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.
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