0, 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 1000 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 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 router An internal router is one that resides completely within a single area within a single OSPF autonomous system. Area border router (ABR) An ABR is one that resides in more than one area within a single OSPF autono- mous system. Autonomous system border router (ASBR) An ASBR is one that connects to multiple OSPF autonomous systems, or to an OSPF autonomous system and another routing protocol's autonomous system. Backbone routers Backbone routers are OSPF routers that reside in area zero. Area zero is considered the backbone in an OSPF network. Specific Routing Protocols Designated router The DR is the router on a broadcast network that is elected to do the brunt of the OSPF processing. The DR will update all the other routers in the area with routes Backup designated router (BDR) The BDR is the router with the most eligibility to become the DR should the DR fail. Unlike other routing protocols, OSPF does not send routes, but rather link state advertisements (LSAs). Each OSPF router determines which routes to use based on an internal database complied from these LSAs. There are six LSA types: 