Label Stack Fields


Label = 20 bits
EXP  = 3 bits  map the standard IP packet ToS into the Experimental field for MPLS CoS
S = Stack bit is use to identify the end of the last label of the stack.
TTL = 8 bits and when set to zero packet is discarded.

The MPLS label stack is can be referred to The Shim Header.
It reside in between the Layer 2 and Layer 3 Payload. ( ip header and Ethernet / PPP / Frame Relay header etc … )

MPLS Architecture   “Control and Forwarding”

Label = Control plane = Label are Tied to routes in the routing table

The control plane is there to bind the label to the network route and distributed them between MPLS routers.
MPLS need a routing protocol so we can exchange the Labels and it’s done through 2 protocols :

TFP : Tag Distribution Protocol ( Cisco Proprietary )
LDP : Label Distibution Protocol ( IETF version of Cisco, the LIB “Label Information base” map incoming packet to outbound labels.

Forwarding Equivalence Class (FEC)
A forwarding equivalence class (FEC) is a grouping of IP packets that are treated in the same way. For example, a destination subnet
could correspond to an FEC. When I say that labels get bound to routes in the routing table, I’m referring to an IP prefix being the equivalent
of an FEC. So, to be specific, labels are bound to FECs.FECs can be based on a number of criteria, including IP ToS bits, IP protocol ID, port numbers, etc.


The routing table is built in the control plane and cached in the forwarding plane. For labels, the LIB is built in the control plane, and only those labels in use reside in the label forwarding information base (LFIB). The LFIB is a subset of the LIB. An additional component that resides in the forwarding plane is the forwarding information base (FIB). The FIB is built by Cisco Express Forwarding (CEF). The FIB is essentially a cached version of the IP routing table that eliminates the need for a route-cache. For Cisco MPLS or tag switching to work, CEF must be enabled.

The control plane comprises the following:
 Routing protocol
 Routing table
 Tag exchange using Tag Distribution Protocol (TDP), resulting in the tag information base (TIB)
The forwarding plane is made up of:
tag forwarding information base (TFIB)
The two technologies are virtually identical.


Configuration Example Step by Step :

1st we have to enable tag switching globally and make sure ip cef is enable, Second we have to enable tag switching on the interfaces.


( assuming you have already configured the addresses )

config t
(config)#ip cef
(config)#mpls ip
(config)#interface X
(config-int)#mpls ip

2rd we need to configure BGP on the Providers Edge “PE” and add the I-BGP  neighbor statement of the others PE loopback Interfaces. and the network advertisement of the loopback.


to be continued


































The info in this document was learn and taken from a book from Sybex , Copyright ©2002 SYBEX, Inc., Alameda, CA and i as have already mention this is for personal reference.

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