OBJECTIVE

    1. The main purposed of this post is to have a deep understanding of the OSPF Non-Broadcast
        Multi Access network Type.
    2.  Determine how a Non-Broadcast Multi-Access (NBMA) can select which router(s) will be
         elected as the Designated Router(DR).
    3. Describe how to configure NMBA over a frame relay and at the same time how to enable
        reachability of networks using OSPF.

   CONCEPTS 

    Configurations of MANILA Router:

 R1#CONFIG T
Enter configuration commands, one per line.  End with CNTL/Z.
R1(config)#hostname MANILA
MANILA(config)#int s3/0
MANILA(config-if)#ip add 192.168.12.1 255.255.255.0
MANILA(config-if)#no shut
MANILA(config-if)#encapsulation frame-relay
MANILA(config-if)#ip ospf network non-broadcast
 MANILA(config-if)#exit
MANILA(config)#int lo0
MANILA(config-if)#ip add 10.10.10.1 255.255.255.0
MANILA(config-if)#exit
 
MANILA#
MANILA#config t
Enter configuration commands, one per line.  End with CNTL/Z.
MANILA(config)#router ospf 1
MANILA(config-router)#network 192.168.12.0 0.0.0.255 area 0
 MANILA(config-router)#network 10.10.10.0 0.0.0.255 area 0
MANILA(config-router)#neighbor 192.168.12.2   <<<< MELBOURNE Router
MANILA(config-router)#neighbor 192.168.12.3  <<<<  SINGAPORE Router
MANILA(config-router)#^Z
MANILA#
  Configurations of MELBOURNE Router:

R2#config t
Enter configuration commands, one per line.  End with CNTL/Z.
R2(config)#hostname MELBOURNE
MELBOURNE(config)#int s3/0
MELBOURNE(config-if)#ip address 192.168.12.2 255.255.255.0
MELBOURNE(config-if)#no shut
MELBOURNE(config-if)#encapsulation frame-relay
MELBOURNE(config-if)#ip ospf network non-broadcast
MELBOURNE(config-if)#ip ospf priority 0  <<< This command ensures that MELBOURNE Router will not participate in DR/BDR Election
MELBOURNE(config-if)#exit
MELBOURNE(config)#intlo0
MELBOURNE(config-if)#ip address 20.20.20.1 255.255.255.0
MELBOURNE(config-if)#exit

MELBOURNE(config)#router ospf 1
MELBOURNE(config-router)#network 192.168.12.0 0.0.0.255 area 0
MELBOURNE(config-router)#network 20.20.20.0 0.0.0.255 area 0
MELBOURNE(config-router)#exit

 Configurations of SINGAPORE Router:

R3#config t
Enter configuration commands, one per line.  End with CNTL/Z.
R3(config)#hostname SINGAPORE
SINGAPORE(config)#int s3/0
SINGAPORE(config-if)#ip address 192.168.12.3 255.255.255.0
SINGAPORE(config-if)#no shut
SINGAPORE(config-if)#encapsulation frame-relay
SINGAPORE(config-if)#ip ospf network non-broadcast
SINGAPORE(config-if)#ip ospf priority 0 <<< This command ensures that SINGAPORE Router will not participate in DR/BDR Election
SINGAPORE(config-if)#^Z
SINGAPORE(config)#intlo0
SINGAPORE(config-if)#ip address 30.30.30.1 255.255.255.0

SINGAPORE(config)#router ospf 1
SINGAPORE(config-router)#network 192.168.12.0 0.0.0.255 area 0
SINGAPORE(config-router)#network 30.30.30.0 0.0.0.255 area 0

Now, let’s check the status of the OSPF neighbors of the three routers.

MANILA#sh ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
30.30.30.1        0   FULL/DROTHER    00:01:59    192.168.12.3    Serial3/0
192.168.12.2      0   FULL/DROTHER    00:01:59    192.168.12.2    Serial3/0
MANILA#

>>> As observed, MANILA have established the full adjacency on both MELBOURNE and SINGAPORE routers. The State is FULL/DROTHER as I have manually set up MELBOURNE and SINGAPORE not to participate the DR/BDR selection.

MELBOURNE#show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
192.168.12.1      1   FULL/DR         00:01:49    192.168.12.1    Serial3/0
MELBOURNE#

>>> We can see that the MANILA Router was the DR.

SINGAPORE#show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
192.168.12.1      1   FULL/DR         00:01:50    192.168.12.1    Serial3/0
SINGAPORE#

>>> Similarly, we can also notice that MANILA was the DR from SINGAPORE router ospf neighbor status.

Let”s check the ROUTING TABLE of the three routers:

MANILA#show ip route ospf
Codes: L – local, C – connected, S – static, R – RIP, M – mobile, B – BGP
       D – EIGRP, EX – EIGRP external, O – OSPF, IA – OSPF inter area
       N1 – OSPF NSSA external type 1, N2 – OSPF NSSA external type 2
       E1 – OSPF external type 1, E2 – OSPF external type 2
       i – IS-IS, su – IS-IS summary, L1 – IS-IS level-1, L2 – IS-IS level-2
       ia – IS-IS inter area, * – candidate default, U – per-user static route
       o – ODR, P – periodic downloaded static route, H – NHRP, l – LISP
       + – replicated route, % – next hop override

Gateway of last resort is not set

      20.0.0.0/32 is subnetted, 1 subnets
O        20.20.20.1 [110/65] via 192.168.12.2, 00:27:03, Serial3/0
      30.0.0.0/32 is subnetted, 1 subnets
O        30.30.30.1 [110/65] via 192.168.12.3, 00:27:03, Serial3/0
MANILA#

>>> As observed from MANILA router’s ROUTING TABLE, we can see the Loopback Address of both the MELBOURNE and SINGAPORE routers.

MELBOURNE#sh ip route ospf
Codes: L – local, C – connected, S – static, R – RIP, M – mobile, B – BGP
       D – EIGRP, EX – EIGRP external, O – OSPF, IA – OSPF inter area
       N1 – OSPF NSSA external type 1, N2 – OSPF NSSA external type 2
       E1 – OSPF external type 1, E2 – OSPF external type 2
       i – IS-IS, su – IS-IS summary, L1 – IS-IS level-1, L2 – IS-IS level-2
       ia – IS-IS inter area, * – candidate default, U – per-user static route
       o – ODR, P – periodic downloaded static route, H – NHRP, l – LISP
       + – replicated route, % – next hop override

Gateway of last resort is not set

      10.0.0.0/32 is subnetted, 1 subnets
O        10.10.10.1 [110/65] via 192.168.12.1, 00:28:05, Serial3/0
      30.0.0.0/32 is subnetted, 1 subnets
O        30.30.30.1 [110/65] via 192.168.12.3, 00:28:05, Serial3/0
MELBOURNE#

>>> As observed from the MELBOURNE router’s  ROUTING’ TABLE , we can see the loopback address of both MANILA and SINGAPORE router.

SINGAPORE#sh ip route ospf
Codes: L – local, C – connected, S – static, R – RIP, M – mobile, B – BGP
       D – EIGRP, EX – EIGRP external, O – OSPF, IA – OSPF inter area
       N1 – OSPF NSSA external type 1, N2 – OSPF NSSA external type 2
       E1 – OSPF external type 1, E2 – OSPF external type 2
       i – IS-IS, su – IS-IS summary, L1 – IS-IS level-1, L2 – IS-IS level-2
       ia – IS-IS inter area, * – candidate default, U – per-user static route
       o – ODR, P – periodic downloaded static route, H – NHRP, l – LISP
       + – replicated route, % – next hop override

Gateway of last resort is not set

      10.0.0.0/32 is subnetted, 1 subnets
O        10.10.10.1 [110/65] via 192.168.12.1, 00:28:38, Serial3/0
      20.0.0.0/32 is subnetted, 1 subnets
O        20.20.20.1 [110/65] via 192.168.12.2, 00:28:38, Serial3/0
SINGAPORE#

>>> As observed from the SINGAPORE router’s ROUTING TABLE, we have both the MANILA and MELBOURNE routers loopback address.

>>> But as we have notice , all the three routers are showing that the Loopback’s are received as /32.

ALTERNATIVES for the “ip ospf priority 0” command:

>>> As Observed, the MANILA Router were elected as the DR as I have manually set the priority to the neighbors as 0. But one thing that I have notice here, if we are not still setting up manually the “ip opsf priority” on the interface mode of the routers, it will try to elect a BDR. Based on the results above, I have set up the OSPF priority of MELBOURNE router to 0 as seen below and it does elect SINGAPORE as the BDR  as it has the HIGHEST LOOPBACK Address.