31 Days Before Your CCNA Exam

156 31 Days Before Your CCNA Examthe 8-bit field is set with a value by the source device of the packet. The TTL is decreased by1 by every router on the route to its destination. If the TTL field reaches 0 before the packetarrives at its destination, the packet is discarded and the router sends an ICMP error messageback to the source of the IP packet.Link-State Routing Protocol FeaturesLike distance vector protocols that send routing updates to their neighbors, link-state protocolssend link-state updates to neighboring routers, which in turn forward that information to theirneighbors, and so on. At the end of the process, like distance vector protocols, routers that uselink-state protocols add the best routes to their routing tables, based on metrics. However, beyondthis level of explanation, these two types of routing protocol algorithms have little in common.Building the LSDBLink-state routers flood detailed information about the internetwork to all the other routers so thatevery router has the same information about the internetwork. Routers use this link-state database(LSDB) to calculate the currently best routes to each subnet.OSPF, the most popular link-state IP routing protocol, advertises information in routing updatemessages of various types, with the updates containing information called link-state advertisements(LSAs).Figure 18-3 shows the general idea of the flooding process, with R8 creating and flooding itsrouter LSA. Note that Figure 18-3 shows only a subset of the information in R8’s router LSA.Figure 18-3Flooding LSAs Using a Link-State Routing ProtocolR8 LSAR8 LSAR2R3R4R8 LSAR8 LSAR8 LSA R8 LSA R8 LSASubnet XR1R5R6R8Fa0/0172.16.3.1/24Cost 10R8 LSAR8 LSAR7R8 Router LSA – Partial ContentsRouter ID: 8.8.8.8Int. IP Address: 172.16.3.1/24State: UPCost: 10