[j-nsp] MPLS MTU Juniper vs. CISCO

[Yastrebov, Valery]

Here i have some information on MTU Cisco vs Juniper, I had investigated it several months ago.

 

1) MTU - JUNIPER

http://www.juniper.net/techpubs/software/junos/junos73/swconfig73-interfaces/html/interfaces-physical-config5.html <http://www.juniper.net/techpubs/software/junos/junos73/swconfig73-interfaces/html/interfaces-physical-config5.html> 

 

NOTE: The actual frames transmitted also contain cyclic redundancy check (CRC) bits, which are not part of the media MTU. For example, the media MTU for a Gigabit Ethernet interface is specified as 1500 bytes, but the largest possible frame size is actually 1504 bytes; you need to consider the extra bits in calculations of MTUs for interoperability.

 

Default media MTU = Default IP MTU + L2 encapsulation overhead

Default IP MTU = Default media MTU -  L2 encapsulation overhead

MPLS MTU = physical interface MTU - L2 encapsulation overhead - 12

if IP MTU is already set, so MPLS MTU = IP MTU + 20 bytes

IP MTU - payload without header!

 

2) MTU Cisco

http://www.cisco.com/univercd/cc/td/doc/product/software/ios124/124tcr/tir_r/int_m1ht.htm#wp999684 <http://www.cisco.com/univercd/cc/td/doc/product/software/ios124/124tcr/tir_r/int_m1ht.htm#wp999684> 

Changing the MTU value with the mtu interface configuration command can affect values for the protocol-specific versions of the command (the ip mtu command, for example). If the value specified with the ip mtu interface configuration command is the same as the value specified with the mtu interface configuration command, and you change the value for the mtu interface configuration command, the ip mtu value automatically matches the new mtu interface configuration command value. However, changing the values for the ip mtu configuration commands has no effect on the value for the mtu interface configuration command.

 

Default media MTU = Default IP MTU    (L2 encapsulation overhead   not included)

MPLS MTU = MPLS overhead + Default IP MTU

MPLS MTU

http://www.cisco.com/en/US/partner/products/ps6566/products_feature_guide09186a0080629db0.html