Introduction to MPLS Label Distribution
and Signaling

Rick Gallaher is course director for CCI, President of Telecommunications Technical Services Inc., and author of  Rick Gallaher's MPLS Training Guide

November 1, 2001

In the first tutorial, we discussed the data flow and the foundational concepts of MPLS networks. In this section, we will introduce the concepts and application of MPLS label distribution and introduce MPLS  signaling.  Moving forward, there will be a tutorial on Advanced MPLS Signaling.

Vocabulary

• Border Gateway Protocol (BGP)
• Binding
• Constrained Router Label Distribution Protocol (CR-LDP)
• Down Stream on Demand (DOD)
• Down Stream Unsolicited (DOU)
• Explicit Routing
• Independent Control
• Implicit Routing
• Intermediate System to Intermediate System (IS-IS)
• Label Distribution Protocol (LDP)
• Next Hop Label Forward Entry (NHLFE)
• Ordered Control
• Open Shortest Path First with Traffic Engineering (OSPF-TE)
• Resource Reservation Setup Protocol with Traffic Engineering (RSVP-TE)

The Early Days of Switching

Circuit switching by label is not new.  A quick look back at telephony shows us how signaling was done in the “old days.”  A telephone switchboard had patch cables and jacks; each jack was numbered to identify its location.  When a call came in, an operator would plug in a patch cord into the properly numbered jack.  This is a relatively simple concept.

Recalling these days, we find that although the process seemed simple enough, it was really hard work. Telephone operators would attend school for weeks and go through an apprenticeship before qualifying to operate a switchboard because the rules for connecting, disconnecting, and prioritizing calls were complex and varied from company to company.

Figure 1 Label Switching in the Early Days

Some of the rules included:

• Never disconnect the red jacks – these are permanent connections.
• Connect only the company executives to the jacks labeled for long distance.
• Never connect an executive to a noisy circuit.
• If there are not enough jacks when an executive needs to make a call, disconnect the lower priority calls.
• When “Mr. Big’s” secretary calls up at 9 a.m. to reserve a circuit for 10 a.m.–noon, make sure that the circuit is ready and that and you’ve placed the call by 9:50 a.m.
• In an emergency, all circuits can be controlled by the fire department.

So one operator had to know the permanent circuits (red jacks), the switched circuits, the prioritization scheme, and the reservation protocols.  When automatic switching came along, the same data and decision-making processes had to be loaded into a software program.

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