MPLS Shim Header Structure

Label

TTL

Exp.

4 Octets

Label: 20-bit value, (0-16 reserved)

Exp.: 3-bits Experimental ( ToS)

S: 1-bit Bottom of stack

TTL: 8-bits Time To Live

Layer 2 Header

IP Packet

...

MPLS "shim" headers

MPLS encapsulations are also defined for ATM and Frame relay.

Header operations

Swap (label)

Push (a new header)

Pop (a header from stack)

Label Switching

Look up inbound label + port (+Exp)

to determine

outbound label + port + treatment