OSPF area topology

FIGURE 5 . 6 OSPF area topology
Only two of the five configurations are shown—otherwise you would just see a lot of redundant
information. Notice the very specific wildcard masks in the network statements. These
facilitate the removal or addition of specific links when troubleshooting. If you have a link that
is flapping, you can easily remove it so that it does not cause LSA flooding within the area. After
the link has stabilized, it will be very easy to add the interface back in.
For example, if all of the router’s interfaces could be summarized by a network statement of
172.16.0.0 0.0.255.255, then you would need only one network statement to add all interfaces
to the OSPF process. However, if one out of the many interfaces was flapping, you could not
easily isolate that interface so that it would not cause unnecessary LSA flooding. Let’s examine
the IOS configuration for this topology:
RouterA#show running-config
Building configuration...
172.16.230.0/24
RouterA RouterB
RouterD
RouterE
RouterC
172.16.32.0/24
172.16.64.0/24
172.16.10.4/30
172.16.10.8/30
172.16.20.0/24
Current configuration:
!
version 11.2
no service password-encryption
no service udp-small-servers
no service tcp-small-servers
!
hostname RouterA
!
enable password cisco
!
!
interface Loopback0
ip address 172.16.240.1 255.255.255.0
!
interface Ethernet0
ip address 172.16.230.20 255.255.255.0
!
interface Serial0
ip address 172.16.10.5 255.255.255.252
clockrate 2000000
dce-terminal-timing-enable
!
interface Serial1
ip address 172.16.10.9 255.255.255.252
clockrate 2000000
dce-terminal-timing-enable
!
interface Serial2
ip address 172.16.32.1 255.255.255.0
clockrate 2000000
dce-terminal-timing-enable
!
interface Serial3
no ip address
shutdown
!
interface BRI0
no ip address
shutdown
Configuring OSPF 165
!
router ospf 1
network 172.16.230.0 0.0.0.255 area 0
network 172.16.32.0 0.0.0.255 area 0
network 172.16.10.5 0.0.0.0 area 0
network 172.16.10.9 0.0.0.0 area 0
!
RouterB#show running-config
Building configuration...
Current configuration:
!
version 12.0
service timestamps debug uptime
service timestamps log uptime
no service password-encryption
!
hostname RouterB
!
enable password cisco
!
ip subnet-zero
!
!
!
interface Loopback0
ip address 172.16.241.1 255.255.255.0
no ip directed-broadcast
!
interface Ethernet0
no ip address
no ip directed-broadcast
shutdown
!
interface Serial0
ip address 172.16.10.6 255.255.255.252
no ip directed-broadcast
no ip mroute-cache
no fair-queue
!
166 Chapter 5  OSPF Operation in a Single Area
interface Serial1
ip address 172.16.20.1 255.255.255.0
no ip directed-broadcast
clockrate 2000000
dce-terminal-timing-enable
!
interface Serial2
no ip address
no ip directed-broadcast
shutdown
!
interface Serial3
no ip address
no ip directed-broadcast
shutdown
!
interface BRI0
no ip address
no ip directed-broadcast
shutdown
!
router ospf 1
network 172.16.10.6 0.0.0.0 area 0
network 172.16.20.0 0.0.0.255 area 0
!
As you can see, these are very simple, straightforward configurations. All interfaces are
assigned to Area 0. The use of a host wildcard mask (0.0.0.0) has the distinct advantage of making
troubleshooting a bit easier, because affected interfaces are immediately identifiable.
An interesting fact about creating a single area is that there are no ABRs. It is possible to have
an ASBR in a single-area configuration, even without having an ABR. If external routes are
injected into the area, the router injecting them will be considered an ASBR. On the other hand,
in order to activate an ABR, any interface on the router must be assigned to a different area.
It is also important to recognize that the neighbor discovery was automatic in this particular
single-area configuration. Now let’s move on to an environment where sometimes neighbors
must be configured manually.