Far and wide -- WAN service notes

By Raymond Keneipp, ITworld.com |  Networking

For the past couple of years, href="http://www.protocols.com/pbook/tag.htm#MPLS">Multiprotocol Label Switching,
or MPLS, has been held up as the solution to many of the performance and scaling
problems service providers are experiencing in their IP networks. But should it be
deployed on your enterprise WAN?

The concept behind MPLS operation is simple. Within an MPLS network, each switching
node (called a label switching router, or LSR) looks at a label attached to an
incoming packet and uses it as an index into a table to determine the outbound link to
which the packet should be forwarded. The LSR then assigns a new label with information
meaningful to the next node and forwards the packet on the outbound link. Thus, each
packet is forwarded hop by hop across the MPLS network, with label swapping occurring
at each LSR node.

While each label has local significance only (that is, the label may be different on
each link), the effect is to create an end-to-end path across the MPLS network. An MPLS
network doesn't route traffic based on packets' IP addresses; they are ignored. Nor
does it route based on asynchronous transfer mode (ATM) virtual circuit or path
identifier (VCI/VPI) information. Thus, a network composed of LSR nodes is not an IP
network or an ATM network -- it's a new and different animal, a MPLS network.

While MPLS is technically a multiprotocol label swapping technique that could work
with any Layer 3 protocol, no one expects to use it with any protocol other than IP;
it's generally thought of as a way to link IP routers. If developers attempted to
expand MPLS to cover other protocols, the result might look very much like ATM, and no
one wants to reinvent that wheel.

The label switching techniques used by LSRs are similar to the ways in which ATM
switches forward traffic based on VCI/VPI information in ATM cells. Many ATM switches
can be reprogrammed as MPLS LSRs. Some vendors' implementations allow both MPLS and ATM
to run on a network simultaneously -- the two disparate types of products can work
together as part of the same backbone.

MPLS is the convergence of connection-based forwarding techniques and IP routing
protocols. MPLS creates label-switched paths over the same shortest-path trees packets
would have traveled had a network been built with conventional routers.

If that's the case, then why run MPLS? MPLS has some distinct advantages when it
comes to creating virtual private networks. With it, you can create the equivalent of
X.25 closed user groups; that is, you can designate a group of ports to be a VPN. The
users on those ports can then run their own addressing schemes. While this is great for
a service provider, it's of little value to most organizations.

But do you need MPLS?

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