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Tutorial
Advanced MPLS Signaling
(continued)
Some early arguments
against RSVP included the problem of scalability: the more paths
that were established, the more refresh messages would be created,
and the network would soon become overloaded with refresh messages.
Methods of addressing this problem include not allowing the traffic
links and paths to become too granular, and aggregating paths.
To view an example of an RSVP-TE path request for yourself,
you can download a protocol analyzer and sample file from www.ethereal.com.
After downloading, install ethereal and open the MPLS-TE.Cap file.
In the sample below (Figure 9), MPLS captures MPLS-TE files. In the
capture, we can see the traffic specifications (TSPEC) for the
controlled load.
See
a large view of this graphic
Figure 9:
RSVP-TE Details
CR-LDP
With CR-LDP
(Constraint-based Routing over Label Distribution Protocol),
modifications were made to the LDP protocol to allow for traffic
specifications. The impetus for this design was to use an existing
protocol LDP and give it traffic-engineering capabilities.
A major effort by Nortel Networks was made to launch the
CR-LDP protocol.
The CR-LDP protocol adds
fields to the LDP protocol. They are called peak, committed, and
excess-data rates – very similar to terms used for ATM networks.
The frame format is shown in Figure 10.
Figure 10:
CR-LDP Frame Format
The call set-up procedure
for CR-LPD is a very simple two-step process: a request and a map,
as shown in Figure 11. The
reason for the simple set-up is that CR-LPD is a hard-state protocol
– meaning that the call, link, or path, once established, will not
be broken down until it is requested that it be done.
Figure 11:
CR-LDP Call Set Up
The major advantage of a
hard-state protocol is that it should be more scaleable, because
there is less “chatter” needed in order to keep the link active.
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