Optimize Segment Routing

The SR-TE optimization tool creates or updates segment lists to minimize the sum total of user-specified metrics for selected SR LSPs, including inter-AS LSPs, using the fewest number of segment list hops as possible. The tool sets the routes based on traffic engineering criteria and optimizes a sequence of node or adjacency hops for the SR LSP paths to follow. For more information, see Optimize SR-TE.

The SR-TE bandwidth optimization tool lets you reduce the traffic utilization of selected interfaces to below a specified threshold. This ability to mitigate congestion is useful when planning for increased traffic and when determining if service-level agreements can be met in the event of congestion. Additionally, you can use this tool for reoptimizing bandwidth after events, such as network failures and subsequent route reconvergence. For more information, see Optimize and Analyze SR-TE Bandwidth.

This section contains the following topics:

Optimize SR-TE

You can use the SR-TE optimization tool (Actions > Tools > SR LSP optimization > SR-TE optimization) to design, capacity plan, and manually configure networks to meet the following objectives. This tool supports both Inter-Area and Inter-AS functionalities.

TE Metric or Delay Minimization—Minimize distance between hops with respect to metrics other than IGP metrics. These can be either TE metrics configured on interfaces (which can be set proportional to circuit latency) or latencies (delays) for each circuit. An example application is a differentiated service in which latency-sensitive network traffic is routed on shortest latency paths, while the bulk of the traffic routes over cost-optimized paths.

Avoidance—Create or optimize segment lists so they avoid routing through specified objects (nodes, interfaces, or SRLGs). An example application is routing pairs of LSPs, each over a different plane in a dual-plane network. The same traffic is routed over both LSPs simultaneously, thus improving availability.

While there is an option to specify a maximum number of hops, doing so might not achieve the lowest possible latency. In this case, the best achievable solution is provided.

You can additionally avoid unnecessary LSP churn by specifying boundaries (bounds) on the path length and margins within which the shortest path must be optimized.

Note

Unless qualified with “TE” or “IGP,” the term metric in this chapter applies to IGP metric, TE metric, or delay.

Specify Inputs for SR-TE Optimization

Optimize the Path Metric

The Minimize path metric section defines whether to optimize SR LSPs based on interface IGP metrics, interface TE metrics, or circuit delays. This minimization is for the sum of the metrics along the path. Note that for inter-AS SR LSPs, these metrics are calculated end-to-end for the LSP, not per AS. These properties are configurable from the Edit Interface window, and delays can also be set in the Edit Circuit window.

Bound and Margin

Bounds and margins identify which paths to optimize, as well as when to stop optimizing a given path. If you enter a value for more than one restriction, Cisco Crosswork Planning uses the strictest limitation as the optimization target. If there is no bound or margin specified, Cisco Crosswork Planning optimizes LSP paths to the best possible solution (lowest total metrics for the LSP path).

Bound: Maximum Acceptable Path Length

The Fixed Bound entry in the Bound on path length section lets you set the maximum path metric that is acceptable. Cisco Crosswork Planning tries to optimize LSP paths with metrics that exceed this bound. If a solution adhering to this bound cannot be found, the best possible solution is provided and bound violations are listed in the report. LSP paths that are less than or equal to this bound are not optimized.

Example: If you select to optimize LSP paths based on TE metrics, the value entered is 50, and the sum of TE metrics on the LSP path is 51, that LSP path is optimized.

Enter a number based on the selected path metric. A TE metric is a property values whose total sum for the LSP path cannot be exceeded. The delay is also a property value, but it is in milliseconds (ms). If you enter “50” and you have selected delay as the metric to optimize, this represents 50 ms as the maximum acceptable delay for the LSP path.

Margin: Maximum Acceptable Metric Above Shortest Path

The Margin entries let you identify the acceptable deviation above the shortest achievable path metric. Any existing LSP path with a metric that is less than or equal to the shortest path metric plus the margin is not optimized.

Fixed—The amount by which a metric must be surpassed before it is optimized.

Example: If an SR LSP route has a delay of 110, a fixed margin set to 10, and the shortest achievable delay path is 100, the current SR LSP is within the margin and will not be updated. If the fixed margin is set to 9, the SR LSP would be optimized.

100 (shortest path) + 10 (fixed margin) = 110, so all paths greater than 110 are optimized.

Percentage—The amount by which a metric must be surpassed, expressed as a percentage of the shortest path, before it is optimized.

Example: If an existing SR LSP route has a TE metric of 210, a percentage margin set to 10%, and the shortest achievable TE metric path is 200, the current SR LSP is within the margin and will not be updated. If the current SR LSP had a metric of 225, it would be optimized.

200 (shortest path) x .10 (percentage margin) = 20, so all paths greater than 220 must be optimized

Example: If an SR LSP route has a delay of 110, a fixed bound is set to 120, a fixed margin is set to 15, a percentage margin is set to 5%, and the shortest achievable delay path is 100, the strictest of these restrictions takes precedence, and the SR LSP is optimized.

Fixed Bound = 120

100 (shortest path) + 15 (fixed margin) = 115

100 (shortest path) x .05 (percentage margin) = 5, so all paths greater than 105 are optimized because it is the strictest margin.

Constraints

The constraints let you specify restrictions for the optimizations.

Maximum segment list hops per SR LSP—The maximum number of segment list hops that any given segment list can contain after optimization. If no value is specified, Cisco Crosswork Planning creates as many hops as needed to optimize the SR LSP.
Avoid—Do not permit optimized segment lists to route through the selected objects (nodes, interfaces, or SRLGs). This constraint is useful when modeling dual-plane topologies that route disjoint LSPs.
Restrict segment node to core nodes—Segment list node hops must be core nodes (nodes that have their Function property set to “core”), and the local node of segment list interface hops must be a core node. An SR LSP could still route using edge nodes if they are not used as hops.

Run SR-TE Optimization

To run the SR-TE optimization tool, do the following:

Procedure

Step 1

Open the plan file (see Open Plan Files). The plan file opens in the Network Design page.

Step 2

From the toolbar, choose any of the following options:

Actions > Tools > SR LSP optimization > SR-TE optimization

OR
Preset workflows > Perform optimization, select SR LSP Optimization as the optimization type, choose SR-TE optimization from the drop-down list, and click Launch.

Step 3

Choose the LSPs you want the optimizer to consider.

Step 4

Click Next.

Step 5

In the Minimize path metric section, choose whether to optimize SR LSPs based on interface IGP metrics, interface TE metrics, or circuit delays. For more information, see Optimize the Path Metric.

Step 6

In the Bound on path length and Margin above shortest path sections, specify the values as per your requirement. For more information, see Bound and Margin.

Step 7

In the Constraints section, specify any restrictions for optimization. For more information, see Constraints.

Step 8

Click Next.

Step 9

(Optional) In the Tag updated LSPs with field, override the defaults for how LSPs are tagged (SROpt).

Step 10

On the Run Settings page, choose whether to execute the task now or schedule it for a later time. Choose from the following Execute options:

Now—Choose this option to execute the job immediately. The tool is run and changes are applied on the network model immediately. Also, a summary report is displayed. You can access the report any time later using Actions > Reports > Generated reports option.

As a scheduled job—Choose this option to execute the task as an asynchronous job. If you choose this option, select the priority of the task and set the time at which you want to run the tool. The tool runs at the scheduled time. You can track the status of the job at any time using the Job Manager window (from the main menu, choose Job Manager). Once the job is completed, download the output file (.tar file), extract it, and import the updated plan file into the user space to access it (for details, see Import Plan Files from the Local Machine).

Note

Ensure that you save the plan file before you schedule the job. Any unsaved changes in the plan file are not considered when you run the tool as a scheduled job.

Step 11

(Optional) If you want to display the result in a new plan file, specify a name for the new plan file in the Display results section.

In the previous step:

If you have selected to run the task immediately, by default, the changes are applied on the current plan file. If you want to display the results in a new file, select the Display results in a new plan file check box and enter the name of the new plan file.
If you have scheduled the task to run at a later time, by default, the results are displayed in the Plan-file-1. Update the name, if required.

Step 12

Click Submit.

Optimization Report

Upon completion, Cisco Crosswork Planning writes a report containing the results of the optimization, as well as verifications that the results met the criteria for avoiding nodes and meeting the specified bounds. To access this information later, choose Actions > Reports > Generated reports and click the Segment Route TE Optimization report link in the right panel.

Optimize and Analyze SR-TE Bandwidth

The goal when reducing traffic is to reroute as few demands as possible. This is achieved by creating private SR LSPs for demands. The SR-TE BW optimization tool also creates LSP paths and segment lists for these LSP paths with the fewest node or interface segment list hops possible, with a maximum of three hops. The final hop is either a node hop or interface whose remote node is the destination of the LSP.

The SR-TE BW optimization tool operates on a set of constraints that determine which paths are selected, whether demands and LSPs can be rerouted, and whether demand latency bounds can be surpassed. If the traffic cannot be reduced to below the threshold given these constraints, private SR LSPs are still created to route demands to reduce congestion as much as possible. If interface utilization surpasses the threshold prior to the optimization, it does not go higher. Utilization of non-congested interfaces might increase, though these will not surpass the threshold.

The tool reroutes existing SR LSPs only if they are private to a demand.

Note

The SR-TE BW optimization tool supports Inter-Area functionality.

Select Operating Modes for SR-TE Bandwidth Optimization

You can run the SR-TE BW Optimization tool in two modes:

Analysis—The SR-TE BW optimization analysis tool performs multiple optimizations, one for each failure scenario within the specified failure set. The results of these optimizations are aggregated in a report. For details on how to run this tool, see Analyze Congestion Under Different Failure Sets.
Operation—The SR-TE BW optimization operation tool performs an optimization using a specified set of constraints. For details on how to run this tool, see Optimize Bandwidth.

An output plan file is produced with the optimization results. Only a single state of the network is considered. For example, if no objects are failed in the input plan file, the optimization is performed for normal operation. If a circuit is failed in the input plan file, the optimization is performed considering this particular failure scenario.

Specify Inputs for SR-TE Bandwidth Optimization

Interface Utilization Thresholds

You can specify utilization threshold of an interface using the SR-TE BW optimization tool. If the utilization for an interface is larger than this threshold value, then that interface is considered to be congested, and the tool reduces traffic on the link.

Global utilization threshold—Use this field to specify the global threshold value which applies to all interfaces on the network.
Utilization threshold tables file—Use this option to upload Utilization threshold tables file which is a .txt file. This file contains the threshold value for a specific interface.

The file contains a table called <InterfaceThresholds> which has the threshold values defined and separated using tabs. The table has three columns namely Node, Interface, and Threshold.

A sample table is shown below:
```
<InterfaceThresholds>
Node	Interface	Threshold
cr1.nyc	to_cr2.wdc	70
```
This above sample table specifies that utilization on interface to_cr2.wdc from cr1.nyc must be less than 70 percent.

Use the Browse button to upload the Utilization Threshold Tables file.

Uploading this file is optional. But when the interface threshold is specified using this file, the tool aims to reduce the utilization on this link to the specified threshold.

Rerouting Demands

Both the analysis and operation modes use the following options for rerouting demands:

Traffic steering mode—If you choose Individual Demands, each demand is individually carried by a private LSP. If you choose Autoroute, multiple demands can be carried by a non-private Autoroute LSP.
Maximum demands split—Use this option to split demands into smaller demands. If n is the value entered, the demands are split into 'n+1' smaller demands. The default value is 0.
Fix demands—Selected or tagged demands are not rerouted. This constraint is useful, for example, when you have previously optimized specific LSPs within the network and want to maintain the route of one or more existing demands.
Enforce latency bounds—If checked, demands cannot surpass their configured latency bound. This property is set in the Edit Demand window. If the latency of rerouted demands exceeds the configured bound, the tool does not reroute demands on congested interfaces.

Constraints

Both the analysis and operation modes use the following options for specifying bandwidth constraints:

Path Selection—When choosing demand routes, the SR-TE BW Optimization tool uses one of the following criteria:
- Maximize available BW—Paths are optimized to achieve the highest available bandwidth on interfaces (Capacity Sim - Traff Sim).
- Minimize metric—Paths are optimized to minimize the sum of the metrics along the path with respect to Delay, TE Metric, or IGP Metric. These properties are configurable from an interface Properties window, and delays can also be set in a circuit Properties window.
- Create interface segment hops—If checked, the tool considers interface hops when trying to drive down utilization.
- Enforce SID depth—If checked, the number of hops in the optimized segment lists are bound by the Max SID Depth defined at the source of the lists.
LSPs:
- Create new LSPs—If checked, new private SR LSPs with optimized routing can be created. If unchecked, new LSPs are not created.
  - Midpoint operation mode—Choose:
    - Disabled: Default mode indicating that the new SR LSP source/destination nodes must match demand source/destination nodes.
    - Demand endpoint proximity: Specifies that the new SR LSP source/destination nodes may differ from the demand source/destination nodes. Source/Destination nodes closer to demand endpoints are selected.
    - Congestion proximity: Specifies that the new SR LSP source/destination nodes may differ from the demand source/destination nodes. Source/Destination nodes closer to congestion points are selected.
  - Exclude end node—Nodes selected are not considered as source/destinations nodes for SR LSPs. Default is None. The nodes selected here are ignored if Midpoint operation mode is set to Disabled.
  - LSP metric—When specified, LSPs created by the tool will have their Metric Value set to the specified value.
  - Avoid excluded end nodes—If the option is selected, the specified nodes must not be included in the newly created LSPs.
- Rerouting LSPs
  - Fix LSPs—Controls whether or not LSP routes can be modified. This constraint is useful if you want to reroute existing LSPs to mitigate congestion.

Optimize Bandwidth

To run the SR-TE BW optimization operation tool, do the following:

Procedure

Step 1

Open the plan file (see Open Plan Files). The plan file opens in the Network Design page.

Step 2

From the toolbar, choose any of the following options:

Actions > Tools > SR LSP optimization > SR-TE BW optimization operation

OR
Preset workflows > Perform optimization, select SR LSP Optimization as the optimization type, choose SR-TE BW optimization operation from the drop-down list, and click Launch.

Step 3

Select whether to optimize bandwidth for all interfaces or selected interfaces.

Step 4

Click Next.

Step 5

Specify the utilization threshold of an interface and the routing demand options. For details, see Interface Utilization Thresholds and Rerouting Demands.

Step 6

(Optional) In the Tag updated LSPs with field, override the defaults for how LSPs are tagged (SRBWOpt).

Step 7

Click Next.

Step 8

Specify constraints. See Constraints.

Step 9

Click Next.

Step 10

Select the required segment hop constraints from the Node segment hop and Interface segment hop drop-down lists, and click Next.

Step 11

On the Run Settings page, choose whether to execute the task now or schedule it for a later time. Choose from the following Execute options:

Now—Choose this option to execute the job immediately. The tool is run and changes are applied on the network model immediately. Also, a summary report is displayed. You can access the report any time later using Actions > Reports > Generated reports option.

Note

Ensure that you save the plan file before you schedule the job. Any unsaved changes in the plan file are not considered when you run the tool as a scheduled job.

Step 12

(Optional) If you want to display the result in a new plan file, specify a name for the new plan file in the Display results section.

In the previous step:

If you have selected to run the task immediately, by default, the changes are applied on the current plan file. If you want to display the results in a new file, select the Display results in a new plan file check box and enter the name of the new plan file.
If you have scheduled the task to run at a later time, by default, the results are displayed in the Plan-file-1. Update the name, if required.

Step 13

Click Submit.

Analyze Congestion Under Different Failure Sets

You can perform multiple optimizations, one for each failure scenario within a specified failure set. The results of these optimizations are aggregated in a report. This way, you can evaluate the ability of the optimizer to mitigate congestion under different failure scenarios.

Procedure

Step 1

Open the plan file (see Open Plan Files). The plan file opens in the Network Design page.

Step 2

From the toolbar, choose any of the following options:

Actions > Tools > SR LSP optimization > SR-TE BW optimization analysis

OR
Preset workflows > Perform optimization, select SR LSP Optimization as the optimization type, choose SR-TE BW optimization analysis from the drop-down list, and click Launch.

Step 3

By default, the analysis tool presents you with the bandwidth optimization options. Configure these options, as required. For details. see Optimize Bandwidth.

Step 4

Click Next to go to the Advanced Setting page. Click the Failure set panel.

Step 5

Select the failure sets that you want the optimizer to consider (circuits, nodes, sites, and so on).

Entries appear dimmed if they are not available in your design plan.

Step 6

(Optional) Specify the maximum number of threads.

By default, the optimizer tries to set this value to the optimal number of threads based on the available cores.

Step 7

Select the required segment hop constraints from the Node segment hop and Interface segment hop drop-down lists, and click Next.

Step 8

On the Run Settings page, choose whether to execute the task now or schedule it for a later time. Choose from the following Execute options:

Now—Choose this option to execute the job immediately. The tool is run and changes are applied on the network model immediately. Also, a summary report is displayed. You can access the report any time later using Actions > Reports > Generated reports option.

Note

Ensure that you save the plan file before you schedule the job. Any unsaved changes in the plan file are not considered when you run the tool as a scheduled job.

Step 9

Click Submit.

This runs a simulation analysis and creates the optimization report.

Bandwidth Optimization Report

Upon completion of a basic optimization, the SR-TE BW optimization tool generates a summary report to identify key changes due to the optimization. The tool tags the LSPs with SRBWopt and generates a new plan file with an -SRBWopt suffix. This plan file opens, showing the LSPs table that is filtered to these rerouted (and newly tagged) LSPs. Saving this plan file then simplifies the process of identifying which LSPs to reconfigure in the network. Both the tags and the new plan filenames are editable.

After running the bandwidth optimization, the optimizer creates a summary report of the:

Number of interfaces that exceeded the utilization both before and after the optimization.
Maximum utilization both before and after the optimization.
Number of demands that were rerouted.
Number of LSPs that were created and rerouted.
Average segment list length.
Number of latency bound violations both before and after the optimization.

Cisco Crosswork Planning Design 7.0 User Guide

Bias-Free Language

Book Title