3Com SuperStack 4 Configuration Manual page 107

RIP
Use the
■ display ip routing-table
corresponding route is valid.
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. This is called the 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 ranging from 0 and 15. A
hop count equal to or exceeding 16 is defined as infinite, which indicates that the
destination network or the host is unreachable.
RIP sends a routing refresh message every 30 seconds. If no routing refresh
message is received from a network neighbor in 180 seconds, RIP tags all routes of
the network neighbor as unreachable. If no routing refresh message is received
from a network neighbor in 300 seconds, RIP removes the routes of the network
neighbor from the routing table.
To improve network performances and avoid routing loops, RIP supports split
horizon, poison reverse, and allows importing of 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.
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Next hop address — The address of the next router that an IP packet will pass
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through for reaching the destination.
Output interface — The interface through which the IP packet should be
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forwarded.
Cost — The cost for the router to reach the destination, which should be an
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integer in the range of 0 to 16.
Timer — The length of time from the last time that the routing entry was
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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
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routing protocol or by an exterior routing protocol.
The process of RIP startup and operation is as follows:
1 If RIP is enabled on a router for the first time, the router broadcasts or multicasts a
request packet to the 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.
2 After receiving the response packets, the router that sent the request modifies its
own routing table.
3 RIP broadcasts its routing table to the adjacent routers every 30 seconds. The
adjacent routers maintain their own routing table after receiving the packets and
elect an optimal route. They then advertise the modification information to their
adjacent network to make the updated route globally available. RIP uses the
command to view whether the
RIP 119