You have been introduced to the basic concepts of OSPF.
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Open Shortest Path First (OSPF) is an Interior Gateway Protocol (IGP) that is based on the SPF algorithm. It was designed for use on the Internet. The OSPF specification is in the public domain, thus making it an open protocol.

OSPF routers use a protocol called the @RHello@r protocol to identify and establish relationships with adjacent routers.

OSPF is a link-state protocol. LSAs are either event driven or generated automatically every 30 minutes as a method of ensuring topological database synchronization.
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You have been introduced to the structure of OSPF networks.
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The OSPF routing protocol supports three types of network connection:
 
    • point-to-point
    • non-broadcast multiaccess
    • multiaccess

In a multiaccess network, the Hello protocol elects a @Rdesignated router@r (DR) and a @Rbackup designated router@r (BDR). The DR is responsible for generating LSAs for the entire network.

OSPF defines @Rareas@r within @Rautonomous systems@r (ASs). Routers that are connected to more than one area are known as area border routers (ABRs).

All ABRs are connected together to form an OSPF backbone which is in itself an OSPF area.

The OSPF packet contains a 24-byte header. This carries the following information:
 
    • version number
    • packet type
    • router ID
    • area ID
    • checksum
    • authentication type
    • authentication information
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You have been introduced to OSPF route summarization and route selection.
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In order to effectively implement route summarization you must use an appropriate addressing scheme.

OSPF requires manual route summarization. Summarization occurs in area border routers (ABRs).

There are four different types of route:
 
    • default route
    • intra-area route
    • inter area route
    • external route

The default value for OSPF metrics is based on bandwidth. Link metrics are normalized to give FDDI a metric value of 1. However if you implement a faster media, such as ATM, you can manually tune the metrics using the IP OSPF Cost interface configuration command.

You should control inter area traffic. If you want the traffic to leave the area as soon as possible then the ABRs should inject only the default route into the area. However, if you want the traffic to leave the area as late as possible, you should configure the ABRs to inject summary information into the area also.

Cisco routers automatically balance the load in an OSPF internetwork. The default method is to distribute packets based on destination.
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You have been introduced to configuring and monitoring OSPF routing on a network.
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In order to enable OSPF routing on a network you must:

    • enable the OSPF process at each router
    • configure the interfaces on each router
      that will run OSPF
    • configure the OSPF areas
    • configure route summarization between 
      the OSPF areas

There are several commands that you can use to monitor OSPF operation.

The Show IP OSPF command displays general information about the OSPF routing process.

The Show IP Route command displays the current state of the routing table.

The Show IP Protocol command displays the parameters and current state of the active routing protocol process.

You can monitor OSPF events as they happen by turning OSPF debugging on using the Debug IP OSPF Events command.
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