Routing Control Plane - Building The Unicast Routing Table - Installing Unicast Routes

Installing Unicast Routes

Different implementations have different sets of preferences for routing information, and these are not standardized among IP routers. It is fair to say that subnets on directly connected active interfaces are always preferred. Beyond that, however, there will be differences.

Implementers generally have a numerical preference, which Cisco calls an "administrative distance", for route selection. The lower the preference, the more desirable the route. Cisco's IOS implementation makes exterior BGP the most preferred source of dynamic routing information, while Nortel RS makes intra-area OSPF most preferred.

The general order of selecting routes to install is:

1. If the route is not in the routing table, install it.
2. If the route is "more specific" than an existing route, install it in addition to the existing routes. "More specific" means that it has a longer prefix. A /28 route, with a subnet mask of 255.255.255.240, is more specific than a /24 route, with a subnet mask of 255.255.255.0.
3. If the route is of equal specificity to a route already in the routing table, but comes from a more preferred source of routing information, replace the route in the table.
4. If the route is of equal specificity to a route in the routing table, comes from a source of the same preference,
   1. Discard it if the route has a higher metric than the existing route
   2. Replace the existing route if the new route has a lower metric
   3. If the routes are of equal metric and the router supports load-sharing, add the new route and designate it as part of a load-sharing group. Typically, implementations will support a maximum number of routes that load-share to the same destination. If that maximum is already in the table, the new route is usually dropped.