how to verify if MTU 9000 configured properly on all component

Question

we want to test the network on all Linux machines

after all machines and switches configured to MTU=9000

the reason for that is because inconsistent MTU configuration can cause huge problem

so in our hadoop cluster we have ~50 machines and also switches that configured also

what are the Linux CLI that can approve that all linux / switches configured with MTU=9000

as all know ifconfig -a , show the MTU value

but we want to test it on each machine that MTU is real working

related unix.stackexchange.com/questions/451647/… and unix.stackexchange.com/questions/426113/… — Rui F Ribeiro, Sep 5, 2018 at 11:04

Daniele Santi · Accepted Answer · 2018-09-05 10:31:07Z

4

You can ping across your infrastructure forcing a suitable packet size and setting the DF (Don't Fragment) bit.

Relevant ping flags are -s to set the packet size and -Mdo to set the DF bit.

For example on my system (normal 1500 MTU):

$> ping 10.10.10.1 -s 9000 -Mdo
PING 10.10.10.1 (10.10.10.1) 9000(9028) bytes of data.
ping: local error: Message too long, mtu=1500

it gives an error (because my PC is not configured with Jumbo frames).

Please keep in mind that if Layer2 MTU is 9000, the ping size should be 28 bytes less, so use 8972 as packet size:

ping A.DD.RE.SS -s 8972 -Mdo

answered Sep 5, 2018 at 10:31

Daniele Santi

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first thank you so much your answer is very clear , so what we get for example from ping $IP -s 9000 -Mdo ( consider that IP configured to MTU=9000 )
– yael
Sep 5, 2018 at 10:57
@yael simply ping will respond instead of returning an error, for example ping 192.168.0.5 will respond 64 bytes from 192.168.0.5: icmp_seq=2 ttl=255 time=0.180 ms
– Daniele Santi
Sep 5, 2018 at 11:01
so , if MTU=9000 was configured on all machines in the cluster , include the switches / router etc ( as you know we have 50 machines ) then from each machine to every each machine we need to get --> respond 64 bytes from 192.168.0.5: icmp_seq=2 ttl=255 time=0.180 ms , am I right ?
– yael
Sep 5, 2018 at 11:03
@yael Sure, please keep in mind that with an MTU of 9000 (layer2), the packet size for ping should be <=8972, so use ping -s 8972, you should get a response like 8972 bytes from ....
– Daniele Santi
Sep 5, 2018 at 11:06
so lets summary , in order to complete verification of MTU=9000 we need to do ping xxx.xxx.xxx.xxx -s 8972 -Mdo , from each machine to all other machine in the cluster , so this will covered all test , am I right ?
– yael
Sep 5, 2018 at 11:09

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