OSPF Next Hop IP with different Network Types

OSPF uses different next-hop IP addresses depending on the configured network type, which affects routing behavior in Frame Relay networks. Broadcast and Non-Broadcast network types use the advertising router’s IP as the next hop, while Point-to-Point, Point-to-Multipoint, and Point-to-Multipoint Non-Broadcast types use the IP address of the router from which the route was received. When you use the wrong network type in Frame-Relay hub-and-spoke topologies, it can cause reachability issues, requiring additional Frame-Relay maps to resolve. In this lesson, you’ll learn how each OSPF network type affects next-hop behavior and how to choose the right type for your Frame-Relay topology to avoid connectivity problems.

Key Takeaways

• Broadcast and Non-Broadcast network types use the advertising router’s IP as the next-hop address
• Point-to-Point, Point-to-Multipoint, and Point-to-Multipoint Non-Broadcast types use the receiving router’s IP as the next hop
• Broadcast/Non-Broadcast types require DR/BDR election and assume full-mesh connectivity
• Point-based network types treat the topology as multiple point-to-point links (no DR/BDR needed)
• Using Broadcast/Non-Broadcast types in hub-and-spoke Frame-Relay may require additional frame-relay maps between spoke routers
• Point-to-Multipoint is typically the best choice for Frame-Relay hub-and-spoke topologies as it avoids reachability issues

Prerequisites

To follow this lesson, you should have:

• Basic understanding of OSPF routing protocol
• Familiarity with Frame-Relay concepts (PVCs, DLCIs, hub-and-spoke topologies)
• Understanding of next-hop routing concepts

Explanation

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.

Configuration

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: