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Routing Policies 301
Routing policies can control the entire flow of routing information among
the network, the protocols, and the routing table manager.
Routing Policies are often referred to as Route Filters because defining
policies for accepting and forwarding routes is very much like defining
filters to screen which routes may be forwarded or accepted.
How Routing Policies
Work
Each router keeps a table of current routing information, called the
routing table . The router protocols on the system receive routes from or
advertise routes to the network.
When a route needs to be added to the routing table:
1 The protocol (OSPF or RIP) that receives the route sends that route to the
routing table manager.
2 The routing table manager searches the Import policies.
3 If the import policy allows the route to be accepted, the routing table
manager adds the route to the routing table; otherwise, the route is
discarded. See Figure 60.
The router also needs to periodically advertise routes to other routers:
1 The protocol (OSPF or RIP) polls the routing table manager for routes to
advertise to other routers.
2 The routing table manager searches the Export policies.
3
If the export policy allows the route to be advertised, the routing table
manager advertises the route on the network; otherwise, the route is not
sent. See Figure 60.