MTU issues can be caused by the extra overhead of PPPoE, VLANs, MPLS, VPLS, etc.

MTU issues can cause Microsoft Exchange to not accept mail.

MTU issues can interfere with SSH connections.

MTU is the maximum packet size.

MSS is the maximum data bytes carried within a packet. 1472 is fairly safe. Less than that is problematic due to fragmentation.

The following command example tests for Jumbo Frames capability, which is usually an MTU (max transmission unit) of 9000 bytes.

We use '8972' because that plus the overhead equals 9000 (as seen in the output of the ping command).

At the Linux CLI:

ping -M do -c 1 -s 8972 <hostname or IP>

• -M do (don't fragment)
• -c 1 (send one packet)
• -s 8972 (send 8972 byte packet, plus overhead)

http://www.cyberciti.biz/faq/centos-rhel-redhat-fedora-debian-linux-mtu-size/

http://www.ubuntugeek.com/how-to-change-mtu-maximum-transmission-unit-of-network-interface-in-ubuntu-linux.html

Once you know the maximum MTU you can use, set it like this:

sudo ifconfig eth0 mtu 1464

ping -f -l 8792 <hostname or IP>

These are tools which will figure out the MTU across the path between the host and the target:

mturoute

tracepath
traceroute --mtu