Operation Manual – Routing ProtocolH3C S5600 Series Ethernet Switches Chapter 3 RIP Configuration3-2z Next hop: IP address of an interface on the adjacent router that IP packetsshould pass through to reach the destination.z Interface: Outbound interface on this router, through which IP packets should beforwarded to reach the destination.z Metric: Cost from the local router to the destination.z Route time: Time elapsed since the routing entry was last updated. The time isreset to 0 every time the routing entry is updated.III. RIP timersAs defined in RFC 1058, RIP is controlled by three timers: Period update, Timeout,and Garbage-collection.z Period update timer: The period update timer defines the interval betweenrouting updates.z Timeout timer: The timeout timer defines the route aging time. If no update for aroute is received after the aging time elapses, the metric of the route is set to 16in the routing table.z Garbage-collection timer: The garbage-collect timer defines the interval fromwhen the metric of a route becomes 16 to when it is deleted from the routing table.During the Garbage-Collect timer length, RIP advertises the route with therouting metric set to 16. If no update is announced for that route after theGarbage-Collect timer expires, the route will be deleted from the routing table.IV. Routing loops preventionRIP is a distance-vector (D-V) based routing protocol. Since a RIP router advertisesits own routing table to neighbors, 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 arouting 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 aneighbor back to the neighbor to prevent routing loops and save the bandwidth.3.1.2 RIP Startup and OperationThe whole process of RIP startup and operation is as follows:z Once RIP is enabled on a router, the router broadcasts or multicasts a requestpacket to its neighbors. Upon receiving the packet, each neighbor running RIPanswers a response packet containing its routing table information.z When this router receives a response packet, it updates its local routing tableand sends a triggered update packet to the neighbors. Upon receiving thetriggered update packet, the neighbor sends the packet to all its neighbors. Aftera series of update triggering processes, each router can get and keep theupdated routing information.
