RouteExplorer™ Tutorial – Monitoring and Alerts

See which routers are advertising default routes in your multi-domainnetwork

Advertisement of default routes in your network can be veryuseful.  But misconfigured defaultscan cause unreachable destinations or dropped packets. 

 

By advertising default routes from one or more BGP borderrouters you can avoid configuring default gateways throughout the network.Additionally, multiple advertisements of default from different gateways can bebeneficial for redundancy and as a means of load balancing and trafficengineering between ASes.  Butmisconfigured default routes can disrupt reachability and change the trafficbalance at peering points.  In evena modest size network, verifying which routers are advertising default route inthe IGP and BGP can be difficult.

 

Route Explorer always knows where default routes areadvertised throughout your network and can show them instantly. Figure1illustrates how Route Explorer can show a complete prefix list for all yournetwork domains from its Tools menu (at left).  At right, the example shows that this network has tworouters advertising default with different IGP metrics and type.  The first router (with lower metric) ispreferred.  But the secondary isalso present and available, should the primary router go down.

 

  

Figure 1

In the next example (Figure 2),the prefix list shows both IGP and BGP default routes.  Again, two IGP core routers areadvertising default for redundancy, but with equal cost.  Also for redundancy, two border routersare advertising BGP default, again with equal cost in terms of Local-Pref andMED attributes.  To locate eachrouter advertising default in the topology map, simply click on the prefixentry in the list and the corresponding source router(s) will be highlighted inflashing yellow (see Figure 2at right).

 

Figure 2

HOW TO:

  1. Open the Route Explorer UI
  2. Open an online and recording network topology
  3. Click on Tools->List Prefixes.  The prefix list will appear.  The list should be sorted in ascending order by prefix.  If not, simply click on the “Prefix” heding of the first column
  4. The default prefix (0.0.0.0/0) should appear at the top.  If none, your network does not advertise default.
  5. To highlight the routers advertising the defaults in the topology map, simply click on the row in the list.
  6. To show all routers advertising defaults everywhere in your network, click on the top most row of the default prefixes.
  7. If you have a multi-protocol network, defaults in all monitored protocols should appear.
  8. To see only defaults in a particular protocol e.g. BGP, click on the “Filter By” drop down at the top of the window. 
  9. Select the protocol item.
  10. Click on the protocol desired. And click “Show”
  11. To show all prefixes again, use the “Filter By” menu and select “Any”.  Click “Show”.

 

 

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