ed to as Hitless Run and Graceful Restart), which means that in the event of a network disruption, routers keep forwarding packets as the OSPF process is restarted. Two more work- ing areas are the ability to support multiple address families (IPv4 and IPv6) as well as OSPF extensions accommodating Traffic Engineering DiffServ (a statistical technique used to predict and engineer the behavior or the network) and optical technologies. Chapter 8 The Internet and IP Infrastructures IS-IS IS-IS is another protocol that routers can use to determine the best way to forward packets through the network (i.e., to perform routing). It is also an IGP, which means it is used within an administrative domain or network, not between networks. (Routing between administrative domains or networks is the job of BGP.) Like OSPF, IS-IS is also a link-state routing protocol and uses Dijkstra's algorithm. One difference between IS-IS and OSPF is that IS-IS does not use IP to carry the routing information messages. They also differ in the way they define areas and routers. In IS-IS, routers are designated as being Level 1 (intra-area), Level 2 (inter- area), or Level 1-2 (both). Level 1 routers exchange information only with other Level 1 routers, Level 2 routers exchange information only with other Level 2 rout- ers, and Level 1-2 routers exchange information with both levels and are used to connect the interarea routers with the intra-area routers. OSPF networks look some- what like a star topology, or spider web, of many areas all attached to the backbone area. IS-IS looks more like a central spine of Level 2 routers with branches of Level 1-2 and Level 1 routers forming the individual areas or networks. IS-IS was developed by the telecommunication group of the International Tele- communication Union (ITU-T; www.itu.int/ITU-T) around the same time that the IETF was developing OSPF. As discussed previously, OSPF is the dominant IGP routing protocol today, although IS-IS has become more widely used as a viable alternative to OSPF in th