- Planet WAN Router User's Manual

Chapter 4 Router Communication

Protocol

4.1 RIP- Router Information Protocol

The routing information Protocol (RIP) is a distance-vector protocol that used to exchange

routing information between routers. RIP uses broadcast User Datagram Protocol (UDP) data

packets to exchange routing information and rip is based on distance-vector algorithm. This

routing protocol is determines the best path through an Internet by looking at the number of

hops between the two end nodes. The maximum hops count for RIP is 15 hops.

4.1.1 Routing loops

There is problem with distance-vector routing protocol, which is router cannot acquaint with the

whole status of network. Routers have to get network reachable information depending on

neighboring routers and RIP also comes up against slow convergence, which will introduce

inconsistence. The following methods that used by RIP to decrease possibility of routing loop:

spilt horizon, spilt horizon with poison reverse, Holddown timer and triggered update.

4.1.1.1 Spilt Horizon

The spilt horizon is a technique for preventing reverse routes between two routers. The rule of

spilt horizon is that router never advertised the cost of a destination to neighbor if it is the

current next-hop for the destination.

4.1.1.2 Spilt Horizon with Poison Reverse

The rule for “split horizon” is when sending updates out a particular interface, designate any

networks that were learned from updates received on that interface as unreachable. This

mean is when an interface is up; the router records from which interface a route comes, and

not sends the route back to this interface.

4.1.1.3 Holddown timer

Holddown timer is able to prevent a router from receiving new routing information that was just

removed from routing table. The default holddown timer is 180 seconds.

4.1.1.4 Triggered update

Split horizon with poisoned reverse will break any loop of two routers. However, it is still

possible for loops of three or more routers, to occur. This loop will break only when infinity

(presented as 16) will be reached. Triggered updates are an attempt to speed up this