HP 6125G HP 6125G & 6125G/XG Blade Switches Layer 3 - IP Routing Confi - Page 27
Configuring RIP, Overview, RIP route entries, RIP timers, Routing loop prevention
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Configuring RIP Overview Routing Information Protocol (RIP) is a distance-vector interior gateway protocol suited to small-sized networks. It employs UDP to exchange route information through port 520. The term "router" in this chapter refers to both routers and Layer 3 switches. RIP route entries RIP stores routing entries in a database. Each routing entry contains the following elements: • Destination address-IP address of a destination host or a network. • Next hop-IP address of the next hop. • Egress interface-Egress interface of the route. • Metric-Cost from the local router to the destination. RIP uses a hop count to measure the distance to a destination. The hop count from a router to a directly connected network is 0. The hop count from a router to a directly connected router is 1. To limit convergence time, RIP restricts the metric range from 0 to 15. A destination with a metric value of 16 (or greater) is considered unreachable. For this reason, RIP is not suitable for large-sized networks. • Route time-Time elapsed since the last update. The time is reset to 0 every time the routing entry is updated. • Route tag-Used for route control. For more information, see "Configuring routing policies." RIP timers RIP employs the following timers: • Update timer-Specifies the interval between routing updates. • Timeout timer-Specifies the route aging time. If no update for a route is received within the aging time, the metric of the route is set to 16. • Suppress timer-Specifies how long a RIP route stays in suppressed state. When the metric of a route is 16, the route enters the suppressed state. A suppressed route can be replaced by an updated route that is received from the same neighbor before the suppress timer expires and has a metric less than 16. • Garbage-collect timer-Specifies the interval from when the metric of a route becomes 16 to when it is deleted from the routing table. RIP advertises the route with a metric of 16. If no update is announced for that route before the garbage-collect timer expires, then the route is deleted from the routing table. Routing loop prevention RIP uses the following mechanisms to prevent routing loops: • Counting to infinity-A destination with a metric value of 16 is considered unreachable. When a routing loop occurs, the metric value of a route will increment to 16 to avoid endless loopings. 17