Please also find the below explanation by cisco
There are three processes involved in building and maintaining the routing table in a Cisco router:
Various routing processes, which actually run a network (or routing) protocol, such as Enhanced Interior Gateway Routing Protocol (EIGRP), Border Gateway Protocol (BGP), Intermediate System-to-Intermediate System (IS-IS), and Open Shortest Path First (OSPF).
The routing table itself, which accepts information from the routing processes and also replies to requests for information from the forwarding process.
The forwarding process, which requests information from the routing table to make a packet forwarding decision.
Let's examine the interaction between the routing protocols and the routing table to understand how the routing table is built.
The main considerations while building the routing table are:
Administrative distance - This is the measure of trustworthiness of the source of the route. If a router learns about a destination from more than one routing protocol, administrative distance is compared and the preference is given to the routes with lower administrative distance. In other words, it is the believability of the source of the route.
Metrics - This is a measure used by the routing protocol to calculate the best path to a given destination, if it learns multiple paths to the same destination. Each routing protocol uses a different metric.
Prefix length
Please follow the link
http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094823.shtml
for frequently asked questions
http://www.cisco.com/en/US/tech/tk365/technologies_q_and_a_item09186a008012d8f7.shtml
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Regards
Thanveer
"Everybody is genius. But if you judge a fish by its ability to climb a tree, it will live its whole life believing that it is a stupid."
