The routing algorithm of OSPF maintains a complex database of topology information, which routers use to maintain full knowledge of distant routers. OSPF is a classless, link-state routing protocol that is widely deployed in many networks. This chapter will introduce OSPF technologies. OSPF load-balances across four equal metric paths by default on Cisco routers. | Single-Area OSPF Implementation Implementing OSPF OSPF Overview Creates a neighbor relationship by exchanging hello packets Propagates LSAs rather than routing table updates Link: Router interface State: Description of an interface and its relationship to neighboring routers Floods LSAs to all OSPF routers in the area, not just directly connected routers Pieces together all the LSAs generated by the OSPF routers to create the OSPF link-state database Uses the SPF algorithm to calculate the shortest path to each destination and places it in the routing table Purpose: This figure presents the IGRP metric with its five possible components. Emphasize : Bandwidth and delay are the two metrics that are most commonly used. They also comprise the default metric. Note: Changing IGRP metrics can have great impact on network performance. Describe the IGRP 24-bit metric field, as follows: Bandwidth—Minimum bandwidth on the route, in kilobits per second. Delay—Route delay, in tens of microseconds. | Single-Area OSPF Implementation Implementing OSPF OSPF Overview Creates a neighbor relationship by exchanging hello packets Propagates LSAs rather than routing table updates Link: Router interface State: Description of an interface and its relationship to neighboring routers Floods LSAs to all OSPF routers in the area, not just directly connected routers Pieces together all the LSAs generated by the OSPF routers to create the OSPF link-state database Uses the SPF algorithm to calculate the shortest path to each destination and places it in the routing table Purpose: This figure presents the IGRP metric with its five possible components. Emphasize : Bandwidth and delay are the two metrics that are most commonly used. They also comprise the default metric. Note: Changing IGRP metrics can have great impact on network performance. Describe the IGRP 24-bit metric field, as follows: Bandwidth—Minimum bandwidth on the route, in kilobits per second. Delay—Route delay, in tens of microseconds. Reliability—Likelihood of successful packet transmission, expressed as an integer from 0 to 255. Loading—Effective bandwidth of path. MTU—Minimum MTU in path, expressed in bytes. The following equation calculates the metric. It is presented for instructors and is not required to be taught: metric = [k1 x bandwidth + (k2 x bandwidth) / (256 - load) + k3 x delay] If k5 does not equal 0, an additional operation is done: metric = metric x (k5/(reliability + k4)) The default constant values are k1 = k3 = 1 and k2 = k4 = k5 = 0. Again, if default values are set, metric = bandwidth + delay. The constants (k1, k2, k3) can be changed using the metric weights command. Changes to the IGRP constant values should be made with great care. OSPF Hierarchy Example Minimizes routing table entries Localizes the impact of a topology change within an area Summarizing is the consolidation of multiple routes into one single advertisement. Proper summarization requires contiguous addressing. Route .
