sh run int tu162
Current configuration : 251 bytes
description GRE over MPLS to Dow
ip address 10.54.63.50 255.255.255.252
ip mtu 1476**********************
ip policy route-map clear-df-bit
tunnel source Vlan60
tunnel destination 192.168.163.222
sh int tu162
Tunnel162 is up, line protocol is up
Hardware is Tunnel
Description: GRE over MPLS to Dow
Internet address is 10.54.63.50/30
MTU 1514 bytes, BW 9 Kbit, DLY 500000 usec,******************
reliability 255/255, txload 174/255, rxload 28/255
Encapsulation TUNNEL, loopback not set
Keepalive not set
Can someone explain me what is differnce
between two MTU config shows 1476 and sh int tu162 shows 1514????
As i've stated earlier, IP MTU affects only the IP traffic, accordingly there is also MPLS MTU, CLNS MTU and so on, while MTU affects all kind of traffic, thus if you change the IP MTU you'll affect only the IP traffic, while if you change the MTU you'll affect all the types of traffic, all the MTU values match the interface MTU unless explicitly changed, and if a specific MTU is changed it will override the interface MTU for this specific traffic type, accordingly in your example all the IP traffic will use the IP MTU 1476.
To prevent potential problems (as per Cisco precautions; data corruption, dropped packets, and high CPU conditions can occur), recent IOSs prevents the IP MTU (or any MTU) to exceed the interface MTU.