Qtech QSW-3900-48-SFP-DC [86/245] Rip working mechanism

QTECH Software Configuration Manual

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7.2.1 RIP Working Mechanism

7.2.1.1 Basic concept of RIP

RIP is a Distance-Vector-based routing protocol, using UDP packets for exchanging information through port 520.

RIP uses a hop count to measure the distance to a destination. The hop count is known as metric. The hop count from

a router to a directly connected network is 0. The hop count from one router to a directly connected router is 1. To

limit convergence time, the range of RIP metric value is from 0 to 15. A metric value of 16 (or bigger) is considered

infinite, which means the destination network is unreachable. That is why RIP is not suitable for large-scaled

networks.

RIP prevents routing loops by implementing the split horizon and poison reverse functions.

7.2.1.2 RIP routing table

Each RIP router has a routing table containing routing entries of all reachable destinations, and each routing entry

contains :

· Destination address : IP address of a host or a network.

· Next hop : IP address of the adjacent router’s interface to reach the destination.

· Egress interface : Packet outgoing interface.

· Metric : Cost from the local router to the destination.

· Route time : Time elapsed since the routing entry was last updated. The time is reset to 0 every time the

routing entry is updated.

· Route tag : Identifies a route, used in routing policy to flexibly control routes.

7.2.1.3 RIP initialization and running procedure

The following procedure describes how RIP works.

· After RIP is enabled, the router sends Request messages to neighboring routers. Neighboring routers return

Response messages including all information about their routing tables.

· The router updates its local routing table, and broadcasts the triggered update messages to its neighbors. All

routers on the network do the same to keep the latest routing information.

· RIP ages out timed out routes by adopting an aging mechanism to keep only valid routes.

7.2.1.4 RIP timers

RIP employs four timers, Update, Timeout, Suppress, and Garbage-Collect.

· The update timer defines the interval between routing updates.

· The timeout timer defines the route aging time. If no update for a route is received after the aging time

elapses, the metric of the route is set to 16 in the routing table.

· The suppress timer defines how long a RIP route stays in the suppressed state. When the metric of a route is

16, the route enters the suppressed state. In the suppressed state, only routes which come from the same

neighbor and whose metric is less than 16 will be received by the router to replace unreachable routes.

· The garbage-collect timer defines the interval from when 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 the

routing metric set to 16. If no update is announced for that route after the Garbage-Collect timer expires, the

route will be deleted from the routing table.

7.2.1.5 Routing loops prevention

RIP is a distance-vector (D-V) based routing protocol. Since a RIP router advertises its own routing table to neighbors,

routing loops may occur.