Would appreciate help changing MTU

[Vinh]

Recently I was not able to access some websites and think it has to do with the router MTU so I put a wan-start script in JFFS but still not able to access sites with high MTU.
I have an RT-AC66u using merlin's 3.0.0.4.374.32 firmware.

Would appreciate confirmation that I did this correctly (or maybe I need to do something different altogether):
1) enabled JFFS, formatted it
2) SCP wan-start to jffs/scripts
3) changed its property to a,x
4) reboot

The contents of wan-start are as follows:
"#!/bin/sh
ifconfig eth0 mtu 1400"

If correct, what else can I be doing? Thanks ahead

 

[PrivateJoker]

Can you share what kind of connection you have (cable modem, ppoe, etc) and also some of the websites in question and others can try to confirm. Jumping right to scripts adjusting your MTU is a big leap.

Also, if you have IPV6 enabled and the site is IPV6, IPV6 has an auto-discovery of MTU.

In my (not by any means authoritarian) experience it is very uncommon to need to adjust your MTU based on an "inability to access some websites."

In Linux/Unix/OSX you can check for fragmentation via command line: ping -D -s [1460-1470 based on header tacked on, no brackets] "destination, no quotes" or Win command line: ping "destination, no quotes" -f -l [1460-1472, or other, no brackets].

 

[KAD]

might help if we knew what you are trying to do

if I remember correctly

eth0 is WAN
eth1 is 2.4ghz
eth2 is 5ghz

and switch ports are part of a vlan

 

[CooCooCaChoo]

First off, you need to figure out what your correct MTU is. Setting it to some arbitrary number that you believe to be low enough isn't going to help you.

Second. You would need to change the MTU for all interfaces to that value. This is so that in the future if you add devices to 2.4GHz or 5GHz interfaces you won't have to do it again.

Third. You would have to go and change the MTU for each computer/device that is connected to the router so that they send out the right sized packets.

By doing number one, you know exactly what the maximum size of the packets you send out are so that one of two or both these things don't happen:

1. Your packets don't get fragmented. This can degrade your network performance. Say your MTU actual is 1492 bytes and you are sending 1500 byte packets. The network would have to fragment your packets so that they fit into two packets instead of one.
2. Your packets don't get dropped. I've never seen this with a default Windows/Linux MTU of 1500.

By doing number two. The router will ever only send out packets with the MTU size you designate so that the two things in the previous don't happen.

And by doing number three. Your computers or devices will never try to send out packets that are too big and thus would have to be fragmented.

You won't be able to configure your modem's MTU because that is set to what the provider wants it to be. You can only set your router and computer's MTU values.

So ... If those quotes are actually in the wan-start config file, I'd take them out.

Also can you run ifconfig again to confirm that the MTU values that you have set are being used