OSPF employs area-based segregation and use of passive interfaces to conserve CPU resources. Modify the OSPF priority (zero to disable participation in DR/BDR election) within interface configuration mode.

An OSPF router will always prefer an intra-area route over an inter-area route. Consider the circumstance where two ABRs are connected between the same two OSPF areas, and advertise both Type 2 and Type 3 LSAs, as in the following scenario: ABR1 has O metric 21 to reach Network BABR1 has O IA metric 16 … Continue reading 6.23 Selecting Intra-Area and Inter-area Routes →

For inter-area OSPF cost calculations, a router looks at two things. The path cost to reach its local ABR (the ABR in the path toward the destination network)The cost provided by the ABR's Type 3 LSA for the destination network.

This process begins as the router analyzes its LSDB and identifies all subnets in its local area. (Note the process for inter-area route calculation is different and discussed briefly in the next section.) OSPF adds all costs of outbound interfaces in a path toward a non-connected network to obtain its metric. All paths toward a … Continue reading 6.21 Calculating the Cost of Intra-Area Routes →

Note the following: OSPF uses cost as the metric to determine shortest path; the lowest cost wins.In Cisco IOS, the OSPF cost is based upon the interface bandwidth.The default reference bandwidth in Cisco IOS is 100Mbps.On high-bandwidth links of 100Mbps or greater, the cost must be manually set per interface.Cost is only applied in the … Continue reading 6.20 OSPF Path Selection →

The SPF algorithm is very CPU intensive. Therefore, a goal of a well-designed OSPF network topology should be to minimize the number of times a router is forced to calculate the SPF algorithm. Note the following: By default, a router sends an LSU every 30 minutes (1800 seconds) that forces the recalculation of the SPF … Continue reading 6.19 Running the SPF Algorithm →

On a broadcast, multiaccess segment where multiple routers communicate on a common network: (After DR/BDR election, which uses Priority then highest RID...)R4 notes a link-state change. It sends LSU to the multicast address of 224.0.0.6 (All DR/BDR routers group).DR router is the only router that floods LSUs for this notification.All other routers send LSAck to … Continue reading 6.18 Synchronizing LSDB on Multiaccess Networks →

Here is the process for forming neighbor adjacency and synchronizing link state databases: The interface comes up on the local router.Local router sends its hello packet to the multicast address 224.0.0.5, which includes the RID. These hello packets are sent out of every OSPF-enabled interface - State: Down > InitNeighbors receive the initial hello and … Continue reading 6.17 Exchanging and Synchronizing LSDBs →

OSPF carries more overhead than other routing protocols (such as EIGRP) because of its periodic and indefinite need to refresh LSAs every 30 minutes (1800 seconds). When you issue a show ip ospf database to view the LSAs within the AS, check the Age value next to the database entries for the value, which counts … Continue reading 6.16 Periodic OSPF Database Changes →

show ip ospf database router Provides detailed information about the Type 1 LSAs that are flooded within an area. Each router advertisement may have details about multiple links in that area and information about the type of link it is: Transit (multi-access), P2P, or Stub. *** There are several more detailed derivatives of the show … Continue reading 6.15 Building the Link-State Database →