MPLS Label Distribution and Signaling
(continued)

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.

DOU

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 as down-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.

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