6.4 Scaling Dial-on-Demand Routing
6.4.8 Configuring routing at the central site
OSPF Routing Configuration
Each central-site access router uses the commands shown in Figure to configure OSPF. These commands limit the routes that are redistributed into OSPF to the major Class B static route and any dynamic subnet routes that may exist for currently connected remote sites. Limiting the routes that are redistributed into OSPF simplifies the routing table on the Cisco 4500 significantly.

The router ospf global configuration command enables an OSPF routing process and assigns to it a process ID of 110.

The first redistribute router configuration command causes static IP routes to be redistributed into OSPF. The subnets keyword specifies that subnets are to be redistributed, and the route-map keyword specifies that only those routes that successfully pass through the route map named static-to-ospf are to be redistributed. The static-to-ospf route map permits the redistribution of routes that match access list 21. Access list 21 permits only major network 172.16.0.0.

The second redistribute router configuration command causes RIP routes to be redistributed into OSPF. The subnets keyword specifies that subnets are to be redistributed, and the route-map keyword specifies that only those routes that successfully pass through the route map named rip-to-ospf are to be redistributed. The rip-to-ospf route map permits the redistribution of routes that match access list 20. Access list 20 permits only routes that start with 172.16 and end with .0 (the third octet is wild). In effect, the RIP-TO-OSPF route map allows only subnets that match 172.16.x.0.

For each asynchronous interface, there is a passive-interface router configuration command, which means that OSPF routing information is neither sent nor received through the asynchronous interfaces. The distance router configuration command assigns the OSPF routing process an administrative distance of 210. This allows the central-site access routers to prefer their static routes (with an administrative distance of 200) over routes learned by OSPF.

Note: When a remote site logs in and a dynamic route is established for it, the other access routers retain their static routes for that remote site. When a remote site logs out, the other access routers do not need to update their routing tables---their routing tables still contain the static routes that are necessary for dialing out to the remote site.

RIP Routing Configuration
Each access router uses the commands shown in Figure to configure RIP. The
timers basic router configuration adjusts the RIP update, invalid, holddown, and flush timers. The command specifies the following: 

  • RIP updates are to be sent every 30 seconds
  • a route is to be declared invalid if an update for the route is not received within 35 seconds after the previous update
  • the time during which better routes are to be suppressed is 0 seconds
  • one second must pass before an invalid route is removed from the routing table 

These timer adjustments produce the fastest possible convergence when a remote site logs out.

The network router configuration command specifies that network 172.16.0.0 is to participate in the RIP routing process. There is no need to propagate RIP routes to the Cisco 1020s, so the distribute-list out router configuration command specifies that access list 10 is to be used to control the advertisement of networks in updates. Access list 10 prevents RIP routes from being sent to the remote site.

Static Routing Configuration
The first
ip route global configuration command creates a static route for major network 172.16.0.0 and assigns it to the dialer interface 20. The route, when redistributed into OSPF, tells the Cisco 4500 that this central-site access router can get to the remote sites. If the access router goes down, the Cisco 4500 learns that the route is not longer available and removes it from its routing table. This route is redistributed into OSPF by the STATIC-TO-OSPF route map. Pairs of static routes follow the first ip route command, one pair for each remote site: In unnumbered IP environments, two static routes are required for each remote site as shown in Figure .

  • One static route points to the next hop on the dialer map. Note that the "200" makes this route a floating static route, but that it is lower than OSPF routes (which are set to 210 by the distance command, earlier in the configuration). This means that a RIP route triggered by a connection to a remote site (whether the remote site or the central site initiates the connection) will override the static route. An OSPF update initiated by a remote site that dials in will not override a static route that points to the next hop address on the dialer map.
  • One static route that defines the interface at which the next hop can be found (in this case, dialer interface 20). This static route is required for unnumbered interfaces. There is no need to make this a floating static route.