Figure
illustrates how each
of the RIP clouds can be converted into an OSPF area. All three
routers are ABRs. ABRs control network information distribution
between OSPF areas and the OSPF backbone. Each router keeps a
detailed record of the topology of its area and receives summarized
information from the other ABRs on their respective areas.
Figure
also illustrates VLSMs. VLSMs use different
size network masks in different parts of the network for the same
network number. VLSM conserves address space by using a longer mask
in portions of the network that have fewer hosts.
Figure
lists the network address assignments for the
network, including the network number, subnet range, and subnet
masks. All interfaces indicate network 130.10.0.0.
SPF address assignments
To conserve address space, a mask of 255.255.255.248 is used for
all the serial lines in area 0. If an area contains a contiguous
range of network numbers, an ABR uses the range keyword with the
area
command to summarize the routes that are injected into the
backbone:
router ospf 109
network 130.10.8.0 0.0.7.255 area 1
area 1 range 130.10.8.0 255.255.248.0
These commands allow Router A to advertise one route, 130.10.8.0
255.255.248.0 that covers all subnets in Area 1, into Area 0.
Without the range keyword in the area command, Router A would
advertise each subnet individually; for example, one route for
130.10.8.0 255.255.255.0, one route for 130.10.9.0 255.255.255.0,
and so forth.
Because Router A no longer needs to redistribute RIP routes, the
router rip
command can now be removed from the configuration file;
however, it is common in some environments for hosts to use RIP to
discover routers. When RIP is removed from the routers, the hosts
must use an alternate technique to find the routers. Cisco routers
support the IRDP and ARP alternatives to RIP.
The ICMP Router Discovery Protocol (IRDP) technique is
illustrated in the example at the end of this section. IRDP is the
recommended method for discovering routers. The
ip irdp
command
enables IRDP on the router. Hosts must also run IRDP.
With proxy Address Resolution Protocol (ARP), if the router
receives an ARP request for a host that is not on the same network
as the ARP request sender, and if the router has the best route to
that host, then the router sends an ARP reply packet giving the
router's own local data link address. The host that sent the ARP
request then sends its packets to the router, which forwards these
packets to the intended host. Proxy ARP is enabled on routers by
default. Proxy ARP is transparent to hosts.
The proxy ARP alternative is not illustrated in this case study.
See the Cisco IOS command references for details on this feature.
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