Forwarding packets, Cr-lsp, Strict and loose explicit routes – H3C Technologies H3C S7500E Series Switches User Manual
Page 86: Traffic characteristics, Preemption, Route pinning
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Only RSVP-TE is supported on S7500E series.
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:
z
Strict and loose explicit routes
z
z
z
z
Administrative group and affinity attribute
z
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