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Constrained-Path LSP Computation
The Constrained Shortest Path First (CSPF) algorithm
is an advanced form of the shortest-path-first (SPF) algorithm used
in OSPF and IS-IS route computations. CSPF is used in computing paths
for LSPs that are subject to multiple constraints. When computing
paths for LSPs, CSPF considers not only the topology of the network,
but also the attributes of the LSP and the links, and it attempts
to minimize congestion by intelligently balancing the network load.
The constraints that CSPF considers include:
- LSP attributes
- Administrative groups (that is, link color requirements)
- Bandwidth requirements
- Explicit route (strict or loose)
- Hop limitations
- Priority (setup and hold)
- Link attributes
- Administrative groups (that is, link colors assigned to
the link)
- Reservable bandwidth of the links (static bandwidth minus
the currently reserved bandwidth)
The data that CSPF considers comes from the following
sources:
- Traffic engineering database (TED)—Provides CSPF
with up-to-date topology information, the current reservable bandwidth
of links, and the link colors. For the CSPF algorithm to perform its
computations, a link-state IGP (such as OSPF or IS-IS) with special
extensions is needed. For CSPF to be effective, the link-state IGP
on all routers must support the special extensions. While building
the topology database, the extended IGP must take into consideration
the current LSPs and must flood the route information everywhere.
Because changes in the reserved link bandwidth and link color cause
database updates, an extended IGP tends to flood more frequently than
a normal IGP. See Figure 3 for a diagram of
the relationships between these components.
- Currently active LSPs—Includes all the LSPs that
should originate from the router and their current operational status
(up, down, or timeout).
Figure 3: CSPF Computation Process
This section discusses the following topics:
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