After Studying the First Chapter of the Ivan and Jim’s MPLS and VPN Architecture Book, i come with a fantastic post regarding the MPLS Architecture, so this will be the first post from the first book that i have started for my SP track:

We know that the IP Propagation in normal router is base on the Destination IP address, which is reachable from the Routers,  and due to some minor drawbacks MPLS came to resolve it, so now Propagation of the IP Packets across the MPLS backbone occur in Three Major Steps:

  1. The Ingress Edge-LSR ( Facing the Customer End ) receives an IP Packet, classifies the packet into the Forward Equivalence Class (FEC)*, and labels the packets with the outgoing label stack corresponding to the FEC. For unicast destination-base IP Routing, the FEC Corresponds to the Destination Subnet and the Packet classification is traditional Layer 3 Look up in forwarding Table.
  2. Core LSRs receives this labeled packet and use label forwarding table to exchange the inbound label in the incoming packet with the outbound label corresponding to the same FEC (IP Subnet in this case).
  3. When Egress Edge-LSR ( Other Customer End ) for this particular FEC receives the labeled packet, it removes the label and perform traditional Layer 3 look up on the resulting IP Packet.

Below is how the it looks while Customer Site-A send the Packets to Customer Site-B:





The Ingress Edge-LSR POP router receive a Packet with the Destination which is located to Site-B and performs a traditional Layer 3 lookup through the IP forwarding Table which is called FIB ( Forwarding Information Table ).

NOTE: CEF is the only Layer 3 switching mechanism that uses FIB Table, and it has to be enabled on all the routers which are running the MPLS. although Core routers don’t perform CEF Switching, they just switch the labeled packets, but still CEF has to be enabled.

When Ingress Edge-LSR router receives the IP Packet it imposes the label “30 in this case” into the packet before it’s forwarded to Core Router ( HOW the Label are Imposes will be discussed in another post ).

The Core Router received Layer 2 PPP Frame from the Ingress Edge-LSR router and immediately identifies the received Packet as a labeled packet based on its PPP Protocol field value and performs label lookup on its label forwarding information base (LFIB), the LFIB entry corresponding to inbound label 30 directs to the Core Router to replace the Label 30 with an outbound label of 29 toward the second core router. it performs the swapping of the label and forward the labels to the Egress Edge_LSR router which is connected to Site-B and LFIB entry tells the Egress Router to POP the label and forward the unlabeled packets.

Now the Egress-LSR router perform the Layer 3 IP Lookup and forward the Packet to it’s destination.


* Routers Select the Next-HOP for the IP Packets base on two Functions:

1. The First Function Partitions the entire set of possible packets into a set of IP destination Prefix.

2. The Second Function maps each IP destination prefix to an IP next-hop address.

Within the MPLS Architecture the result of First Function are known as a Forwarding Equivalence Classes (FEC). which can be visualized as describing a group of IP Packets that are forwarded int the same manner, over the same path, with the same forwarding treatment.


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