Protocols

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Finding a transport vehicle with which to build these complex tables is of the utmost concern to network designers. What is needed is a protocol that can carry all the necessary data while being fast, being self-healing, and maintaining excellent reliability.

Label Distribution Protocol, or LDP, was created by design engineers and the MPLS workgroup as a means of addressing such transport needs. This protocol works much like a telephone call: When labels are bound, they remain bound until a command appears 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. Border Gateway Protocol (BGP) and Intermediate System-to-Intermediate System (IS-IS) work well for distributing label information along with routing information.

The LDP, BGP, and IS-IS protocols establish the Label Switch Path (LSP but do little in the service of traffic engineering, because routed traffic can potentially be redirected onto a high-priority LSP, thereby causing congestion.

To overcome this problem, signaling protocols were established to create traffic tunnels (explicit routing) and allow for better traffic engineering. These protocols 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 widely used as of this writing.

Table 2.5:

Protocol	Routing	Traffic Engineering
LDP	Implicit	No
BGP	Implicit	No
IS-IS	Implicit	No
CR-LDP	Explicit	Yes
RSVP-TE	Explicit	Yes
OSPF-TE	Explicit	Yes

Checkpoint 

Choose one of the three terms in parentheses to answer each of the following questions.

1. Traffic tunnels provide for (implicit, explicit, signal-based) routing.

2. The process of relating a label to an (FEC, OSPF, NHLFE) is known as binding.

3. (BGP, IS-IS, OSPF) does not distribute label information with routing information.

4. NHLFE is a (protocol, standard, table) that works within an LSR.

Answers: 1. explicit; 2. FEC; 3. OSPF; 4. table.

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