Propagating default route and route summarization in OSPF

Default route

To redistribute default route int ospf you have two choices:

  1. Use command
    default-information originate
  2. Use command
    default-information originate always

Difference between them is that in case you put „alway“ after the command you will advertise default route also in case you dont have any configured. The metric for default network is 1 and it does not change through the path! But you can change the type via the default-information originate metric-type command.

Summarization

Route summarization is consolidation of mutliple routes into single advertisement. Summary LSAs ( type 3) and external LSAs (type 5) by default do not contain summarized routes. There are two types of summarization:

  1. Interarea route summarization – occurs at ABRs
  2. External route summarization – occurs at ASBRs

OSPF cannot summarize at any router like in EIGRP. OSPF can summarize for external routes only at ASBRs and for regular routes only at ABRs.

Interarea OSPF route summarization

This is done via command area area-id range address mask [advertise|not-advertise][cost cost]. Only ABRs can summarize networks that are within its areas (LSA type 1,2) Not all ABRs can summarize all networks (For example LSA3,5 networks). When you do the summ ios automatically creates the null0 interface as a loop prevention. Imagine that one interface regarding the network goes down but summary route is still advertising. In case there is back routing to the source via def. root for example, you got routing loop in network. In the command advertise word is default, not-advertise is when you dont wanna advertise this summary route and cost you specify metric – default is 1. Metric type used by this summarization is type 1 with considering the path metric (like regular LSA 3)

External OSPF route summarization

Each external route advertised by different protocol is advertised individually with an external LSA. Summarization can be done by ASBRs for type 5 LSAs (external routes). A null0 interface is automatically created for each summary range.

Use command summary-address ip-address mask [not-advertise] [tag tagto instruct ASBR for summarizing external routes before inserting them int OSPF.

Note: In NSSA ABR can summarize external routes when it creates type 5 summary LSA from a type 7 LSA representing external route.

 

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