OSPF Next Hop IP with different Network Types

OSPF will use different IP addresses for the next hop depending on the network type that you use. This can be confusing when you try configuring OSPF on top of a frame-relay network. In this short lesson, I want to show you the difference between the next-hop IP address and the OSPF network type that we use.

There are 5 OSPF network types:

• Non-Broadcast
• Broadcast
• Point-to-Multipoint
• Point-to-Multipoint Non-Broadcast
• Point-to-Point

I made non-broadcast and broadcast blue because these two network types have something in common. They both require a DR/BDR election and basically, you are telling OSPF that you have a multi-access network. In other words…every router can reach any other router. This can be challenging with frame-relay because this is not always the case if you have a partial mesh or hub and spoke configuration.

Point-to-multipoint, point-to-multipoint non-broadcast, and point-to-point are in red because they have one thing in common: We tell OSPF that we have a “bunch of point-to-point” links. As you will see in my demonstration, the “blue” network types have different next-hop behavior from the “red” network types.

Let’s take a look at a configuration so I can demonstrate it to you:

Above you see a frame-relay hub and spoke network. We have two PVCs, but there is only a single subnet, so this is a frame-relay point-to-multipoint network. You can also see that router Spoke1 has network 2.2.2.0 /24 behind it. We will advertise this network into OSPF and see what the next hop IP address is like. Let’s configure our routers:

Hub(config)#interface serial 0/0
Hub(config-if)#ip address 192.168.123.1 255.255.255.0
Hub(config-if)#encapsulation frame-relay 
Hub(config-if)#ip ospf network broadcast 
Hub(config-if)#exit
Hub(config)#router ospf 1
Hub(config-router)#network 192.168.123.0 0.0.0.255 area 0

Spoke1(config)#interface serial 0/0
Spoke1(config-if)#ip address 192.168.123.2 255.255.255.0
Spoke1(config-if)#encapsulation frame-relay 
Spoke1(config-if)#ip ospf network broadcast 
Spoke1(config-if)#ip ospf priority 0
Spoke1(config-if)#exit
Spoke1(config)#router ospf 1
Spoke1(config-router)#network 192.168.123.0 0.0.0.255 area 0

Spoke2(config)#interface serial 0/0
Spoke2(config-if)#ip address 192.168.123.3 255.255.255.0
Spoke2(config-if)#encapsulation frame-relay 
Spoke2(config-if)#ip ospf network broadcast 
Spoke2(config-if)#ip ospf priority 0
Spoke2(config-if)#exit
Spoke2(config)#router ospf 1
Spoke2(config-router)#network 192.168.123.0 0.0.0.255 area 0

Above you see my configuration for the hub and spoke routers. I configured frame-relay, used the broadcast network type, and made sure that the spoke routers won’t become the DR/BDR with the priority 0 command. Now let’s advertise network 2.2.2.0 /24 in OSPF on router Spoke1:

Spoke1(config)#interface loopback 0
Spoke1(config-if)#ip address 2.2.2.2 255.255.255.0    
Spoke1(config-if)#exit
Spoke1(config)#router ospf 1
Spoke1(config-router)#network 2.2.2.0 0.0.0.255 area 0

We will add a loopback interface to advertise the network in OSPF. Let’s check router Spoke2 to see what it looks like in the routing table: