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LSP Routing Behavior

 

You can configure NorthStar Controller to automatically reroute LSPs based on interface traffic or link delay conditions. To access these configuration parameters, navigate to Administration > Analytics.

To use LSP Rerouting, you must select Reroute: Enabled. For LSP rerouting based on link utilization (bandwidth), you can then specify a reroute interval (in minutes) and a link utilization threshold (%). The reroute interval is used to pace back-to-back rerouting events. LSPs are rerouted when both of the following conditions are true:

  • A link utilization threshold has been crossed.

  • No previous utilization-triggered reroute has occurred within the configured reroute interval (in this sense, this timer specifies the minimum time interval between successive reroute actions).

For delay-based rerouting, the Link Delay Increase parameter controls when the LSP delay calculation (and reroute) are triggered. Only if the delay measured on a link increases by more than the link delay increase value (milliseconds), are the LSPs re-optimized.

Note

For delay-based rerouting, the LSPs must also be configured with a Max Delay constraint (on the Provision LSP window, Design tab).

Figure 1 shows the Provision LSP Design tab. The thresholds use the delay information to derive the metrics of the LSPs, which are, in turn, used by the devices when choosing which LSPs to use to forward traffic to a given destination.

Figure 1: Provision LSP, Design Tab Showing Delay Thresholds
Provision
LSP, Design Tab Showing Delay Thresholds

Max Delay is used by the NorthStar Path Computation Server (PCS) to constrain the routing path of an LSP. If this constraint is not met, the LSP is not routed by PCS. Max Delay is also used by the NorthStar Telemetry module to trigger LSP rerouting.

High Delay Threshold is used to penalize the LSP so it is not used by the data plane as long as there are other parallel LSPs with lower metrics. The availability of the LSP is not restored once the delay is lower than the High Delay Threshold, until the LSP delay reaches Low Delay Threshold. This prevents excess impact on the network. When the LSP delay drops below the Low Delay Threshold, its metric is set to Low Delay.

Figure 2 shows the LSP Routing Behavior window (Administration > Analytics).

Figure 2: LSP Routing Behavior
LSP Routing Behavior
Note

The link utilization threshold is currently defined on a global level and cannot be configured on a per-link basis. When utilization for a link crosses a configured threshold, it appears in the Timeline as an event, as does any subsequent rerouting.

Table 1 summarizes the Analytics parameters that affect LSP routing behavior.

Table 1: Analytics Parameters Affecting LSP Routing Behavior

Parameter

Description

How to Access

Reroute Interval

User-defined, global parameter applied to both Layer 3 link utilization and LSP delay violations. It is the minimum interval after which the controller reacts to any traffic/delay violations. The minimum value is 1 minute and there is no maximum. The smaller the value, the higher the number of rerouting processes, and consequently, the greater the impact on the network. It is a mandatory parameter to trigger a Layer 3 link utilization violation or LSP delay violation rerouting process.

Administration > Analytics

Link Utilization Threshold

User-defined, global parameter applied to all links for Layer 3 link utilization violation scenarios. Whenever this threshold is exceeded, the controller will starts moving LSPs away from the congested links. It is a mandatory parameter to enable this controller behavior when Layer 3 link utilization violations occur. Once the link utilization crosses the defined threshold and no previous rerouting processes have occurred within the defined Reroute Interval, the rerouting process is triggered.

Administration > Analytics

Link Delay Increase

User-defined, global parameter applied to all the links. The controller continuously monitors the link delays and computes the delta for all links. The delay increase is the absolute difference between two consecutive received link delays. It is a mandatory parameter to enable this controller behavior when LSP delay violations occur.

To reduce unnecessary LSP delay computation, the PCS server calculates all LSPs delays only when this delta is exceeded. If any LSP calculated delay exceeds its own Max Delay settings, and no previous rerouting process has occurred within the defined Reroute Interval, then the controller attempts to perform LSP rerouting.

Note: LSP delay is the sum of all the delays of the links that belong to the LSP routing path. The controller does not directly monitor LSP delays.

Administration > Analytics

Max Delay

User-defined, local parameter applied to each LSP. It is a mandatory parameter to trigger any LSP delay violation rerouting process. When an LSP is configured with a Max Delay, and there is also a global link utilization threshold value, the controller checks the LSP upon both Layer 3 link utilization and LSP delay violations.

Applications > Provision LSP (Design Tab), or modify an existing tunnel from the network information table by selecting the tunnel row and clicking Modify at the bottom of the window.

The REST API can also be used.