RFC 3785 (rfc3785) - Page 2 of 8
Use of Interior Gateway Protocol (IGP) Metric as a second MPLS Traffic Engineering (TE) Metric
Alternative Format: Original Text Document
RFC 3785 IGP Metric as a second MPLS TE Metric May 2004
These IGP routing protocol extensions currently include advertisement
of a single additional MPLS TE metric to be used for Constraint Based
Routing of TE tunnels.
However, the objective of traffic engineering is to optimize the use
and the performance of the network. So it seems relevant that TE
tunnel placement may be optimized according to different optimization
criteria. For example, some Service Providers want to perform
traffic engineering of different classes of service separately so
that each class of Service is transported on a different TE tunnel.
One example motivation for doing so is to apply different fast
restoration policies to the different classes of service. Another
example motivation is to take advantage of separate Constraint Based
Routing in order to meet the different Quality of Service (QoS)
objectives of each Class of Service. Depending on QoS objectives one
may require either (a) enforcement by Constraint Based Routing of
different bandwidth constraints for the different classes of service
as defined in [DS-TE], or (b) optimizing on a different metric during
Constraint Based Routing or (c) both. This document discusses how
optimizing on a different metric can be achieved during Constraint
Based Routing.
The most common scenario for a different metric calls for
optimization of a metric reflecting delay (mainly propagation delay)
when Constraint Based Routing TE Label Switched Paths (LSPs) that
will be transporting voice, while optimizing a more usual metric
(e.g., reflecting link bandwidth) when Constraint Based Routing TE
LSPs that will be transporting data.
Additional IGP protocol extensions could be defined so that multiple
TE metrics could be advertised in the IGP (as proposed for example in
[METRICS]) and would thus be available to Constraint Based Routing in
order to optimize on a different metric. However this document
describes how optimizing on a different metric can be achieved today
by existing implementations and deployments, without any additional
IGP extensions beyond [ISIS-TE] and [OSPF-TE], by effectively using
the IGP metric as a "second" TE metric.
2. Common Practice
In current MPLS TE deployments, network administrators often want
Constraint Based Routing of TE LSPs carrying data traffic to be based
on the same metric as the metric used for Shortest Path Routing.
Where this is the case, this practice allows the Constraint Based
Routing algorithm running on the Head-End LSR to use the IGP metric
advertised in the IGP to compute paths for data TE LSPs instead of
the advertised TE metric. The TE metric can then be used to convey
Le Faucheur, et al. Best Current Practice