Discussion

The main difference between LDP and RSVP is that for LDP, any router in the path
can be the ingress for traffic destined to the LSP tail end, while for RSVP, only the
head-end router can push traffic onto the LSP.
When deciding which signaling protocol to use, you should consider configuration
complexity and features. From a configuration point of view, LDP is easier to configure.
For the initial configuration, you enable LDP on the router’s interfaces and,
when adding new routers to the network, you configure only the new box. For initial
RSVP configuration, you must explicitly configure the LSP on the ingress router
and, when adding a new router, you must explicitly configure all LSPs originating at
that router. Because LSPs are unidirectional, they must also be configured from each
of the other routers toward the new one. From a feature point of view, only RSVP
supports traffic engineering and fast reroute. If these are not required by your network,
LDP is a better choice.
CSPF
RSVP uses the CSPF algorithm when computing paths for LSPs. CSPF is based on
the SPF algorithm used in OSPF and IS-IS route calculations. In addition to network
topology, CSPF considers other factors constraining the LSP computation—such as
link attributes, bandwidth requirements, current bandwidth reservations, and hop
limitations—when choosing paths that minimize congestion, balance traffic, and
avoid node failures.
CSPF uses the router’s traffic engineering database (TED) when computing paths.
The TED maintains information about network topology and current link state. It
learns this information from an IGP, either IS-IS or OSPF, that has been extended to
carry additional information, such as available link bandwidth, that is used in the
CSPF calculation. In the JUNOS IS-IS implementation, traffic engineering extensions
are enabled by default; for OSPF, you must explicitly enable them.
14.1 Configuring LSPs Using LDP as the Signaling
Protocol
Problem
You want to set up an LSP path through an IP network. Instead of setting up the
path manually, you want to use LDP as the signaling protocol to establish and maintain
the path.
Solution
For each LDP-signaled LSP, configure the ingress, transit, and egress routers on the
path. On the ingress router, first configure the interface to support the MPLS address
family:
Configuring LSPs Using LDP as the Signaling Protocol | 485
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