How cspf calculates a traffic-engineered path, For more information, refer to, How cspf calculates a – Brocade Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide (Supporting R05.6.00) User Manual
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Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide
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Traffic engineering database
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How CSPF calculates a traffic-engineered path
Using information in the TED in addition to the attributes and requirements of the LSP, CSPF
calculates a traffic-engineered path for the LSP by performing the tasks listed below.
1. When more than one LSP needs to be enabled, CSPF selects the LSP for path calculation
based on the LSPs setup priority and bandwidth requirement.
When multiple LSPs are enabled simultaneously, such as when the device is booted, CSPF
calculates the paths one at a time. CSPF starts with the LSP that has the highest configured
setup priority. When more than one LSP has the same setup priority, CSPF calculates the path
first for the LSP with the highest configured bandwidth requirement.
2. Eliminate unsuitable links from consideration.
The device examines the topology information in its TED and uses this information to eliminate
links from consideration for the traffic-engineered path. A link is eliminated when any of the
following are true:
•
The link is half duplex
•
The link does not have enough reservable bandwidth to fulfill the LSPs configured
requirements
•
The LSP has an include statement, and the link does not belong to an administrative group
in the statement
•
The LSP has an exclude statement, and either the link belongs to an administrative group
specified in the exclude statement or the link does not belong to any administrative group
at all
3. Using the remaining links, calculate the shortest path through the MPLS domain.
Using the links that were not eliminated in the previous step, the device calculates the shortest
path between the ingress and egress LERs. When the LSP is configured to use an explicit path,
the device individually calculates the shortest path between each node in the path. Refer to
for more information on explicit paths.
By default, the path calculated by CSPF can consist of no more than 255 hops, including the
ingress and egress LERs. The user can optionally change this maximum to a lower number.
Refer to
“Limiting the number of hops the LSP can traverse”
4. When multiple paths have the same cost, select one of them.
The shortest path calculation performed in the previous step may result in multiple, equal-cost
paths to the egress LER. In this case, the device chooses the path whose final node is the
physical address of the destination interface.
When more than one path fits this description, by default, the device chooses the path with the
fewest hops. When multiple paths have this number of hops, the device chooses one of these
paths at random. The user can optionally configure the device to choose the path that has
either the highest available bandwidth or the lowest available bandwidth. Refer to
a tie-breaker for selecting CSPF equal-cost paths”
.
The output of the CSPF process is a traffic-engineered path, a sequential list of the physical
interfaces that packets assigned to this LSP pass through to reach the egress LER. Once the
traffic-engineered path has been determined, RSVP signaling attempts to establish the LSP on
each LSR in the path. Refer to the next section,
“How RSVP establishes a signaled LSP”
description of how this works.