– The following is an excerpt from the upcoming edition of the:
Introduction to Label Distribution
Thus far in this book we have discussed the values and characteristics of labels, we have dealt with their meaning and how they affect the creation of both the control and forwarding planes used in MPLS. Throughout this entire process we have only loosely referred to the exchange of labels between label switching devices. Now it is time to discuss the process and mechanism behind this exchange or advertising process.
There are two methods at our disposal to exchange labels and their associated bindings between devices that are peered. The key word here is “peered”. Whether we are using the Cisco proprietary Tag Switching or the industry standard Label Distribution Protocol (LDP) as the mechanism of choice to communicate label information, it all still boils down simply to peering relationships and the rules governing those peers. For the purpose of our discussion we will deal specifically with LDP from this point forward.
When MPLS is enabled on an integrated services router (ISR) LDP is the default label distribution protocol. It is this protocol that dynamically creates and governs the label switched path in our network. So it must be noted that everything we have discussed up to this point regarding label generation, assignment, and exchange is maintained by this single protocol. Additionally LDP is also responsible for the formation of the peering relationships we mentioned earlier. It is these relationships that facilitate the exchange of our local label bindings between peers.
At this time we should define and innumerate the major operational roles this protocol will be responsible for in terms our discussions in this chapter. Primarily we can outline three such functions:
- Dynamic discovery of adjacent LDP Peers
- Peering and session establishment
- Regulation of peer-to-peer communication and label exchange
We will now initiate an introduction to the concepts associated with these major operational functions in their own individual sections.