TCP/IP Tutorial and Technical Overview - IBM Redbooks

► The associated cost (distance) of the most attractive path to reach this
destination. This provides the ability to differentiate between multiple paths to
a destination. In this context, the terms distance and cost can be misleading.
They have no direct relationship to physical distance or monetary cost.
► The IP address of the next-hop device used to reach the destination network.
Each time a routing table advertisement is received by a device, it is processed
to determine if any destination can be reached by a lower cost path. This is done
using the RIP distance vector algorithm. The algorithm can be summarized as:
► At router initialization, each device contains a distance vector table listing
each directly attached networks and configured cost. Typically, each network
is assigned a cost of 1. This represents a single hop through the network. The
total number of hops in a route is equal to the total cost of the route. However,
cost can be changed to reflect other measurements such as utilization,
speed, or reliability.
► Each router periodically (typically every 30 seconds) transmits its distance
vector table to each of its neighbors. The router can also transmit the table
when a topology change occurs. Each router uses this information to update
its local distance vector table:
– The total cost to each destination is calculated by adding the cost reported
in a neighbor's distance vector table to the cost of the link to that neighbor.
The path with the least cost is stored in the distance vector table.
– All updates automatically supersede the previous information in the
distance vector table. This allows RIP to maintain the integrity of the
routes in the routing table.
► The IP routing table is updated to reflect the least-cost path to each
destination.
Chapter 5. Routing protocols 183