OSPF multi-area link configuration

What is OSPF Multi-area feature?

basically as the name indicates, it will make one interface work and form multiple OSPF adjacencies
over a single link

for example, consider the below topology:

as you can see, we need to make the link between R4-R3 to work in backbone area and another two area's ( Area 101, Area 999)

ok, how to configure this?
well, simply by going to interface level and use the command #ip ospf multi-area xx

where X is the new Area number.
ok lets configure it:

R3:
interface Ethernet0/0
 ip address 43.0.0.3 255.255.255.0
 ip ospf network point-to-point
 ip ospf multi-area 999
 ip ospf multi-area 101
 ip ospf 1 area 0
 ip ospf cost 1
end
R4:
 interface Ethernet0/0
 ip address 43.0.0.4 255.255.255.0
 ip ospf network point-to-point
 ip ospf multi-area 999
 ip ospf multi-area 101
 ip ospf 1 area 0
end

Note: IP OSPF NETWORK POINT-TO-POINT is a must.

ok, how to verify this is working or not?
well, the multi-area command should create a new virtual link that can be seen using:

R3#show ip ospf inter br
Interface    PID   Area            IP Address/Mask    Cost  State Nbrs F/C
Lo0               1     0               3.3.3.3/32         1     LOOP  0/0
Et0/0             1     0               43.0.0.3/24        1     P2P   1/1
MA1             1     101             Unnumbered Et0/0   1     P2P   1/1
Et0/1             1     101             73.0.0.3/24        1     P2P   1/1
MA2             1     999             Unnumbered Et0/0   1     P2P   1/1

also in OSPF neighbor command:

R3#show ip os nei

Neighbor ID     Pri   State           Dead Time   Address         Interface
4.4.4.4           0   FULL/  -        00:00:39    43.0.0.4        Ethernet0/0
4.4.4.4           0   FULL/  -        00:00:39    43.0.0.4        OSPF_MA1
7.7.7.7           0   FULL/  -        00:00:32    73.0.0.7        Ethernet0/1
4.4.4.4           0   FULL/  -        00:00:38    43.0.0.4        OSPF_MA2



 and in here:
#show ip ospf

OSPF_MA2 is up, line protocol is up
  Interface is unnumbered. Using address of Ethernet0/0 (43.0.0.3), Area 999, Attached via Multi-area
  Process ID 1, Router ID 3.3.3.3, Network Type POINT_TO_POINT, Cost: 1

OK, So now why we need to use this feature?

OK, one of the reasons is that, OSPF prefer routes learned internally over the routes learned from other area, for example: R2 will prefer the routes coming from R4 over the routes coming from R6 via R3 border router, even if the metric is much better, OSPF will use the routes learned from (O over O IA.)

So what to do in this case?
well, we can configure the OSPF Multi-area command on R4-R3 link to make it work in Area 101,
in this case R2 will compare the metric and choose the best one to reach R5 loopback

lets check the routing table:
 R2#show ip route 5.5.5.5
Routing entry for 5.5.5.5/32
  Known via "ospf 1", distance 110, metric 20
  Tag 111111, type extern 2, forward metric 4
  Last update from 26.0.0.6 on Ethernet0/2, 00:19:39 ago
  Routing Descriptor Blocks:
  * 26.0.0.6, from 4.4.4.4, 00:19:39 ago, via Ethernet0/2
      Route metric is 20, traffic share count is 1
      Route tag 111111
R2#



a trace route from R1:
R1#traceroute 5.5.5.5 num
Type escape sequence to abort.
Tracing the route to 5.5.5.5
VRF info: (vrf in name/id, vrf out name/id)
  1 12.0.0.2 0 msec 0 msec 1 msec
  2 26.0.0.6 1 msec 0 msec 0 msec  (R6)
  3 67.0.0.7 1 msec 1 msec 1 msec (R7)
  4 73.0.0.3 1 msec 1 msec 1 msec (R3)
  5 43.0.0.4 2 msec 1 msec 1 msec (R4)
  6 192.168.1.5 2 msec *  2 msec (R5)
R1#


 I hope this was useful!


Samer R. Saleem