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Shortest Path First
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
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
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.
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