OSPF Point-to-Multipoint over Frame-Relay

In previous lessons, I explained the OSPF non-broadcast and broadcast network types. Now we are going to look at the OSPF point-to-multipoint network type. This is the topology that we will use:

OSPF Network Type Topology

There are a couple of things that you need to be aware of:

  • Automatic neighbor discovery, so there is no need to configure OSPF neighbors yourself.
  • No DR/BDR election since OSPF sees the network as a collection of point-to-point links.
  • Only a single IP subnet is used in the topology above.
  • Make sure your frame-relay network is configured with the broadcast keyword.

Let’s take a look at the configuration:

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 point-to-multipoint
Hub(config-if)#exit
Hub(config)#router ospf 1
Hub(config-router)#network 192.168.123.0 0.0.0.255 area 0

This is the hub configuration. I changed the network type to point-to-multipoint and advertised the 192.168.123.0 /24 network in OSPF. Let’s look at the spoke router configuration:

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 point-to-multipoint
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 point-to-multipoint
Spoke2(config-if)#exit
Spoke2(config)#router ospf 1
Spoke2(config-router)#network 192.168.123.0 0.0.0.255 area 0

Just a basic configuration. You’ll have to change the OSPF network type and type in the correct network statements to become OSPF neighbors.

Don’t forget that you’ll require the broadcast keyword for your frame-relay maps, or this will not work. By default Inverse ARP will do this, but if you disabled Inverse ARP, you’ll have to create the correct frame-relay maps yourself.

Let’s check if we have OSPF neighbors:

Hub#show ip ospf neighbor 

Neighbor ID     Pri   State           Dead Time   Address         Interface
192.168.123.3     0   FULL/  -        00:01:35    192.168.123.3   Serial0/0
192.168.123.2     0   FULL/  -        00:01:56    192.168.123.2   Serial0/0

You can see that the hub router has two OSPF neighbors and there is no DR/BDR election.

  • Configurations
  • Hub
  • Spoke1
  • Spoke2

Unit 1: Introduction to OSPF

Unit 2: OSPF Neighbor Adjacency

Unit 3: OSPF Network Types

Unit 4: OSPF Stub Areas

Unit 5: Advanced OSPF Topics