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The trouble with MPLS

High-end carrier services can mean high-end config issues (and risks)

By Jon Espenschied
March 12, 2007 12:00 PM ET

Computerworld - Multisite and outsourced IT operations are making good use of Multiprotocol Label Switching (MPLS), but strange trouble is turning up more and more. Often in discussion with local network staffers, we come to the point when I ask about backhaul lines or internet service providers over which they presumably run a site-to-site virtual private network (VPN). They happily reply, "Oh, we have MPLS" and provide a network diagram consisting of a suitably inscrutable cloud.

Life is not so simple.  Increasingly, those IT infrastructures appear functional, but a simple scan turns up many times the number of hosts that ought to be visible. Finding rogue devices on a network is cause for a bit of alarm, but unknown subnets?

What's going on? MPLS is supposed to simplify wide-area networking with carrier-grade service, not increase the risks of exposing sensitive data. Finding one's network cross-connected with another organization is not something that can be dealt with tomorrow, and a serious address-space collision can put networks completely out of commission.
IT managers and technologists looking for a simple way to connect distant LANs turn to MPLS as a solution that has more currency and expandability than older offerings. The trouble is many of them make the decision to adopt MPLS without enough information.

Blue car, yellow car

As its name implies, MPLS embeds network switching or routing information into lower network layers. Like frame relay, it allows low-latency transit of network traffic between two distant points but leaves error handling up to the endpoints.  Like asynchronous transfer mode, it embeds transit information into lower network layers, but the variable-length packets of MPLS are more suited for encapsulating IP traffic than ATM's fixed-length cells.

By labeling traffic at a lower network level, less processing has to happen at each waypoint between source and destination. It's analogous to color-coding cars on the highway and allowing only blue cars to enter at the Los Angeles on ramp and exit at San Francisco and vice versa. Yellow cars might share the same road from San Diego to Santa Barbara, but they would enter or exit only on ramps flagged for yellow cars. 

The transit speed is unchanged -- MPLS doesn't make a 10Mbit/sec. link go to 11 -- but the entry, routing and exit decisions can be made much more simply and quickly. To send someone to San Francisco, you could give them a blue car, and the network (the road) would get him there without needing to get on and off the highway to ask for directions.

The devil's in the details. RFC 3031 defines MPLS, but it takes a subsequent half-dozen RFCs to cover the more drowsy topics of label distribution, handling, application and interfaces with other networks. MPLS configuration is, as others have noted, expensive and tedious, which is why the technology has been the domain of carriers for the better part of a decade.

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