Case Study: - Running OSPF without AREA 0
Author: - Amarjit Singh
Document Type: - Informational
Introduction
Topology used is as shown below
3 Routers are connected with ABR on 3 different AREAs as shown in above topology diagram.
ABR Router connectivity summary:
| Interface | IP Address | OSPF AREA | Remote Connected Router |
| Interface F0/0 | 1.1.1.1/30 | Area 1 | AREA-1 |
| Interface S1/0 | 2.2.2.1/30 | Area 2 | AREA-2 |
| Interface E2/0 | 3.3.3.1/30 | Area 3 | AREA-3 |
| Interface E2/1 | 4.4.4.1/24 | Reserved for Area 0 | AREA-4 |
Now without configuring AREA 0, no OSPF routes are showing in the routing table. Only direct connected routed are available in the routing table as shown below.
FIGURE 1
FIGURE 2
Also from routers at Area 1, Area 2 & Area 3 only directly connected IPs are pinging.
FIGURE 3
Now lets have a look after configuring AREA 0 at ABR. Here I am configuring the IP address 4.4.4.1/24 on the Interface E2/1 in OSPF Area 0 (Refer to ABR Router connectivity summary table)
FIGURE 4
And output of show ip ospf databse on ABR before (FIGURE 5) and after (FIGURE 6) configuring AREA 0
FIGURE 5
FIGURE 6
SUMMARY
OSPF will not run without AREA 0. We must have to configure "AREA 0" to run the OSPF. No OSPF routes will be shown in "show ip route". Reason behind this is, each area has its own link state databases which will be shared with AREA 0 only by default. And AREA 0 will then flood the IA routes to other AREA's.
Exception: If using only one area say area 11 and no other area is there then no need to create AREA0
It means to flood LSA 1 and LSA 2, no need to create area 0, but for all other LSA's to be flooded properly, say LSA 3, LSA 4, LSA 5 & LSA 7 AREA 0 is must.
I want to bring one more thing in your notice... and its amazing.... Without creating AREA 0 on ABR, the output of the command "show ip ospf neig" is
ABR#sh ip ospf nei Neighbor ID Pri State Dead Time Address Interface 10.10.10.2 1 FULL/DR 00:00:39 1.1.1.2 FastEthernet0/ 0 10.10.10.3 0 FULL/ - 00:00:37 2.2.2.2 Serial1/0 10.10.10.4 1 FULL/DR 00:00:37 3.3.3.2 Ethernet2/0
Here the interesting thing is "FULL" state. It means if the neighbor ship is in "FULL" state and if OSPF routes are not showing in the routing tabe, then you are not connected with AREA 0. SIMPLE...
The router which is calling as ABR actually is not an ABR. Because ABR is that router whose one leg is connected in area 0. So it clears in my scenario that no area 0 is used then no ABR is present. If no ABR is present then no LSA will be flooded (except LSA1 & LSA2).
In my scenario, the router in multiple areas would know all the destinations (subnets in various areas), but would not propagate them between the areas (as it's not an ABR). Therefore, a router residing solely in area 1 would not receive subnets from area 2 and would not be able to communicate with another router that resides solely in area 2.
Configuration of ABR:
!
version 12.4
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname ABR
!
boot-start-marker
boot-end-marker
!
!
no aaa new-model
memory-size iomem 5
!
!
ip cef
!
!
!
!
!
!
!
interface Loopback0
ip address 10.10.10.1 255.255.255.255
!
interface FastEthernet0/0
ip address 1.1.1.1 255.255.255.252
duplex auto
speed auto
!
interface Serial1/0
ip address 2.2.2.1 255.255.255.252
serial restart-delay 0
!
interface Serial1/1
no ip address
shutdown
serial restart-delay 0
!
interface Serial1/2
no ip address
shutdown
serial restart-delay 0
!
interface Serial1/3
no ip address
shutdown
serial restart-delay 0
!
interface Ethernet2/0
ip address 3.3.3.1 255.255.255.252
half-duplex
!
interface Ethernet2/1
ip address 4.4.4.1 255.255.255.0
half-duplex
!
interface Ethernet2/2
no ip address
shutdown
half-duplex
!
interface Ethernet2/3
no ip address
shutdown
half-duplex
!
router ospf 1
log-adjacency-changes
network 1.1.1.0 0.0.0.3 area 1
network 2.2.2.0 0.0.0.3 area 2
network 3.3.3.0 0.0.0.3 area 3
network 4.4.4.0 0.0.0.255 area 0
!
ip http server
!
!
!
!
control-plane
!
banner motd
***********************
* *
*THIS IS ABR ROUTER*
***********************
!
line con 0
password cisco123
login
line aux 0
line vty 0 4
password cisco123
login
!
!
end
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