Dell PowerConnect B-RX BigIron RX Series Configuration Guide v02.7.03 - Page 280

7 Configuring forwarding parameters Administrative distance The administrative distance is a unique value associated with each type (source) of IP route. Each path has an administrative distance. It is used when evaluating multiple equal-cost paths to the same destination from different sources, such as RIP, OSPF and so on, but not used when performing IP load sharing. The value of the administrative distance is determined by the source of the route. The device is configured with a unique administrative distance value for each IP route source. When the software receives paths from different sources to the same destination, the software compares their administrative distances, selects the one with the lowest distance, and puts it in the IP route table. For example, if the device has a path learned from OSPF and a path learned from RIP for a given destination, only the path with the lower administrative distance enters the IP route table. Here are the default administrative distances on the device: • Directly connected - 0 (this value is not configurable) • Static IP route - 1 (applies to all static routes, including default routes and default network routes) • Exterior Border Gateway Protocol (EBGP) - 20 • OSPF - 110 • RIP - 120 • Interior Gateway Protocol (IBGP) - 200 • Local BGP - 200 • Unknown - 255 (the router will not use this route) Lower administrative distances are preferred over higher distances. For example, if the router receives routes for the same network from OSPF and from RIP, the router will prefer the OSPF route by default. NOTE You can change the administrative distances individually. Refer to the configuration chapter for the route source for information. Since the software selects only the path with the lowest administrative distance, and the administrative distance is determined by the path's source, IP load sharing does not apply to paths from different route sources. IP load sharing applies only when the IP route table contains paths from the same IP route source to the same destination. Path cost The cost parameter provides a basis of comparison for selecting among paths to a given destination. Each path in the IP route table has a cost. When the IP route table contains multiple paths to a destination, the device chooses the path with the lowest cost. When the IP route table contains more than one path with the lowest cost to a destination, the device uses IP load sharing to select one of the lowest-cost paths. The source of a path's cost value depends on the source of the path: • IP static route - The value you assign to the metric parameter when you configure the route. The default metric is 1. Refer to "Configuring load balancing and redundancy using multiple static routes to the same destination" on page 202. • RIP - The number of next-hop routers to the destination. 208 BigIron RX Series Configuration Guide 53-1001986-01