tions, and a router may be classified as more than one type. The main types of routers are as follows: Area border router (ABR)-The ABR is a router that connects one or more OSPF areas to the backbone network. Autonomous system boundary router (ASBR)-The ASBR is a router that is connected to more than one autonomous system and exchanges informa- tion with routers in other autonomous systems. It is used to distribute rout- ing information from other autonomous systems throughout its own autonomous system. Internal router (IR)-An IR is a router that exchanges routing information only with routers in the same area. Backbone router (BR)-The BR has a connection into the backbone area. Designated router (DR)-The DR is a router elected by the network based on calculating the router's priority number and router ID. The basic building block of the OSPF routing protocol for IP is the link-state advertisement (LSA). The LSA provides a description of a router's local routing topology that the router advertises (distributes) to all other routers. Because OSPF is designed with scalability in mind, not all LSAs are flooded (i.e., advertised) on all the interfaces; instead, they are sent only to the interfaces that belong to the appropriate area. This allows the detailed information to be kept local, while sum- mary information is provided to the rest of the network. There are 11 LSAs defined in OSPF, each applicable to a particular area and type of router. Routers that belong to the same broadcast domain (i.e., that belong to the same network) form what are called adjacencies, or the relationship of being neigh- bors. The routers elect a DR, which acts as a hub to reduce traffic between routers. OSPF makes use of both unicast and multicast (discussed later in this chapter) to send "hello" packets and link-state updates. In keeping with advances in Internet protocols, a number of updates have been added to OSPF, including OSPFv3 (version 3), which allows support for IPv6. Another update introduces Nonstop Forwarding (also referr