Hi,
The main difference is interface MTU defined max packet size supported by an interface, while IP MTU is used to set MTU size of IP PACKET.
1. 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.
Media Type --------------- Default MTU (Bytes)
Ethernet --------------- 1500
Serial --------------- 1500
Token Ring --------------- 4464
ATM --------------- 4470
FDDI --------------- 4470
HSSI (HSA) --------------- 4470
2. IP MTU:
To set the maximum transmission unit (MTU) size of IP packets sent on an interface. Minimum is 128 bytes; maximum depends on the interface medium.
Changing the MTU value (with the mtu interface configuration command) can affect the IP MTU value. If the current IP MTU value is the same as the MTU value, and you change the MTU value, the IP MTU value will be modified automatically to match the new MTU. However, the reverse is not true; changing the IP MTU value has no effect on the value for the mtu command.
http://www.cisco.com/univercd/cc/td/doc/product/software/ios124/124tcr/tiap_r/apl_i2ht.htm#wp1202749
Pls rate useful post(s).
Rgds,
AK
