RIP 89

RIP

Routing Information Protocol (RIP) is a simple, dynamic routing protocol, that is

 

Distance-Vector (D-V) algorithm-based. It uses hop counts to measure the distance

 

to the destination host, which is called routing cost. In RIP, the hop count from a

 

router to its directly connected network is 0. The hop count to a network which

 

can be reached through another router is 1, and so on. To restrict the time to

 

converge, RIP prescribes that the cost value is an integer that ranges from 0 to 15.

 

The hop count equal to or exceeding 16 is defined as infinite, or the destination

 

network or host is unreachable.

 

RIP exchanges routing information using UDP packets. RIP sends a routing refresh

 

message every 30 seconds. If no routing refresh message is received from one

 

network neighbor in 180 seconds, RIP tags all routes of the network neighbor as

 

unreachable. If no routing refresh message is received from one network neighbor

 

in 300 seconds, RIP removes the routes of the network neighbor from the routing

 

table. RIP v2 has the MD5 cipher authentication function while RIP v1 does not.

 

To improve performance and avoid routing loops, RIP supports split horizon,

 

poison reverse, and allows for importing routes discovered by other routing

 

protocols.

Each router that is running RIP manages a route database, which contains routing entries to all the reachable destinations in the network. These routing entries contain the following information:

Destination address — The IP address of a host or network.

Next hop address — The address of the next router that an IP packet will pass through to reach the destination.

Output interface — The interface through which the IP packet should be forwarded.

Cost — The cost for the router to reach the destination, which should be an integer in the range of 0 to 15.

Timer — The length of time from the last time that the routing entry was modified until now. The timer is reset to 0 whenever a routing entry is modified.

Route tag — The indication whether the route is generated by an interior routing protocol, or by an exterior routing protocol.

The whole process of RIP startup and operation can be described as follows:

1If RIP is enabled on a router for the first time, the router broadcasts a request packet to adjacent routers. When they receive the request packet, adjacent routers (on which RIP is also enabled) respond to the request by returning response packets containing information about their local routing tables.

2After receiving the response packets, the router that sent the request modifies its own routing table.

3RIP broadcasts its routing table to adjacent routers every 30 seconds. The adjacent routers maintain their own routing tables after receiving the packets and elect an optimal route, then advertise the modification information to their adjacent network to make the updated route globally available. Furthermore, RIP uses timeout mechanism to handle timed-out routes to ensure the timeliness and

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3Com 10014298 manual Rip, Network or host is unreachable