| The "Foundation Summary" section of each chapter lists the most important facts from the chapter. Although this section does not list every fact from the chapter that will be on your exam, a well-prepared candidate should, at a minimum, know all the details in each "Foundation Summary" before going to take the exam. The following characteristics describe not only RIPv1, but in essence, any distance vector routing protocol. -
Count to infinity A router (A) hears about networks from its neighbors (B and C) and updates the routing table with the new networks. The routing table is then sent to all neighbors (B and C). However, if a neighbor (B) is sent information about networks for which it was the originating source, confusion can occur, referred to as a routing loop. The problem occurs when the path to a network goes down; each router might believe that there is an alternative path through its neighbor. The ramifications of this problem are limited because each router increments the hop count before it sends out the update. When the hop count reaches 16, the network is rejected as unreachable because the diameter of a RIPv1 network cannot be greater than 15. This is called counting to infinity, where "infinity" equals 16. Although the liability is controlled, it will still slow convergence of the network. -
Split horizon This is a mechanism to prevent loops . If split horizon works, the need for "count to infinity" is eliminated. The split horizon rule states that the routing process will not advertise networks out of the interface through which those networks were heard . This prevents information about networks being repeated to the source of those networks. -
Split horizon with poison reverse Split horizon on its own might not prevent loops, though it prevents networks being advertised out of the interface from which they were learned. However, poison reverse overrides split horizon when a network is lost. Poison reverse includes all the networks that have been learned from the neighbor, but it sets the metric to infinity (16). By changing the metric value to 16, the networks are reported to be unreachable. The routing process acknowledges the network but denies a valid path. Although this increases network overhead by increasing the update size , split horizon with poison reverse can prevent loops. -
Holddown After deciding that a network in the routing table is no longer valid, the routing process waits for three routing updates (by default) before it believes a routing update with a less-favorable metric. Again, this is to prevent routing loops from generating false information throughout the network. -
Triggered updates As soon as a routing process changes a metric for a network in its routing table, it sends an update with the metric set to a value that states it is unusable. In RIP, this value is infinity, that is, 16. Triggered updates inform the other routers immediately. If there is a problem in the network, all the affected routers go into holddown immediately instead of waiting for the periodic timer. This mechanism increases convergence and helps prevent loops. -
Load balancing If the routing process sees multiple paths of equal cost to a remote network, it distributes the routed (datagram) traffic evenly among the paths. It will allocate datagrams to the different paths on a round-robin basis. The type of switching that is usedprocess switching or fast switchingwill determine whether the load balancing is done on a round-robin or session basis. Round- robin load balancing is used when there is process switching in effect. Table 4-3 summarizes default administrative distances. Table 4-3. Default Administrative Distance | Routing Source | Administrative Distance | | Connected interface or static route that identifies the outgoing interface rather than the next hop | | | Static route | 1 | | EIGRP summary route | 5 | | External BGP | 20 | | EIGRP | 90 | | IGRP | 100 | | OSPF | 110 | | RIP | 120 | | External EIGRP | 170 | | Internal BGP | 200 | | An unknown network | 255 or infinity | |
