Te Metric And Igp Metric - Nokia 7705 SAR-W Series Manual

MPLS and RSVP-TE
3.2.1.1
When the TE metric is selected for an LSP, the shortest path computation will select
an LSP path based on the TE metric constraints instead of the IGP metric (for OSPF
and IS-IS), which is the default metric. The user configures the TE metric under the
router>mpls>interface context and the IGP metric under the router>ospf>area>
interface context (for OSPF) and the router>isis>if>level context (for IS-IS). Both
the TE and IGP metrics are advertised by OSPF and IS-IS for each link in the
network.
The TE metric is part of the traffic engineering extensions of the IGP protocols. For
more information on the OSPF and IS-IS routing protocols, refer to the
7705 SAR Routing Protocols Guide.
Typically, the TE metric is used to allow Constrained Shortest Path First (CSPF) to
represent a dual TE topology for the purpose of computing LSP paths, where one TE
topology is based on the RSVP-TE database and the other is based on the IGP-TE
database.
An LSP dedicated to real-time and delay-sensitive user and control traffic has its path
computed by CSPF using the TE metric. The user configures the TE metric to
represent the amount of delay, or combined delay and jitter, of the link. In this case,
the shortest path satisfying the constraints of the LSP path will effectively represent
the shortest-delay path.
An LSP dedicated to non-delay-sensitive user and control traffic has its path
computed by CSPF using the IGP metric. The IGP metric could represent the link
bandwidth or some other value as required.
When the use of the TE metric is enabled for an LSP, the CSPF process will first
eliminate all links in the network topology that do not meet the constraints specified
for the LSP path; the constraints include bandwidth, admin-groups, and hop limit.
CSPF will then run the SPF algorithm on the remaining links. The shortest path
among all the SPF paths will be selected based on the TE metric instead of the IGP
metric. The TE metric is only used in CSPF computations for MPLS paths and not in
the regular SPF computation for IP reachability.
Operational metrics of LSPs that use the TE metric in CSPF path calculations can be
overridden with the user-configured administrative LSP metric.
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TE Metric and IGP Metric

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