1-2z Egress interface: Packet outgoing interface.z Metric: Cost from the local router to the destination.z Route time: Time elapsed since the routing entry was last updated. The time is reset to 0 every timethe routing entry is updated.z Route tag: Identifies a route, used in a routing policy to flexibly control routes. For information aboutrouting policy, refer to Routing Policy Configuration in the IP Routing Volume.RIP timersRIP employs four timers, update, timeout, suppress, and garbage-collect.z The update timer defines the interval between routing updates.z The timeout timer defines the route aging time. If no update for a route is received within the agingtime, the metric of the route is set to 16 in the routing table.z The suppress timer defines how long a RIP route stays in the suppressed state. When the metric ofa route is 16, the route enters the suppressed state. In the suppressed state, only routes whichcome from the same neighbor and whose metric is less than 16 will be received by the router toreplace unreachable routes.z The garbage-collect timer defines the interval from when the metric of a route becomes 16 to whenit is deleted from the routing table. During the garbage-collect timer length, RIP advertises the routewith the routing metric set to 16. If no update is announced for that route after the garbage-collecttimer expires, the route will be deleted from the routing table.Routing loops preventionRIP is a distance vector (D-V) routing protocol. Since a RIP router advertises its own routing table toneighbors, routing loops may occur.RIP uses the following mechanisms to prevent routing loops.z Counting to infinity. The metric value of 16 is defined as unreachable. When a routing loop occurs,the metric value of the route will increment to 16.z Split horizon. A router does not send the routing information learned from a neighbor to theneighbor to prevent routing loops and save bandwidth.z Poison reverse. A router sets the metric of routes received from a neighbor to 16 and sends backthese routes to the neighbor to help delete such information from the neighbor’s routing table.z Triggered updates. A router advertises updates once the metric of a route is changed rather thanafter the update period expires to speed up network convergence.Operation of RIPThe following procedure describes how RIP works.1) After RIP is enabled, the router sends request messages to neighboring routers. Neighboringrouters return Response messages including information about their routing tables.2) After receiving such information, the router updates its local routing table, and sends triggeredupdate messages to its neighbors. All routers on the network do the same to keep the latest routinginformation.3) By default, a RIP router sends its routing table to neighbors every 30 seconds.4) RIP ages out routes by adopting an aging mechanism to keep only valid routes.RIP VersionRIP has two versions, RIPv1 and RIPv2.
