Dec 302012

Each OSPF routers have a unique identification number which represents the router to the OSPF domain. If the ID is configured using the router-id command, it will be preferred over any highest loopback IPv4 address.

When looking out at the detailed output of a route in the routing table, the advertising routers RID will be displayed. Thus looking at the RIB we can determine to which router the network is attached.

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Jun 042012

R4 does redistribution of the network as a Type-7 LSA with metric-type of 1. In this case the forwarding address will be an interface attached to R4. R1 and R2 which are Border routes will retain the forwarding address when translating Type-7 to Type-5 LSA.

The forwarding address will be:

  • The highest IP address from among the loopback interfaces on which OSPF is activated
  • If there are no loopback interfaces, then the IP address of the physical interface on which OSPF is activated

With this theory, let’s check the current packet flow towards the external network. Each interface in the topology has a cost of 1.
Route calculation for R1: R3 which has a higher RID ( than R1 ( will do the Type-5 translation. R1 will get a Type-5 route and a Type-7 route each having a cost of 20. Then it calculates the cumulative cost to reach the forwarding address which will be 22 for both. Thus R1 will have an E1 route in its RIB.

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Jun 032012

An external route could be injected into OSPF domain either as a Type-5 or Type-7. The Type-7 route will be injected by an ASBR from a NSSA area. The Type-5 route will be injected by an ASBR from any normal area.

A Type-7 LSA will be translated to Type-5 by an NSSA ABR and flooded it to the whole OSPF domain. If there are multiple NSSA ABRs, the one with higher RID will do the translation. The forwarding address and the metric-type will remain the same as the Type-7 route. If the forwarding address is non-zero, then metric / cost to the forwarding address is accounted or else the metric / cost is to reach the advertising router (ASBR).

If there are multiple external paths to an identical destination, the NSSA ABR will use the following rules for path selection.

1.       E1 and N1 routes are preferred over E2 or N2
2.       If both are Type-1 or Type-2 routes, the route with the lower metric / cost will be installed in the RIB
3.       If both are Type-1 or Type-2 routes, and both routes have the same metric / cost E1 will be preferred over N1 and E2 will be preferred over N2
4.       If maximum-paths 1 command is configured, the route advertised from highest RID will be considered

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May 222012

As all routers in an area must have the same copy of the database and each route builds its RIB by running SPF algorithm , Intra area routes can only be filtered locally on a router.

Type  1 LSA Filtering can be done in 2 different ways:

  • Changing the Administrative Distance of the route
  • Using the distribute-list command

Case 1: The loopback networks of R1 must be filtered from entering the RIB of R2. The network must be reachable via R1 and R3.


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