Max and Default MTU Size on MX WAN ports

[MarkBrown]

Can anyone confirm the Max and Default MTU size on MX100. Does it differ on the MX models? 

I know support can change the size, I am just looking for information on the sizes.

Thank you.

[alemabrahao · Accepted solution]

The Maximum Transmission Unit (MTU) is the maximum frame size that can be sent between two hosts without fragmentation. The MX uses an MTU size of 1500 bytes on the WAN interface. When a packet is sent from a local host to a host in a remote network, the frame may traverse multiple router hops. If an intermediate router is configured with an MTU size that is too small and the IP header in the datagram has the "Do-not-fragment" bit set, the router informs the sender of an unacceptable maximum packet size with an ICMP "Destination Unreachable-Fragmentation Needed and DF Set" message. The sender will then transmit a smaller frame taking into account the smaller MTU size.

Some routers are configured to drop certain ICMP traffic. If the ICMP error message never makes it back to the sender, it can cause intermittent connectivity issues between the source and destination hosts. 

https://documentation.meraki.com/General_Administration/Tools_and_Troubleshooting/Troubleshooting_MT...

[KarstenI]

To add on what is already said:

• If PPPoE is used on the WAN, the MTU is automatically adjusted for the overhead
• If you need a different WAN MTU, Support can set this for you. But this reduced MTU is not accounted for when using non-Meraki IPsec VPNs and you will certainly run into problems.