Difference between the mtu and ip mtu command?

[Diego Sousa]

Hi,

I am running labs and I came across the following situation:
I have four R1 routers using the Fast 0/0 interface connected to R2 on fast 0/0, R2 using fast 0/1 connected to R3 on fast 0/1 and connecting from R3 to R4 via the 0/0 fas . When I run the "mtu 700" command on the fast 0/1 interface of R3, performing ping tests with 800 bytes will display giants errors and no fragmentation will occur, but when the "ip mtu 700" command is broken, fragmentation occurs. What's the difference between these two commands, is there an MTU value for Layer 3 and an MTU value for Layer 2?

[Deepak Kumar · Accepted solution]

Hi,

IP MTU:

As with Ethernet frames, the MTU can be adjusted for IP packets. However, the IP MTU is configured per interface rather than system-wide, with the ip mtu command.

 Interface MTU - default MTU size for an interface:

Each interface has a default maximum packet size or MTU size. This number generally defaults to the largest size possible for that interface type. On serial interfaces, the MTU size varies, but cannot be set smaller than 64 bytes.

http://packetlife.net/blog/2008/nov/5/mtu-manipulation/

https://supportforums.cisco.com/t5/vpn/difference-between-interface-mtu-and-ip-mtu/td-p/650311

Regards,

Deepak Kumar

[Diego Sousa]

but both commands are used in interface configuration mode, it is still very confusing the difference between these commands and those values.