Rip Overview - 3Com 7757 Configuration Manual

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RIP Overview

Basic Concepts
RIP C
ONFIGURATION
Routing information protocol (RIP) is a simple interior gateway protocol (IGP)
suitable for small-sized networks.
RIP
RIP is a distance-vector (D-V) algorithm-based protocol. It exchanges routing
information via UDP packets.
RIP uses hop count (also called routing cost) to measure the distance to a
destination address. In RIP, the hop count from a router to its directly connected
network is 0, and that 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 is an
integer ranging from 0 and 15. The hop count equal to or exceeding 16 is defined
as infinite; that is, the destination network or host is unreachable.
To improve performance and avoid routing loop, RIP supports split horizon.
Besides, RIP can import routes from other routing protocols.
RIP routing database
Each router running RIP manages a routing database, which contains routing
entries to all the reachable destinations in the internetwork. Each routing entry
contains the following information:
Destination address: IP address of a host or network.
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Next hop address: IP address of an interface on the adjacent router that IP
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packets should pass through to reach the destination.
Interface: Interface on this router, through which IP packets should be
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forwarded to reach the destination.
Cost: Cost for the router to reach the destination.
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Routing time: Time elapsed after the routing entry is updated last time. This
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time is reset to 0 whenever the routing entry is updated.
RIP timers
As defined in RFC 1058, RIP is controlled by three timers: Period update, Timeout,
and Garbage-collection.
Period update timer: This timer is used to periodically trigger routing
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information update so that the router to send all RIP routes to all the
neighbors.
Timeout timer: If a RIP route is not updated (that is, the switch does not receive
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any routing update packet from the neighbor) within the timeout time of this
timer, the route is considered unreachable.