 to be alternate possibilities for routes: R3#sho ip eigrp topology IP-EIGRP Topology Table for AS(100)/ID(10.100.100.100) Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply, r - reply Status, S - sia Status P 5.5.5.5/32, 1 successors, FD is 409600 via 10.10.10.5 (409600/128256), Etherneto/o P 10.10.10.0/24, 1 successors, FD is 281600 via Connected, Etherneto/o P 192.168.3.0/24, 1 successors, FD is 2195456 via 10.10.10.5 (2195456/2169856), Etherneto/o In a nutshell, the premise of the OSPF routing protocol is that the shortest or fastest path that is available is the one that will be used. OSPF is the routing protocol of choice when: There are routers from vendors other than Cisco in the network. The network requires segmentation into areas or zones. You want to avoid proprietary protocols. OSPF is a link-state routing protocol. The metric it uses is bandwidth. The bandwidth of each link is calculated by dividing 100,000,000 by the bandwidth of the link in bits per second. Thus, a 100 Mbps link has a metric or "cost" of 1, a 10 Mbps link has a cost of 10, and a 1.5 Mbps link has a cost of 64. A 1 Gbps (or faster) link also has a cost of 1 because the cost cannot be lower than 1. The costs for each link in the path are added together to form a metric for the route. In networks that include links faster than 100 Mbps, the formula for link cost can be changed using the auto-cost reference-bandwidth command. The default reference bandwidth is 100. In other words, by default, a 100 Mbps link has a cost of 1. To make a 1,000 Mbps link have a cost of 1, change the reference bandwidth to 1,000: R3 (config)#router ospf 100 R3 (config-router): auto-cost reference-bandwidth 1000 Specific Routing Protocols If you change the reference bandwidth, you must change it on every router communicating in the OSPF process. Failure to do SO will cause unstable networks and unpredictable routing behavior. OSPF classifies routers according to their function in the network. These are the types of OSPF routers: Internal route