MTU size and Cisco DPC3939B

The correct MTU or Maximum Transmission Unit can be found using ping.

Choose a publicly accessible site like google.com or comcast.net and try values until the response is not fragmented then add 28. Substitute xxxx's for the size of the packet.

to see ping's options on a windows pc open a cmd prompt: /start/run/cmd and type in the black window ping /?

ping google.com -f -n 1 -l xxxx

to see options for ping in linux: man ping

ping google.com -c 1 -s xxxx

much more complete tutorials are found on the web with pictures.

the Cisco DPC3939B gateway provided by Comcast does not allow options to ping other than count.

can anyone share the MTU size this unit is set to? please

Responses

train_wreck

Gold Problem solver

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610 Messages

10 years ago

The MTU on the DPC's ethernet ports is 1500. Shown below is a set of increasingly larger "dont fragment"-enforced pings from one of my Linux servers:

[root@pLAN9-Server2 /]# ping -M do -s 1471 10.1.10.1
PING 10.1.10.1 (10.1.10.1) 1471(1499) bytes of data.
1479 bytes from 10.1.10.1: icmp_seq=1 ttl=64 time=4.87 ms
^C

[root@pLAN9-Server2 /]# ping -M do -s 1472 10.1.10.1
PING 10.1.10.1 (10.1.10.1) 1472(1500) bytes of data.
1480 bytes from 10.1.10.1: icmp_seq=1 ttl=64 time=7.90 ms
^C

[root@pLAN9-Server2 /]# ping -M do -s 1473 10.1.10.1
PING 10.1.10.1 (10.1.10.1) 1473(1501) bytes of data.
ping: local error: Message too long, mtu=1500
^C

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aic

New problem solver

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39 Messages

10 years ago

thanks train_wreck for taking this on.

I get the same results you do, regardless I had difficulties getting an email and I solved it by lowering my MTU to 1380. I am back to 1472+28=1500, thanks for confirming.

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