MPLS Label Distribution and Signaling
(continued)

MPLS Label Distribution

The MPLS switches must also be trained – they must learn all the rules and when to apply them. Two methods are used to make these switches.  One method uses hard programming; it is similar to how a router is programmed for static routing.   Static programming eliminates the ability to dynamically reroute or manage traffic.

Modern networks change on a dynamic basis.  To accommodate this need, many network engineers have chosen to use the second method:  dynamic signaling and label distribution.  Dynamic label distribution and signaling can use one of several protocols, with each its given advantages and disadvantages. Because this is an emerging technology, we have not seen the dust fully settle on the most dominant label and signaling protocols.  Yet despite the selection of protocols and their tradeoffs, the basic concepts of label distribution and signaling remain consistent across the protocols.

At a minimum, MPLS switches must learn how to process packets with incoming labels. Sometimes this is called a cross-connect table.   For example, label 101 in at port A will go out port B with a label swapped for 175.  The major advantage of using cross-connect tables instead of routing is that cross-connect tables can be processed at the “data link” layer, where processing is considerably faster than routing.

We will start our discussion using a simple network (figure 2) with four routers.  Each router has designated ports. For the sake of illustration, the ports have been given a simple letter a, b, s, h, a, and e. These port identifications are router specific.  The data flows from the input a of r1 to the input of r4.  This basic network diagram will be enhanced as we progress through MPLS signaling.

Figure 2:  Basic MPLS Network with 4 Routers

CONTROL OF LABEL DISTRIBUTION

There are two modes used to load these tables.  Each router could listen to routing tables, make its own cross-connect tables, and inform others of its information.  These routers would be operating independently.  Independent control occurs when there is no designated label manager, and when every router has the ability to listen to routing protocols, generate cross-connect tables, and distribute them.  (Figure 3)

Figure 3:  Independent Control

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