When I started exploring the details of MTU handling in Cisco IOS, I quickly got tired of analyzing various long printouts to extract the MTU sizes, so I wrote a Tcl script that display hardware, IP and MPLS MTUs in a tabular format. To install it on your router:

1. Download it from my web site and copy it to your router's flash or NVRAM.
2. Define an alias, for example alias exec mtu tclsh flash:displayMTU.tcl.

The script recognizes two parameters: the ip parameter displays only the interfaces that have IP configured and the mpls parameter displays only the MPLS-enabled interfaces.

Here is a sample printout from one of my lab routers:

R2#mtu
Interface            Hardware     IP   MPLS
=========================================================
FastEthernet0/0          1500          1600
FastEthernet0/1          1500
Serial1/0                1500   1500
Serial1/1                1380   1380   1430
Serial1/2                1500   1500
Serial1/3                1500
Loopback0                1514   1514
Tunnel0                  1514   1476   1476

R2#mtu mpls
Interface            Hardware     IP   MPLS
=========================================================
FastEthernet0/0          1500          1600
Serial1/1                1380   1380   1430
Tunnel0                  1514   1476   1476