One of the most widely used routing protocols in IP networks is the Routing Information Protocol (RIP). RIP is the canonical example of a routing protocol built on the ___25___ algorithm. Routing protocols in intemetworks differ slightly from the idealized graph model. In an internetwork, the goal of the routers to forward packets to various ___26___.

Routers running RIP send their advertisement about cost every ___27___ seconds. A router also sends an update message whenever an update from another router causes it to change its routing table.

It is possible to use a range of different metrics or costs for the links in a routing protocol. RIP takes the simplest approach, with all link costs being equal ___28___. Thus it always tries to fend the minimum hop route. Valid distances are 1 through ___29___. This also limits RIP to running on fairly small networks.

28（）

A.1 

B.15 

C.16

D.length of the link