Routing
Dlink DRO-210i User Guide 23
4 Routing Routing determines how to transport packets from the initiating host to the receiving host.
The packet needs to determine a path through which it can travel from the sender to the
receiver. The Routing Table in a router provides such a map to all packets. Each entry (or
route) in the routing table indicates the destination address of the packet, where the next
hop (or gateway) should be, which interface of the router the packet should go out to
reach the destination, and hence provides a path selection. When a packet arrives at a
router, it looks up the routing table to decide which path to take next. The router
compares the destination address of the packet with the entries in the routing table, and
finds out the associated interface and next hop from the matching route to forward the
packet.
As the networks grow large and complex, the entire domain of routing can be divided
into sub areas to provide better functionality and control. This routing hierarchy divides
all the routers in the n etwork into administrative regions called the Autonomous System
(AS). Routers inside AS (i.e. intra-AS routers) run the same routing algorithm and only
need to know the topology of their network. RIP and OSPF are examples of intra-AS
routing algorithms. These algorithms are also referred to as Interior Gateway Routing
Protocols. This router supports RIP routing algorithm.
In the internetworking environment there are typically more than one path connecting the
two end hosts. The dynamic routing algorithm selects the “best” path that has the “least
cost” for transporting. RIP uses Path Length routing metric to determine the best path.
Path length is the sum of the costs associated with each link. Path Length is commonly
known as the hop count, or the number of routing devices (i.e. routers) that a packet takes
to travel from the source to its destination. The cost of a link may be determined based on
the below parameters:
• Bandwidth – Bandwidth is the traffic capacity of a path, rated by “Mbps”.
• Load – Load refers to the usage of a router. The usage can be evaluated by CPU
utilization and the throughput.
• Delay – Delay is the time it takes to move a packet from the source to the destination.
The time depends on many factors, such as the bandwidth, load, and the length of the
path.
In case of a device or a link failure, the router looses its “best” route (associated with that
link) and relies on the routing algorithm to select the next best route available in the
routing table. This is a continuous process to keep the internetworking functional and
help the router select the correct path at all times.