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#1 (**permalink**) 01-05-2008, 11:14 AM

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 (**permalink**) 01-05-2008, 11:14 AM

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 (**permalink**) 01-05-2008, 11:14 AM

On Feb 7, 6:26 am, "brother_Matthius" <maciek.ma...@gmail.com> 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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