Editing Routing Information Protocol (section)

===RIP version 1===
The original specification of RIP was published in 1988.{{Ref RFC|1058}} When starting up, and every 30 seconds thereafter, a router with RIPv1 implementation [[Broadcasting (networking)|broadcasts]] to {{IPaddr|255.255.255.255}} a request message through every RIPv1 enabled interface. Neighbouring routers receiving the request message respond with a RIPv1 segment, containing their [[routing table]]. The requesting router updates its own routing table, with the reachable IP network address, hop count and next hop, that is the router interface IP address from which the RIPv1 response was sent. As the requesting router receives updates from different neighbouring routers it will only update the reachable networks in its routing table, if it receives information about a reachable network it has not yet in its routing table or information that a network it has in its routing table is reachable with a lower hop count. Therefore, a RIPv1 router will in most cases only have one entry for a reachable network, the one with the lowest hop count. If a router receives information from two different neighbouring router that the same network is reachable with the same hop count but via two different routes, the network will be entered into the routing table two times with different next hop routers. The RIPv1 enabled router will then perform what is known as equal-cost load balancing for IP packets.<ref name="ciscopress.com"/>

RIPv1 enabled routers not only request the routing tables of other routers every 30 seconds, they also listen to incoming requests from neighbouring routers and send their own routing table in turn. RIPv1 routing tables are therefore updated every 25 to 35 seconds.<ref name="ciscopress.com"/> The RIPv1 protocol adds a small random time variable to the update time, to avoid routing tables synchronizing across a LAN.<ref>{{Cite book|title=CCIE Professional Development: Routing TCP/IP Volume I, Second Edition |author1=Jeff Doyle |author2=Jennifer Carroll |publisher=ciscopress.com|year=2005 |isbn=9781587052026 |page=171}}</ref> It was thought, as a result of random initialization, the routing updates would spread out in time, but this was not true in practice. Sally Floyd and Van Jacobson showed in 1994 that, without slight randomization of the update timer, the timers synchronized over time.<ref>[http://www.icir.org/floyd/papers/sync_94.pdf The Synchronization of Periodic Routing Messages], S. Floyd & V. Jacobson,April 1994</ref>

RIPv1 can be configured into silent mode, so that a router requests and processes neighbouring routing tables, and keeps its routing table and hop count for reachable networks up to date, but does not needlessly send its own routing table into the network. Silent mode is commonly implemented to hosts.<ref>{{Cite book|title= CCIE Professional Development: Routing TCP/IP Volume I, Second Edition|author1=Jeff Doyle |author2=Jennifer Carroll |publisher= ciscopress.com|year=2005 |isbn= 9781587052026|page=175}}</ref>

RIPv1 uses [[classful address|classful]] routing. The periodic routing updates do not carry [[subnetwork|subnet]] information, lacking support for [[VLSM|variable length subnet masks]] (VLSM). This limitation makes it impossible to have different-sized [[subnetwork|subnet]]s inside of the same [[network class]]. In other words, all subnets in a network class must have the same size. There is also no support for router authentication, making RIP vulnerable to various attacks.