MTU size on router

[vikas-arora]

Hi,

Is there a recommended value for WAN MTU size in router configuration? Should it be set to 1472 or 1500?

Thanks!

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@vikas-arora wrote:

Is there a recommended value for WAN MTU size in router configuration? Should it be set to 1472 or 1500?

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What are you trying to configure (or optimize) and why?  There are already standardized ways for systems to perform MTU Path Discovery.

1472 bytes happens to be the largest data payload that you can send over ICMPv4 on a 1500-MTU network without fragmenting the packet.  (1500 - 20 (IPv4 header) - 8 (ICMPv4 Header) = 1472)  That doesn't mean that you would want to set the WAN MTU on your router to that value.

[vikas-arora]

@-G- I am not trying to optimize. I have been troubleshooting an issue with Netgear support (random WAN connection drops) and they suggested changing MTU size from 1500 to 1479. This didn't make sense to me for the same reasons that you have mentioned, and it was easy to confirm by doing a simple ping test.  Nevertheless, I thought of checking it here on this forum.

WAN drop happens in both cases when Rogers modem (Hitron CODA) is in bridge mode or RG mode (double NAT - not an ideal configuration, but just for testing). I don't find any logs/events in modem whenever drop happens.

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@vikas-arora  For testing, you can drop the WAN MTU to whatever they recommend.  You would normally only drop the MTU on a router to optimize for cases where your network traffic was being encapsulated; e.g. 1492 when transmitting over PPPoE links.

If you are running Windows, and the "ping -f -l 1472 8.8.8.8" command results in a reply, then that confirms that a Network MTU of 1500 does not result in fragmented packets.

However, in your case, you won't get very far with support if you don't follow their recommendations and test whatever they ask you to test.