OSPF Non-Broadcast Multiple-Access (NBMA) Networks
Share this page:On the Multipoint Frame-Relay network the default OSPF type is NON-BROADCAST. This means that the OSPF Neighbors will not be formed like on the standard Broadcast Network Segment.
Cisqueros_R1#show ip ospf inter s1/0
** Serial1/0 is up, line protocol is up **
** Internet Address 10.1.1.1/24, Area 0 **
** Process ID 1, Router ID 1.1.1.1, Network Type NON_BROADCAST, Cost: 64**
** Topology-MTID Cost Disabled Shutdown Topology Name**
** 0 64 no no Base**
** …**
So in order to establish the OSPF Neighbors, we can for example use the “network” command in order to transform the OSPF link from MULTICAST to UNICAST
** (config-router)#neighbor 172.128.185.66**
*In HUB-AND-SPOKE the Spokes do not have the Layer 2 reachability, so this command makes no sense. Instead just be sure to set their OSPF priority to 0, so that they dont participate the DR/BDR Election
(config-if)#ip ospf priority 0
The HUB Router will be elected as DR on every Link and exchange OSPF Database with each of the Spokes:
** Cisqueros_R1#show ip ospf neighbor <— R1 IS THE HUB**
** Neighbor ID Pri State Dead Time Address Interface**
** 2.2.2.2 0 FULL/DROTHER 00:01:51 10.1.1.2 Serial1/0**
** 3.3.3.3 0 FULL/DROTHER 00:01:51 10.1.1.3 Serial1/0**
** 4.4.4.4 0 FULL/DROTHER 00:01:56 10.1.1.4 Serial1/0**
*In this kind of OSPF Topology - it’s not necessary to have the Frame-Relay interface configured with the “broadcast” keyword, because we are manually defining the OSPF Neighbor and turning the Links into UNICASTS.
OSPF BROADCAST Networks v.s. POINT-TO-POINT Networks
If you wish to convert the previous network into the Broadcast Network, the following command needs to be applied:
** (config-if)#ip ospf network broadcast**
And in case it needs to be Point-to-Point:
** (config-if)#ip ospf network point-to-point**
The main difference here is the NEXT HOP:
BROADCAST: Next Hop is the router that ORIGINATED the Route
POINT-TO-POINT: Next Hop is the router that ADVERTISED the Route