths to specific hosts - and sometimes makes a mistake.

    How to check and disable PMTU discovery:
    no -a | grep pmtu
    grep pmtu /etc/tunables/lastboot

    no -p -o udp_pmtu_discover=0
    no -p -o tcp_pmtu_discover=0

    no -a | grep pmtu
    grep ptmu /etc/tunables/nextboot

    Hope this will help.


    Regards,
    brother Matthius


  3. Re: Lost connectivity

    On Feb 7, 6:26 am, "brother_Matthius" wrote:
    > I've seen something like this a few times, mostly on servers with
    > HACMP (redundant NICs in failover config).
    > For reason I can only guess, AIX routing table redirected outbound
    > traffic to 127.0.0.1. Only connection within the same subnet worked
    > OK.
    >
    > Once I disabled PMTU discovery and manually corrected routing table,
    > outbound connections started working fine.
    > Apparently, PMTU discovery dynamically updates AIX routing table with
    > optimized paths to specific hosts - and sometimes makes a mistake.
    >
    > How to check and disable PMTU discovery:
    > no -a | grep pmtu
    > grep pmtu /etc/tunables/lastboot
    >
    > no -p -o udp_pmtu_discover=0
    > no -p -o tcp_pmtu_discover=0
    >
    > no -a | grep pmtu
    > grep ptmu /etc/tunables/nextboot
    >
    > Hope this will help.
    >
    > Regards,
    > brother Matthius


    Thanks Bro!

    I'll give it a shot, although no HACMP here, but pmtu_discovery was
    on.


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