Problems with MTU

[Stefan Gabel]

Hi,

I have some problems with my AIX 5.2.0.0 and the MTU. I changed the MTU to 1500, but the I still see on my network packets greater than 1500. Some packets have the size up to 50.000.

The switch also use MTU size 1500.

With netstat -r command it lists the server to whom I want to send the packets. It also use MTU 1500.

Have anybody some explanations.

[TwoProc]

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[TwoProc]

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[Stefan Gabel]

Thanks for the fast answer.

Do you have some axplanations or just some jokes.

[RAC_1]

There can be MTU for NIC, for netwotk path. so even if you set nic mtu at 1500 and path has something else, packets going on that path will have mtu set for that path.

[Stefan Gabel]

I use smitty to configure my AIX. I can't find any path values. Only some settings for the NIC

[RAC_1]

route -nr
lanadmin -msx "nm_id"

to know mtus of route path and network.

[Stefan Gabel]

this ist the answer for the routing tabel when I use netstat -r


Destination Gateway Flags Refs Use If PMTU Exp Groups

s415mt36.tss.dai alcatraz.tss.daim UGHMW 2 2711 en0 1500 -

All signs which I can find says MTU 1.500.
But I captured my traffic with to different tools and can see packages with size of 50.000

[Pat Lieberg]

Were the packets you saw comming into your host? The MTU set on your system's network card controls only that device, not the entire network. Are your network switches and routers also set to an MTU of 1500?

[Stefan Gabel]

Thanks for help.

I don't get the packets in my host. This host is sending this big packets. And all switches and routers a setup with MTU 1500.

Somtehing inside the AIX server overrides the settings. It seems to be weird.

[Pat Lieberg]

Can you tell what kind of packets they are?

PMTU packets are used by the system to discover the MTU size of the network and are often large and have the Do Not Fragment flag set.

[Stefan Gabel]

All packets have don't fragement flag set to 1.

And all packets have packet Fragement offset==0.

On the destination I can't find this big packets. And I also get for this particular big packet 10-15 acknowleges from the destination server.

Should I disable MTU discovery? Do think this would solve the problem?

[Pat Lieberg]

Disabling MTU Discovery might well get rid of the packets, but might affect network performance. You could of course disable it to see if that's the source of the large packets.

I guess the real question is, are these large packets that you are seeing now affecting performance? If they are MTU Discovery packets and there are no network performance problems, then I don't see any issue.

[Stefan Gabel]

I have disabled MTU discovery.

The behaviour changed a little, but I still see packet size greater than 1.500. The maximum is right now 4.434 which some packets have.
This packets I also see when I enable MTU discovery.

So I don't see the real big packets not anymore, but the packets are still greater than the MTU in the system and on the switch.

[Stefan Gabel]

The solution lies on NIC. The NIC has an controller on it an was enabled. This wasn't the really traffic on the network

[Stefan Gabel]

See my answer before