OSPF: Configuring NBMA Network Type



There are 5 network tpyes in OSPF. They are: NBMA (Non-Broadcast MultiAccess), Point-to-Multipoint which are RFC standard and Broadcast, Point-to-Point, Point-to-Multipoint (static) which are Cisco proprietary. The default network type for interfaces in OSPF for multipoint network or when you are configuring Frame-Relay on physical interfaces is NBMA.


The characteristics of NBMA network type are there will be DR/BDR elections, what has to be noted here is that DR and BDR should have direct connectivity with the rest OSPF router in the network. In NBMA network there will be no broadcast, so that we have to define our OSPF neighbor statically. We only have to specify the neighbors statically at one router (one-way), and the peer router will respond and the neighborship will be formed. Usually, the static neighbor configuration is done at the hub-router.



We will try to configure OSPF in NBMA network type based on this topology



First, configure the serial1/0 interface, give an ip address, set the encapsulation to frame-relay and perform no shutdown


ip address 10.10.10.1 255.255.255.0
encapsulation frame-relay
no shutdown
ip ospf priority 255


Remember that in an NBMA network, there is a DR/BDR election. DR/BDR routers should have direct connectivity with the rest of the routers in the segment. So here, we want to make sure that R1 becomes the DR router. Therefore we set the OSPF priority of the interface to 255. You can go to R2 and R3 and configure the same configuration at the serial1/0 interface


R2:


ip address 10.10.10.2 255.255.255.0
encapsulation frame-relay
no shutdown
ip ospf priority 0


R3:


ip address 10.10.10.3 255.255.255.0
encapsulation frame-relay
no shutdown
ip ospf priority 0


R2 and R3 have no direct connectivity, therefore both of them couldn't be the BDR. We specify the OSPF interface priority of R2 and R3 serial1/0 to 0, making them inelligible for DR/BDR election.


I also configure a loopback0 interface on R1 for OSPF connectivity purpose, so if later after OSPF has been configured, we can see at least a route learned from OSPF on R2 and R3 in its routing table. On R1:


interface loopback0
ip address 10.10.10.3 255.255.255.0


Then we can start configuring OSPF. We will use OSPF with process id of 1.


R1:


router ospf 1
router-id 1.1.1.1
network 10.10.10.1 0.0.0.0 area 0
network 192.168.1.0 0.0.0.255 area 0


R2:


router ospf 1
router-id 2.2.2.2
network 10.10.10.2 0.0.0.0 area 0


R3:


router ospf 1
router-id 3.3.3.3
network 10.10.10.3 0.0.0.0 area 0


You can do the "show ip ospf interface" command to see the network type of serial1/0 interface. Here's the partial output of the command.



It should list the network type as NON_BROADCAST. If not, you can change the OSPF network type with the command "ip ospf network non-broadcast" at the interface configuration command level. At this point, you will not have any OSPF neighbor, since in non-broadcast network, neighbors should be specified manually. Remember that we only have to type the neighbor command one-way, only on one router and it is a good idea to configure it at the hub-router, which is R1. So, we go to R1 and type in the following command at the global configuration level:


router ospf 1
neighbor 10.10.10.2
neighbor 10.10.10.3


After entering the command, you can type the "show ip ospf neighbor" command to see that R1 is ATTEMPT-ing to become neighbor with R2 and R3. You might want to wait before the neighbor status becomes FULL. When it comes to FULL, we can verify that the network of R1 loopback0 interface is advertised to R2 and R3 and is listed in their routing table. From the picture bellow, for you who might be wondering why does 192.168.1.1 as advertised as a host route with /32 subnet mask when you expect it to be /24. This is because loopback interfaces are advertised with /24 subnet. To get a /24 network in the routing table, the OSPF network type of the loopback interface could be change to point-point with the command "ip ospf network point-to-point".



But, there is still a problem. If you try to ping R3 from R2 and vice-versa, the ping will fail. This is because both R2 and R3 think that they have direct connectivity because both of the reside in the same network. But actually, they have to send the packets to R1 first before reaching each other. To solve this, we add the manual frame-relay map command for them to reach each other using the DLCI used to reach R1. On R2 serial1/0 interface configuration level, type the following command:


frame-relay map ip 10.10.10.3 201 broadcast


On R3 serial1/0 interface configuration level, type the following command:


frame-relay map ip 10.10.10.2 301 broadcast


Now, if you try to ping R3 from R2 or vice-versa, the ping will succeed.



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