Criteria for Inserting Routes in the IP Routing Table
One of the intriguing aspects of Cisco routers is the way the router chooses the best route among those presented by routing protocols, manual configuration , and various other means. While route selection is not difficult, to understand it completely requires some knowledge about the way Cisco routers work. The router must consider the following four criteria:
Valid next-hop IP address: As each routing process receives updates and other information, the router first verifies that the route has a valid next-hop IP address.
Metric: If the next hop is valid, then the routing protocol chooses the best path to any given destination based on the lowest metric. The routing protocol attempts to install this path into the routing table. For example, if EIGRP learns of a path to 10.1.1.0/24 and decides this particular path is the best EIGRP path to this destination, then the routing protocol tries to install the learned path into the routing table.
Administrative Distance: The next consideration is administrative distance. If more than one route exists for the same network, the router decides which route to install based on the administrative distance of the source of the route. If the routing protocol that is presenting the path to a particular destination has the lowest administrative distnace compared to the other ways the router has learned about this network, then the router installs the route in the routing table. If that route does not have the best administrative distance, it is rejected.
Prefix: The router looks at the prefix being advertised. If there is no exact match to that prefix in the routing table, the route is installed. For example, the router has three routing processes running on it, and each processhas received these following routes:
-- EIGRP(internal): 192.168.32.0/16
-- RIP: 192.168.32.0/24
-- OSPF: 192.168.32.0/19
Because each route has a different prefix length, also know as the subnet mask, the routers are considered different destinations and are installed in the rouing table.
Posted by
Lifeng Shen on May 9, 2007 9:41 AM
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