MPLS: The very basic concept on how data is forwarded through a MPLS domain

MPLS is a technology that uses labels to forward data through a network.

Open System Interconnection (OSI) Model

Before we proceed, lets just refer to the OSI model. It has 7 basic layers:
Application
Presentation
Session
Transport
Network
Data
Physical

Each layer processes the data in hand, and adds a layer specific header to it. For eg., if ‘z’ is the data processed by the application layer, then after processing, the application layer  will add a header ‘y’ to ‘z’, and what is passed down to presentation layer is ‘y+z’. Now this layer considers ‘y+z’ as single data, and processes it and adds a header ‘x’ to it; therefore we get ‘x+y+z’. This process continues down to all the layers. The reverse of this happens when the data is processed by the receiver (each header is stripped off).

The Network (layer-3) and Data ( layer-2) layers are associated with routing and switching respectively.

IP Forwarding
Let us suppose that Computer A has to send some data to computer B in some other network. Now, A will send a packet of data with the IP address of B to the network. As this packet reaches the first router, the router performs a Forwarding Information Base (FIB) look-up to learn the IP address present in the table with the longest matched address prefix. If the destination address is 192.0.10.10 and the addresses in the FIB are
192.8.5.15
192.4.5.3
192.0.1.3
; then the the routing algorithm will fix on 192.0.1.3, this being the longest prefix match. This look-up will take more time than trying to find the exact match, and even worse, this has to be repeated at every router.

MPLS (Multi protocol label Switching) forwarding
MPLS uses labels instead of IP address to forward packets.

Lets illustrate the working concept through an example. Computer A(24.168.0.10) & B(25.3.8.4) are located somewhere in the Network (not necessarily the same network). Now A sends a packet destined to B. Suppose the backbone network is MPLS enabled.

lets analyze the packet flow through the network.

(to be continued…)

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