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distance vectors to mathematically compare routes to identify
the best path to any given destination address. These
algorithms emerged from academic research that dates back to
1957. Today's open standard version of RIP, sometimes
referred to as IP RIP, is formally defined in two documents:
Request for Comments (RFC) 1058 and Internet Standard
(STD) 56. As IP-based networks became both more numerous
and greater in size, it became apparent to the Internet
Engineering Task Force (IETF) that RIP needed to be updated.
Consequently, the IETF released RFC 1388 in January 1993,
which was then superseded in November 1994 by RFC 1723,
which describes RIP 2 (the second version of RIP). These
RFCs described an extension of RIP's capabilities but did not
attempt to obsolete the previous version of RIP. RIP 2 enabled
RIP messages to carry more information, which permitted the
use of a simple authentication mechanism to secure table
updates. More importantly, RIP 2 supported subnet masks, a
critical feature that was not available in RIP.
This chapter summarizes the basic capabilities and features
associated with RIP. Topics include the routing update process,
RIP routing metrics, routing stability, and routing timers.
Routing Updates
RIP sends routing-update messages at regular intervals and
when the network topology changes. When a router receives a
routing update that includes changes to an entry, it updates its
routing table to reflect the new route. The metric value for the
path is increased by 1, 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 independently of the regularly scheduled
updates that RIP routers send.
RIP Routing Metric
