RIP stands for Router Information Protocol. It is an old standard for distributing route information between routers on an internal network. RIP has been superseded by OSPF but is still in common use because of its simplicity and wide support. RIP broadcasts RIP announcements by default every 30 seconds. When a router receives a RIP announcement it makes a note of where it came from and sends it out via its other interfaces. If it receives the same packet again it drops it. From these broadcasts each router builds up a map of the shortest routes to each other router and places them in its routing table. When it receives a packet it looks up where to send it in its routing table. If it does not hear a RIP announcement from a router for 180 seconds it drops that router from its routing tables.
Microsoft Windows XP supports RIP making it a good protocol to dynamically provide multiple gateways on a subnet.