OSPF Convergence

Regardless of which fashion OSPF uses, the cost of any given route path is the sum of the costs of all interfaces encountered along the path. OSPF calculates route costs in one of two fashions:

• A non-bandwidth-sensitive default value can be used for each OSPF interface.

• OSPF can calculate automatically the cost of using individual router interfaces.

At a minimum, OSPF uses bandwidth to calculate the cost of a route, using the formula (10⁸/Bandwidth). Table F-1 demonstrates some of these calculated costs.

Table F-1. OSPF Link Costs

Interface	OSPF Cost
100 Mbps FDDI/Ethernet	1
45 Mbps T3	2
10 Mbps Ethernet	10
1.544 Mbps T1	~64 (64.7)
56 Kbps	1,768

OSPF convergence is based on the adjacency mechanism, discussed in the next section.

OSPF Adjacencies

Adjacency is the next step after the OSPF neighboring process. Adjacent routers are routers that go beyond the simple Hello protocol exchange and proceed into the database exchange process. To minimize the amount of information exchange on a particular segment, OSPF elects one router to be a designated router (DR) and one router to be a backup designated router (BDR) on each multi-access segment. The BDR is elected as a backup mechanism in case the DR goes down. The idea behind this is that routers have a central point of contact for information exchange. Instead of each router exchanging updates with every other router on the segment, every router exchanges information with the DR and BDR. The DR and BDR relay the information to everybody else.

OSPF routers become adjacent when each router has the same link-state database. Following is a brief summary of the states an interface passes through before becoming adjacent to another router:

• Down No information has been received from anybody on the segment.

• Attempt On non-broadcast multi-access clouds, such as Frame Relay, this state indicates that no recent information has been received from the neighbor. An effort should be made to contact the neighbor by sending Hello packets at the reduced rate poll interval.

• Init The interface has detected a Hello packet coming from a neighbor, but bi-directional communication has not yet been established.

• Two-way There is bi-directional communication with a neighbor. The router has seen itself in the Hello packets coming from a neighbor. At the end of this stage, the DR and BDR election would have been done. At the end of the two-way stage, routers will decide whether to proceed in building an adjacency. The decision is based on whether one of the routers is a DR or BDR or the link is a point-to-point or a virtual link.

  NOTE

  Area 0 is the backbone area and is connected to each OSPF area in the internetwork. In some rare instances, it is impossible to have an area physically connected to the backbone. In this case, a virtual link is used. The virtual link will provide the disconnected area with logical path to the backbone.

• Exstart Routers are trying to establish the initial sequence number that is going to be used in the information exchange packets. The sequence number ensures that routers always get the most recent information. One router will become the primary router and the other will become secondary. The primary router will poll the secondary for information.

• Exchange Routers will describe their entire link-state database by sending database description packets. At this state, packets could be flooded to other interfaces on the router.

• Loading At this state, routers are finalizing the information exchange. Routers have built a link-state request list and a link-state retransmission list. Any information that looks incomplete or outdated will be put on the request list. Any update that is sent will be put on the retransmission list until it is acknowledged.

• Full At this state, the adjacency is complete. The neighboring routers are fully adjacent. Adjacent routers will have a similar link-state database.