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

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

What is CEF- Cisco Express Forwarding: Load-Balancing with Static Routing

The topology is given below we will be using here :

We made loopbacks each on R1& R2 and configure default static-routing with next hop interfaces on R2.
Configuration is like this :
On R1:

Configuring Default-routes on R1:

On R2:

Now we did trace route from R1 to Loopback 2.2.2.2 on R2 :

Take a look … it is sending one packet from 12.12.12.0/24 network and other from
And this is what R1 shows but what’s happening behind the scenes on R2 ?
Enabling Debug ip packet on R2 will help to know the sources of packets.

So, this is interesting ….
It shows,
For one packet source is 12.12.12.1 (interface S0/0)
And for another 30.15.20.1 (interface S0/1) and the process take place like this and packets has been sent from each serial interface of R1 alternatively.
But here’s the catch ….
With trace route load-balancing is working fine but when I tried with PING to 2.2.2.2

And as usual #debug ip packets was enabled on R2 . Things are not in favor now L
They are only using same route to send and receive packets.

And at the same time on R1

Note: Routed packets via FIB …
I saw it is following only 30.15.20.0/24 network to send packets. No load balancing.
Look at the above picture carefully,
When R1 is sending packet s=30.15.20.1 , d=2.2.2.2, routed through RIB.
And When R2 is sending Acknowledgement reply for this packet when s=2.2.2.2 and d=30.15.20.1, routed via FIB.
Mummble …. Now whats that CEF, FIB, RIB ??
RIB - routing information base
FIB - forwarding information base
CEF- Cisco Express Forwarding
Once CEF is enabled, it will form the FIB table with the help of Routing table. Now router no longer looks on RIB and FIB acts as replacement for RIB.
CEF also generates adjacency table which pre-maps all of the next hop ip as well as MAC address so as we configure Static routing, CEF add routes to FIB (forwarding information base) and send the packets through the interface which is added first into the table and the static route we have configured first will be added first and will be used to send packets .
Since, here 30.15.20.0/24 is stored first as well as configured first for static route so it will be using 30.15.20.0/24

At the same moment and it will not check the routing table when sending packets to another router/network from then on unless there is change in the routing table and will forward all the packets based on CEF table only.
CEF is ON by default on the routers.

Now, try to debug after Disabling CEF on R1

We are successful to load-balance equally from R1-R2 by disabling CEF. J
NOTE: Routed via RIB not FIB i.e. No longer using FIB table created by CEF.
Now, why?? Why ?? traceroute is able to do load-balancing without disabling CEF ?
Better to find out through practical approach
CEF is enable (by default)
Enable #debug ip packet and
Traceroute to 2.2.2.2



Note : sending broad/multicast .

Now, why it was choosing 30.15.20.0/24 not 12.12.12.10/24 network when we are sending packets through PING??
Hope this is informative for you .
Thank you for reading.