Thursday, November 1, 2001

Tutorial: Introduction to MPLS Label Distribution and Signaling

Rick Gallaher, CISSP, is owner of Dragonfly Associates LLC 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.

  • 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.

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


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

The other model is ordered control, as shown in Figure 4.  In the ordered control mode, one router – typically the egress LER – is responsible for distributing labels.

Each of the two models has its tradeoffs. Independent control provides for faster network convergence.  Any router that hears of a routing change can relay that information to all other routers.  The disadvantage is that there is not one point of control making traffic, which makes engineering more difficult.

Ordered control has the advantages of better traffic engineering and tighter network control; however, its disadvantages are that convergence time is slower and the label controller is the single point of failure.

Figure 4: Ordered Control (pushed)

The Triggering of Label Distribution

Within ordered control, there are two major methods to trigger the distribution of labels.
These are called down-stream unsolicited and down-stream on demand.


In figure 4, we saw the labels “pushed” to the down-stream routers.  This push is based upon the decisions of the label manager router. When labels are sent out unsolicited by the label manager, it is known asdown-stream unsolicited (DOU).

For example: The label manager may use a trigger point (such as a time interval) to send out labels or label refresh messages every 45 seconds.   Or, a label manager may use the change of standard routing tables as a trigger – when a router changes, the label manager may send out label updates to all affected routers.


When labels are requested, they are “pulled” down or demanded, so this method has been called pulled or down-stream on demand (DOD).  Note in Figure 5, that in the first step   the labels are requested and in the second step the labels are sent.

Figure 5:  Down-stream on Demand  (DOD)

Whether the labels arrive via independent or ordered control, or via DOD or DOU, the label switch router (LSR) creates a cross-connect table similar to the one shown in Figure 6.

The connect tables are sent to router r3 to r1. The tables heading read: label-in, port-in, label-out, port-out, and instruction (I).  In this case, the instruction is to swap (s).  It is important to note that the labels and cross-connect tables are router specific.

After the cross-connect tables are loaded, the data can flow from router 1 to router 4 with each router following its instructions to swap the labels.

Figure 6: LSR with Cross-connect Tables Populated

After the cross-connect tables are loaded, the data can now follow a designated LSP (label switch path) and flow from route 1 to router 4, as shown in Figure 7.

Figure 7:  Data Flow on LSP


As a brief review, we learned that routers need cross-connect tables in order to make switching decisions.  The routers can receive these tables from their neighbors via independent control or from a label manager via ordered control.

A label manger can send labels on demand (called down-stream on demand) or it can send labels when it decides to, even though it has not been requested by the down-stream routers, by using down-stream unsolicited (DOU).

With these basic concepts understood, there are some more advanced concepts to consider.  For instance, just how are labels sent to routers? What vehicle will be used to carry these labels?  How is the quality of service information relayed or sent to the routers? 
Reviewing from the first article, MPLS packets carry labels; however, the packets do not have an area that tells routers how to process the packet for quality of service (QoS).

Recalling that traffic can be separated into groups called forward equivalence classes (FECs), and that FECs can be assigned to label switch paths (LSP), we can perform traffic engineering to force high-priority FECs on to high-quality LSP and lower priority FECs on to lower-quality LSP.  The mapping of traffic using different QoS standards will cause the distribution of label and maps to be more complex.

Figure 8 shows a drawing of what goes on inside a LSR. There are two planes: the data plane and the control plane.  Labeled packets enter at input a with a label of 1450 and exit port b with a label of 1006. This function takes place in the cross-connect table. This table can also be called the next hop label forwarding entry table (NHLFE).  
Figure 8: A Closer Look at the Router

This database is not a stand-alone database.  It connects to two additional databases in the control plane: the FEC data and the FEC-to-NHLFE database.  The FEC database contains, at a minimum, the the destination IP address, but it can also contain traffic characteristics and packet processing requirements. Data in this database must be related to a label; the process of relating an FEC to a label is called binding.
Here is an example of how labels and FECs are set-up:

FEC Database

FECProtocol Port no packet loss efforts load

Free Label Table
100-10,000 are not in use at this time
FEC to NHLFE Table

FECLabel inLabel out


Label in Label out

So we see that packets with labels can be quickly processed when entering the data plane, if the labels are bound to an FEC.  However, a lot of background processing must be done to the data traffic off line before a cross-connect table can be established.


Finding a transport vehicle to build these complex tables is of the utmost concern to network designers.  What is needed is a protocol that can carry all of the necessary data while, at the same time, be fast, self-healing, and maintain very high reliability.

The MPLS workgroup and design engineers created the Label Distribution Protocol.
(LDP). This protocol works like a telephone call. When labels are bound, they stay bound until there is a command to tear down the call. This hard-state operation is less “chatty” than a protocol that requires refreshing. The LDP protocols provide implicit routing.

Other groups argue against using a new untested label distribution protocol when there exist routing protocols that can be modified or adapted to carry the bindings. Thus, some existing routing protocols have been modified to carry information for labels.  The Border Gateway Protocol (BPG) and IS-IS work well for distributing label information along with routing information.

The LDP, BGP and IS-IS protocols establish the Label Switch Path (LSPs), but do little for traffic engineering, because routed traffic could be redirected onto a high priority LSP, causing congestion.

To overcome this problem, the signaling protocols were established to create traffic tunnels (explicit routing) and allow for better traffic engineering.  They are Constraint Route Label Distribution Protocol (CR-LDP) and Resource Reservation Setup Protocol (RSVP-TE). In addition, the Open Shortest Path First (OSPF) routing protocol has undergone modifications to handle traffic engineering (OSPF-TE); however, it is not currently widely used.

ProtocolRoutingTraffic engineering
     IS-IS ImplicitNO


In this article, we learned that one of several protocols could be used to dynamically program switches to build the cross-connect tables.  In the next article we will further explore the details and tradeoffs of the label distribution and signaling protocols.

Suggested URLs:


George Mason University

Network Training

MPLS Links Page

MPLS Resource Center


Special thanks to:

I would like to thank Uyless Black, Susan Gallaher, and Amy Quinn for their assistance, reviewing, and editing.

A special thank you to all those who assisted me with information and research on the MPLSRC OP mail list, especially: Syed Ali, Adithya Bhat, Krishna Kishore, Irwin Lazar, Christopher Lewis, Vic Nowoslawski, Mario Puras, Mehdi Sif, and Geoff Zinderdine.

Rick Gallaher, CISSP, is owner of Dragonfly Associates LLC and author of  Rick Gallaher's MPLS Training Guide