Thread: MTU on PC / Router / Modem

I have an office network with Dataone connection. I have changed the MTU to 1452 both on the Router and Modem as I was having some problems in getting emails. But I am having problems from time to time and some sites dont open properly at times. My questions are :
1) Do I have to change MTU on Modem + Router + PC or only on Modem will do
2) What is the optimum MTU setting
3) At my home too I have the same configuration but the IP address range is different, though from same telephone exchange. Will the MTU settings be same for home connection.

Could be a DNS problem.. try changing DNS servers to opendns 208.67.222.222 and 208.67.220.220

Originally Posted by superprash2003

Could be a DNS problem.. try changing DNS servers to opendns 208.67.222.222 and 208.67.220.220

I did this and it works. But now I am unable to send mails through my company server, getting timed out. Receiving is ok.
But what should the MTU be, 1500 / 1492 / 1452 or something else. And should I change this in Modem and keep everything else as 1500 or change in router and computers also.

MTU needs to be less or equal than the smallest MTU value in connected remote hosts. As the packet travels through many router to reach the destination any conflict in MTU size will result in packet drop and returning a ICMP "HOST UNREACHABLE" message , using this message usually ur machine will try to correct his own MTU value (RFC1191 ) .But few of the router in the network path may have firewall installed which blockes those ICMP message , preventing MTU correction. (I have seen this with few microsoft sites).

It's best to change the MTU to a optimum low value (but too small value will result in more fragmentation ,i.e. same message will be split in many small parts). Also be clear that if underlying ISP uses ATM to transfer the IP packets then it should be multiple of 48 bytes (max cell/packet size in ATM) for optimum transmission.

Most of the BSNL's 59.xxx series IP doesn't have such MTU releasted issues. Most of the problem is seen in 117.xxx series. (I donno why ).