Glimpse on MPLS

MPLS added SHIM header of 32bits...which includes 20bits label, 3 bits for CoS, 1 bit for Bottom of Stack & 8 bits for TTL.

This overhead will sometimes creates the issue...b/c in different MPLS application. ..different numbers of labels are attached..sometimes 2 or even 3...causing 4B x 3 = 12B overhead...so if framesize increase 1500B than it will cause fragmentation. ..and if application mark the TCP segment as Dont Fragment.... so it will simply drop the packet.

To overcome this issue.....Service Provider used to increase MTU size at Core Interfaces to min of 1532 to carry....1500B information from the Core. But if the core interface is Trunk....than further overhead b added...so we used to implement 1550B Core interface MTU over Trunk ethernet interfaces.