Whereas IGPs use the Shortest Path First (SPF) algorithm to
determine how traffic is routed within a network, RSVP uses the Constrained
Shortest Path First (CSPF) algorithm to calculate traffic paths that
are subject to the following constraints:
LSP attributes—Administrative groups such as link
coloring, bandwidth requirements, and EROs
Link attributes—Colors on a particular link and
available bandwidth
These constraints are maintained in the traffic engineering
database (TED). The database provides CSPF with up-to-date topology
information, the current reservable bandwidth of links, and the link
colors.
In determining
which path to select, CSPF follows these rules:
Computes LSPs one at a time, beginning with the
highest-priority LSP—the one with the lowest setup priority
value. Among LSPs of equal priority, CSPF starts with those that have
the highest bandwidth requirement.
Prunes the traffic engineering database of links
that are not full duplex and do not have sufficient reservable bandwidth.
If the LSP configuration includes the include statement, prunes all links that do not share any included colors.
If the LSP configuration includes the exclude statement, prunes all links that contain excluded colors. If a link
does not have a color, it is accepted.
Finds the shortest path toward the LSP's outbound
router, taking into account any EROs. For example, if the path must
pass through Router A, two separate SPF algorithms are computed: one
from the inbound router to Router A and one from Router A
to the outbound router.
If several paths have equal cost, chooses the one
with a last-hop address the same as the LSP's destination.
If several equal-cost paths remain, selects the
path with the fewest number of hops.
If several equal-cost paths remain, applies CSPF
load-balancing rules configured on the LSP.
