How LSA Works with/without OSPF AREA 0 | Running OSPF without AREA 0 By Anuj Tyagi: Case Study

Hi Friends, I have received a complete case study done by Mr.Anuj Tyagi on OSPF routing protocol. After reading our article Configure OSPF without AREA 0: CCNP OSPF Case Study, he also tested the similar scenario in his testing LAB. I am publishing his case study as it is without any editing so that guys who are learning networking & OSPF, will get some benefit from this.

Running OSPF without AREA 0

Topology 1: Connecting OSPF Area 2 and area 3 without using ABR.

Interface configurations are mentioned above in the topology.

Remember,

· ABR: ABR is a area border router that contain interfaces in atleast two separate area out of which one should always be in AREA 0.

So, surely above topology don’t have any ABR and we will be going to test how routers behavior in absence of backbone area.

After configuring OSPF routes on R1, R2, R3 as in above topology, we analysis all Routers neighbor table .

That shows Full Neighborship Status for neighbors . Take a look again at the neighbor table,

Neighborship is being maintained with it’s directly connected interfaces.

Now, if we will try to PING

· R2 S0/1 , surely we will get successful reply .

· R2 S0/0, we will get successful reply (as it is directly connected)

· R3 S0/0, obviously we R1 can’t reach 2.2.2.1 of R2 we will be unable to get reply from R3 s0/0.

Why? Just take a look at the neighborship table of R1 again .

So, what is happening behind the scenes when R1 trying to communicate with 2.2.2.0/24 network .

R1 can’t even find the path or from which interface R1 should send the packets .

Now lets take a look at the Router OSPF LSA’s,

· R1 having LSA1 (router-id) updates of only those routers in same area.

Reason : There is no ABR to send LSA3 (summary LSA) updates . ( remember ABR? )

After looking at LSA and neighbor-table, you can guess about the routes will be there in Routing Table.

Again, they are also from directly connected interfaces with Routers.

Now make it a bit interesting, we will going to add interfaces on R2 in Area 0 .

Now, compare neighborship table of Router R1,R2,R2 with and without AREA 0 (backbone area) .

R1 topology table with AREA 0

We can now clearly see LSA3 in the table, apart from networks of Area 0 R1 got one more network advertisement in LSA for 192.168.2.0/24 and 2.2.2.0/24 in summary LSA updates from R2(ABR router).

While in

R2 topology table without AREA 0

In the same way, AREA 0 will also make Router R2 as ABR now and that will add a lot to topology table of Router R2 .

Now, it is behaving like ABR as it is acting as both in AREA-2 and AREA-3 completely.

Router R3 also getting LSA1 and LSA3 updates from R2(ABR).

Note: LSA2 updates (network LSA) are only advertised in Non-broadcast where DR & BDR exist.

Now , also PING will be successful from any Router to any network in topology.

Conclusion:

OSPF will not form neighborship with inter-area(IA) routes if there is no backbone area exist or we do not use ABR . In other words, Each area share it’s link state database only through AREA0 to any other AREA. It will make neighborship only with directly connected networks

LSA 1,2 do not need Area0 to share it’s Acknowledgement but LSA2 will form only in non-broacast network (like frame-relay) where DR will be having responsibility to send updates but we must need to have AREA0 to send LSA 3, 4, 5 & 7 updates.

----------------------------------------------------------------------------

R1 Config

Using 1024 out of 57336 bytes
!
version 12.4
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname R1
!
boot-start-marker
boot-end-marker
!
!
no aaa new-model
memory-size iomem 5
ip cef
!
!
ip auth-proxy max-nodata-conns 3
ip admission max-nodata-conns 3
!
multilink bundle-name authenticated
!
!
archive
log config
hidekeys
!
!
!
interface FastEthernet0/0
ip address 192.168.1.1 255.255.255.0
duplex auto
speed auto
!
interface Serial0/0
ip address 1.1.1.1 255.255.255.0
clock rate 2000000
!
interface FastEthernet0/1
no ip address
shutdown
duplex auto
speed auto
!
interface Serial0/1
no ip address
shutdown
clock rate 2000000
!
router ospf 100
router-id 1.1.1.1
log-adjacency-changes
network 1.1.1.0 0.0.0.255 area 2
network 192.168.1.0 0.0.0.255 area 2
!
ip forward-protocol nd
!
!
ip http server
no ip http secure-server
!
!
line con 0
line aux 0
line vty 0 4
!
!
end

-------------------------------------------------------------

R2-ABR Config

R2-ABR#show configuration
Using 1114 out of 57336 bytes
!
version 12.4
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname R2
!
boot-start-marker
boot-end-marker
!
!
no aaa new-model
memory-size iomem 5
ip cef
!
ip auth-proxy max-nodata-conns 3
ip admission max-nodata-conns 3
!
multilink bundle-name authenticated
!
!
archive
log config
hidekeys
!
!
interface FastEthernet0/0
ip address 172.16.1.1 255.255.0.0
duplex auto
speed auto
!
interface Serial0/0
ip address 1.1.1.2 255.255.255.0
clock rate 2000000
!
interface FastEthernet0/1
ip address 172.17.1.1 255.255.0.0
duplex auto
speed auto
!
interface Serial0/1
ip address 2.2.2.1 255.255.255.0
clock rate 2000000
!
router ospf 100
router-id 2.2.2.2
log-adjacency-changes
network 1.1.1.0 0.0.0.255 area 2
network 2.2.2.0 0.0.0.255 area 3
network 172.16.0.0 0.0.255.255 area 0
network 172.17.0.0 0.0.255.255 area 0
!
ip forward-protocol nd
!
!
ip http server
no ip http secure-server
!
!
control-plane
!
!
line con 0
line aux 0
line vty 0 4
!
!
end

-------------------------------------------------------------------------

R3 Config

Using 1024 out of 57336 bytes
!
version 12.4
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname R3
!
boot-start-marker
boot-end-marker
!
!
no aaa new-model
memory-size iomem 5
ip cef
!
!
ip auth-proxy max-nodata-conns 3
ip admission max-nodata-conns 3
!
multilink bundle-name authenticated
!
!
!
!
archive
log config
hidekeys
!
!
!
interface FastEthernet0/0
ip address 192.168.2.1 255.255.255.0
duplex auto
speed auto
!
interface Serial0/0
ip address 2.2.2.2 255.255.255.0
clock rate 2000000
!
interface FastEthernet0/1
no ip address
shutdown
duplex auto
speed auto
!
interface Serial0/1
no ip address
shutdown
clock rate 2000000
!
router ospf 100
router-id 3.3.3.3
log-adjacency-changes
network 2.2.2.0 0.0.0.255 area 3
network 192.168.2.0 0.0.0.255 area 3
!
ip forward-protocol nd
!
!
ip http server
no ip http secure-server
!
control-plane
!
!
!
line con 0
line aux 0
line vty 0 4
!
!
end