Establishing paths, Forwarding packets, Cr-lsp – H3C Technologies H3C SR8800 User Manual
Page 54: Strict and loose explicit routes, Traffic characteristics, Preemption
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CSPF first prunes TE attribute incompliant links from the TEDB and then performs SPF calculation to
identify the shortest path to an LSP egress.
Establishing paths
When setting up LSP tunnels, you may use two types of signaling: CR-LDP and RSVP-TE. Both can carry
constraints such as LSP bandwidth, some explicit route information, and color and deliver the same
function.
They are different in that CR-LDP establishes LSPs using TCP while RSVP-TE using raw IP.
RSVP is a well-established technology in terms of its architecture, protocol procedures and support to
services; while CR-LDP is an emerging technology with better scalability.
Both CR-LDP and RSVP-TE are supported on your router.
Forwarding packets
Packets are forwarded over established tunnels.
CR-LSP
Unlike ordinary LSPs established based on routing information, CR-LSPs are established based on criteria
such as bandwidth, selected path, and QoS parameters in addition to routing information.
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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Administrative group and affinity attribute
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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.