Are there consequences to having MTU set too high on a consumer router?

Question

Are there consequences to having MTU set too high on a consumer router?

Answer 1

The consequence of too high MTU is that some operating systems do not trust fragmented packets and tend to ignore them unless specifically allowed. That is probably not an issue using default MTU 1500 for ethernet unless there is a choke point like PPPoE with 8-byte header, therefore 1492 max MTU, if neither end of a connection is aware of it. Even that is probably not an issue for a network client or a server behind modern single router unless the server behind a 2nd router (double NAT) is unaware of the choke point on the 1st modem/router.



I first became aware of that when playing around with an smtp server (sendmail in Linux). When I sent small test messages to it from work, that was no problem. But when sending messages with attachments from the internet to it through its PPPoE connection, sendmail logs showed "timeout awaiting data transfer" every 60 seconds (because by default Linux ignores fragmented packets) and then the sending server would keep retrying. Linux ifconfig can change mtu on the fly and once I changed the receiving server's MTU to 1492, the mail with attachments arrived.



Even though I have a newer DSL gateway now that lets the network know about the 1492 mtu, I set mtu on Linux to 1492. Even then Linux is smart enough to use mtu 1500 on the LAN, but has no problems negotiating mtu 1492 for internet traffic (mss is used to negotiate mtu).

Answer 2

What that means is Yes that is a Good practice but only within Internal Network where the other Router has a High MTU... or all router have a HIGH MTU Internally...



But for your router that has the Gateway to the internet should use the default MTU size as that is what the Internet uses.

Answer 3

According to: http://compnetworking.about.com/od/networkprotocols/g/mtu-maximum.htm

"If the maximum TCP packet size is set too high, it will exceed the network's physical MTU and also degrade performance by requiring that each packet be subdivided into smaller ones (a process known as fragmentation)."