3Com S7902E, Руководство по настройке

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The mechanism setting up and managing constraints is called Constraint-based Routing (CR).
CR-LSP involves these concepts:
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Strict and loose explicit routes
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Traffic characteristics
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Preemption
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Route pinning
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Administrative group and affinity attribute
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Reoptimization
Strict and loose explicit routes
An LSP is called a strict explicit route if all LSRs along the LSP are specified.
An LSP is called a loose explicit route if the downstream LSR selection conditions rather than LSRs are
defined.
Traffic characteristics
Traffic is described in terms of peak rate, committed rate, and service granularity.
The peak and committed rates describe the bandwidth constraints of a path while the service granularity
specifies a constraint on the delay variation that the CR-LDP MPLS domain may introduce to a path's
traffic.
Preemption
CR-LDP signals the resources required by a path on each hop of the route. If a route with sufficient
resources cannot be found, existing paths may be rerouted to reallocate resources to the new path.
This is called path preemption.
Two priorities, setup priority and holding priority, are assigned to paths for making preemption decision.
Both setup and holding priorities range from 0 to 7, with a lower numerical number indicating a higher
priority.
For a new path to preempt an existing path, the setup priority of the new path must be greater than the
holding priority of the existing path. To initiate a preemption, the Resv message of RSVP-TE is sent.
To avoid flapping caused by improper preemptions between CR-LSPs, the setup priority of a CR-LSP
should not be set higher than its holding priority.
Route pinning
Route pinning prevents an established CR-LSP from changing upon route changes.
If a network does not run IGP TE extension, the network administrator will be unable to identify from
which part of the network the required bandwidth should be obtained when setting up a CR-LSP. In this
case, loose explicit route (ER-hop) with required resources is used. The CR-LSP thus established
however, may change when the route changes, for example, when a better next hop becomes available.
If this is undesirable, the network administrator can set up the CR-LSP using route underpinning to
make it a permanent path.
Administrative group and affinity attribute
The affinity attribute of an MPLS TE tunnel identifies the properties of the links that the tunnel can use.
Together with the link administrative group, it decides which links the MPLS TE tunnel can use.