Discussion

LDP discovers neighbors by sending Hello messages, multicasting them to 224.0.0.2
on UDP port 646. After discovering a neighbor, LDP establishes a TCP connection to
the neighbor and exchanges information regarding labels and routes (called forwarding
equivalence classes [FRCs]) associated with the labels. All packets associated with
the same FEC are treated in the same way by the router. In general, this means that
the packets are sent to the same next hop. The use of TCP ensures reliable delivery of
label information. LDP sends periodic keepalive messages to maintain the TCP connection.
Each LDP router updates its forwarding-path information independently as
it tracks the state of the IGP.
RSVP was developed before MPLS and was designed to create bandwidth reservations
for individual traffic flows. RFC 3209 extends RSVP to allow it to create and
maintain MPLS LSPs and to reserve the bandwidth needed for LSPs. The extensions
are called RSVP-TE. In the rest of this chapter, when we say RSVP, we mean RSVP
with TE extensions. Unlike LDP, which uses the IGP’s shortest path as the transit
path for the LSP, RSVP works with the Constrained Shortest Path First (CSPF) algorithm
to determine the LSP’s path. On the ingress router, CSPF computes the path of
the LSP (the Explicit Route Object [ERO]) and passes the path to RSVP. RSVP then
signals to set up the path. This path-determination mechanism allows RSVP-signaled
LSPs to be used in MPLS-based traffic engineering to explicitly control the path taken
by traffic between specific points in the network. Unlike LDP, which is limited to a
single IGP domain, RSVP-signaled LSPs can cross AS boundaries.
When the ingress router initiates an RSVP-signaled LSP, it sends an RSVP Path message
with the destination address of the egress router. This message also contains
several objects, including the:
Label Request Object (LRO)
Requests an MPLS label for the path
ERO
Contains the addresses of the routers along the LSP
Record Route Object (RRO)
Records the path of the LSP
Sender TSpec
Requests a bandwidth reservation for the LSP
The egress router responds to a Path message by sending a Resv message that contains
the label to be used for the LSP (in the label object) and a record of the path
taken by the Resv message. In turn, the penultimate router sends a Resv message to
its upstream router containing the label value of its choice inside the label object,
because each hop to the downstream router chooses the label value to be used by the
upstream router to forward packets on that hop. This message-exchange scheme
means that an RSVP-signaled LSP requires configuration only on the ingress router.
RSVP periodically sends Path and Resv messages along the LSP to maintain its state.
484 | Chapter 14: MPLS
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