An Introduction to MPLS

This is a post about MPLS (definition below) and a monitoring project we’ve recently helped a Command Center customer with. I’ll start by talking about MPLS in general, and post the monitoring stuff later. I'm going to assume you've never heard of MPLS.

MPLS for Dummies

It stands for Multiprotocol Label Switching. It’s a way of speeding up network traffic by avoiding the time it takes for a router to lookup the address of the next node to send a packet to. In an MPLS network data has a label attached to it, and the path that is taken is based on the label. Additionally, you can also attach a class to the data, which can be used to indicate that data has higher than usual priority.

Whilst the above takes care of the ‘Label Switching’ part of the name, the Multiprotocal part comes from the fact that Asynchronous Transport Mode (ATM), Internet Protocol (IP),  and frame relay network data can all be sent using MPLS.

For most companies, MPLS will be a service that they buy from a network services provider, and it might be beneficial to think of it thus: a pair of routers (the idea of a pair is important) on either side of an ‘MPLS cloud’. Example: you have two offices you want to link – say London and Edinburgh. You have in each office a router which interfaces with the MPLS service. When a device in Edinburgh wants to send data to a device in London it is sent via the Edinburgh router onto the MPLS service (appropriately labelled and classified by the router) where it will appear (eventually) on the London router for passing to the correct device. Between the two routers (referred to as ‘edge routers’ because they sit on the edge of the MPLS service) the data is the responsibility of the network services provider. For this reason, an MPLS service is often referred to as a cloud in network diagrams (‘we don’t know or care what happens here’).

So why bother? From the handful of customers we know using these services, the MPLS service is replacing leased lines. One of the key drivers seems to be cost – the MPLS services are working out cheaper than a leased line. However, another driver seems to be the desire to offer new services to users, one of which is Voice over IP (usually shortened to VoIP).

MPLS can be a sound (heh) choice for VoIP because of the idea of prioritising and classifying data. For VoIP to work, packets need to be sent quickly and at a relatively stable speed – otherwise, you get distortions on the line. Therefore, MPLS will offers the promise of ‘first class mail’ data packets (voice) and ‘2nd class mail’ (data) over the same network path (data is less sensitive to speed of transmission and variance of speed of transmission).

MPLS links:
MPLS Seminar notes – A pretty good introduction
Wiki – not for the faint hearted

I’ll post the monitoring details later in the week.

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