RouteExplorer™ Tutorial – Network Planning and Maintenance

How much do I need to change metrics to affect a change in the trafficflow?

In a previous example, we saw how Route Explorer can provideunique link failure simulation capabilities during the network planning andservice rollout process.  Here weshow how Route Explorer can go further, with simulation capability foradjusting link metrics to affect routing paths.

 

Figure1shows an OSPF network monitored by Route Explorer.  The highlighted route shows the path from the source routerin the top center of the topology. The network has a pair of high capacity core switch/routers, but thisparticular flow is taking a peripheral route, presumably because it is lowercost.  But let us suppose that thistraffic flow is placing an undesirable load on link A shown and it is desirableto steer this flow over link B towards the core of the network.  One way to accomplish this is toincrease the cost (link metric) of link A.

Figure 1

To set the metric of a link right click on the link itselfin Route Explorer’s topology map. In the resulting popup menu, click “Set Metric”.  A popup menu such as that at the leftin Figure 2will be displayed. Enter the new metric you wish to simulate.  In this example, we have used 120 inplace of the existing 100.  TheList of Route/Link Edits displays the changes made (at the right in Figure2).

 

 

Figure 2

As a result of this simulated metric change, Route Explorerrecalculates the highlighted path. Figure 3shows that the new path does indeed follow link B through the core of thenetwork as desired. 

 

Figure 3

Next, we need to find the optimum metric value to affectthis change.  Furtherexperimentation with Route Explorer shows that 110 is that number.(Alternatively, one may simulate the first link going down using Route Exploreras shown above, and examine the link metrics of the secondary route to arrive atthe same result).  At the metricsetting of 110, both paths are of equal cost, as shown in Figure4.  The details of the three ECMP routesare shown in Figure 5.

 

Figure 4

 

Figure 5

Please note that while the three paths shown in Figure5do not look to be “equal cost” in terms of hop count, the total cost of thepaths in terms of the sum of their hop-by-hop link metrics is the same.

 

This example has shown that Route Explorer’s unique linkmetric simulation capability can help network designers experiment with networktuning in real-time, on a production network with virtually no risk.  This in turn can help them respond toshort-term traffic overloading or propose longer-term network changes with highconfidence and reduced risk.

 

HOW TO:

  1. Open an X Windows or VNC session to the Route Explorer.  See Route Explorer User Guide for details.
  2. Click on File->Open Topology
  3. Select the topology domain “UCBJul03a” from menu.
  4. Click Open.
  5. Highlight a route:
    1. Right-click on source router
    2. Click “Route Source” in node pop-up menu
    3. Right-click on destination router
    4. Click “Route Destination” in pop-up
  6. To see the route in hop-by-hop detail, select Tools->List Highlighted Paths
  7. Change metric:
    1. Right-click on link
    2. Click “Set Metric” in link pop-up menu
    3. Enter metrics in resulting dialog and click on “set”
  8. Show all simulated changes: Select Tools->List Router/Link Edits
  9. Restore edits:
    1. Click on “Restore All” in list of edits
    2. Up the individual links or nodes via pop-up menu (right-click on item)

 

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