Perform What-If Analysis

This section contains the following topics:

Examine Failure Scenarios

A frequent question in the planning process is how to test the value of a minor change that may prevent the need for a larger one. For instance, rather than a huge upgrade, a planner may consider whether adding a single link between two sites can mitigate congestion over part of the network.

In Cisco Crosswork Planning, you can examine what happens if you change any aspect of the network model. For example:

  • What happens if a link or a node fails?

  • What happens if you change a metric?

  • What happens if you change the topology?

  • What happens if a new customer or service is added?

Perform Failure Analysis

One of the most useful features of Cisco Crosswork Planning is its ability to show the network behavior when objects fail. For example, your network might show utilizations are well under 100% under normal operation. But what happens under failure?

Cisco Crosswork Planning lets you perform failure analysis, which can be useful in long-term planning and short-term preparation. It simulates reroutings and changes in traffic utilization under individual failure events. For an example of how failure analysis is performed in Cisco Crosswork Planning, see Example: Failure Analysis.

To fail or recover objects, 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

Select an object from its respective table. To see the complete list of objects that can be failed, see the list mentioned in State.

Step 3

Choose the > Fail option under the Actions column.

Once failed, the menu option changes to > Recover. Use this option to recover the objects.

Step 4

Notice the difference in traffic utilization values and how the demand has been rerouted in the network plot.

Example: Failure Analysis

In this example, we determine the effects on the network if a single interface fails.

Procedure

Step 1

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

Step 2

In the network plot (see Before Failing the Interface), notice all traffic utilization is under 100%. You can tell this because none of the interfaces are red, which by default indicates over 100% utilization. Also, the Util sim column in the Interfaces table shows no values over 100.

Step 3

In the Interfaces table, select the cr1.lon-cr1.par interface (LON-PAR circuit) and choose Cross Table Filter icon > Filter to demands > Through all interfaces.

Step 4

Click the demand to see its route. The solid violet arrow indicates the demand path.

Figure 1. Before Failing the Interface

Step 5

In the interfaces table, select the cr1.lon-cr1.par interface and choose > Fail option under the Actions column.

Step 6

Notice how the demand has been rerouted (see After Failing the Interface). The dashed line shows the current demand path under failure.

The red cross on the link indicates a failure.

Step 7

Clear the filter in the Demands table and notice the difference in traffic utilization colors in the plot. For example, the AMS-FRA circuit (cr1.ams-cr2.fra interface) is now congested, as indicated by red color.

Figure 2. After Failing the Interface

Step 8

To restore the failed circuit/interface, select it and choose > Recover option under the Actions column.

Perform Impact Analysis of Topology Changes

A frequent question in the planning process is how to test the value of a minor change that may obviate the need for a larger one. For instance, rather than a huge upgrade, a planner may consider whether adding a single link between two sites can mitigate congestion over part of the network.

Being able to emulate and predict the impact of these changes promotes Service Level Agreement (SLA) adherence and staff efficiency. Making a topology change could impact traffic flows, congestion, and latency. Therefore, knowing the effect such changes would have is valuable for operators, planners, and designers alike, and is critical for those with penalty clauses in customer SLAs.

Cisco Crosswork Planning lets you edit the network topology so that you can add, edit, and delete objects. Once topology changes are made, demands are rerouted, showing the resulting utilization changes in the new network. For an example of how adding a circuit between two sites can help in relieving the congestion, see Example: Impact Analysis of Topology Changes.

Example: Impact Analysis of Topology Changes

In this example, we see how adding a circuit between two sites ("wdc" and "nyc") can help in relieving the congestion.

Procedure

Step 1

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

Step 2

Notice that the link between the sites "wdc" and "nyc" is congested, as indicated by the red interface.

Figure 3. Before Adding a Circuit

Step 3

Add a circuit beween "wdc" and "nyc".

  1. From the toolbar, choose Actions > Insert > Circuit.

    OR

    Go to the Circuits table and click Add icon. If the Circuits tab is not visible, then click the Show/hide tables icon (Show/Hide Tables Icon), select the Circuits check box, and click Apply.

  2. In the Add Circuit window, enter the following details:

    • Circuit name—Enter the name of the circuit.

    • Capacity—Enter the amount of total traffic this circuit can carry. The drop-down list has a selection of the most widely used capacities.

    • Interface A and Interface B panels—Specify two interfaces that are connected by the circuit. In this case, for Interface A, choose the interface belonging to the site, "wdc". For Interface B, choose the one belonging to "nyc".

  3. Click Add.

Step 4

Notice the difference in the traffic utilization color in the link between "wdc" and "nyc". The dark green color indicates that the congestion is relieved. The dotted line indicates the presence of multiple circuits.

Figure 4. After Adding a Circuit

If congestion had not been relieved, a larger circuit or different metric could have been analyzed with a simple change to the circuit properties in the UI.

Perform Impact Analysis of Metric Changes

Cisco Crosswork Planning lets you update a metric and helps you analyze how it affects the network. For an example of how changing the IGP metric of an interface impacts the utilization, see Example: Impact Analysis of Metric Changes.

To optimize the metric of multiple interfaces, use the Metric optimization and Tactical metric optimization tools. For details, see Optimize Metrics in the Network Core.

Example: Impact Analysis of Metric Changes

In this example, we see how changing the IGP metric value of the interfaces (between "kcy" and "hst" sites) changes the traffic utilization.

Procedure

Step 1

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

Step 2

Notice that the color of the link between the sites "kcy" and "hst" is dark green, which indicates that the traffic utilization is between 0-30%.

Figure 5. Impact of Changing the IGP Metric

Step 3

Select this link. In the Interfaces table, the interfaces belonging to this circuit is selected.

Step 4

Click Edit icon.

Step 5

In the IGP metric field, change the value from 63 to 2.

Step 6

Notice the difference in the color of the link between "kcy" and "hst". The blue color indicates that the traffic utilization on these interfaces has increased.