Routing information protocol (RIP)
The routing information protocol is a distance-vector protocol that uses hop count as a
measure of how good a route is. RIP is widely used for routing traffic within a single
autonomous system. Hence, it is considered an internal gateway protocol(IGP). This is
different from external gateway protocol like border gateway protocol (BGP) which
perform routing between different autonomous system.
RIP sends routing-update message at regular intervals and right after network topology
charges. It also listens for RIP update from router on neighboring networks. When a
router receives a routing update that includes changes to an entry, it update its routing
table to reflect the new route. The metric value (number of hops along the path) is
increased by one and the sender is indicated as the next hop. RIP routers maintain only
the best route (the route with the lowest metric value) to a destination. After updating its
routing table, the router immediately begins transmitting routing updates to inform other
network routers of the change. These updates are sent independent of the regular
scheduled updates that RIP routers send.
Figure 5-9 illustrates the information provided in the routing table. The fields in the table
are as follows.
1. Destination- indicates the IP address of the destination host.
2. Metric- indicates the number of hops to the destination host
3. Next hop- indicates the IP address of the next router in the path to the destination
4. Route change flag- indicates whether information about the route has changed
recently
5. Timers- provides information about various timers associated with the path
The routing tables maintain only the best route to destination; they do not
maintain information about multiple paths to a single destination.
RIP uses a single routing metric, hop count, to measure the distance between the
source and destination hosts. Each hop in a path from source to destination is
assigned a hop count value. RIP prevents routing loops from continuing
indefinitely by implementing a limit on the number of hops allowed in a path
from the source to destination. The maximum number of hops in a path is 15. If a
router receives a routing update that contains a new or changed entry and
increasing the metric value by one causes the metric to exceed the maximum, then
the host destination is considered unreachable.
1. Routing update timer- this timer clocks the interval between periodic routing
updates. It is normally set to 30 seconds with a small random number of
seconds added each time the timer is reset to prevent collisions.
2. Route timeout- each routing table entry has a route timer associated with it.
When the timer expires, the route is marked invalid. However, it remains in
the table until the route flush timer expires.
3. Route flush timer- when this timer expires, all invalid routes are removed
from the routing table.
