In a previous lesson I showed you how to configure the OSPF point-to-multipoint network type. This time we’ll look at the OSPF point-to-multipoint non-broadcast network type. It’s the exact same thing but we’ll have to specify OSPF neighbors ourselves. Here is the topology that we’ll use:
There are a couple of things that you need to be aware of:
- No Automatic neighbor discovery so you 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.
Here’s the configuration for the Hub router:
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 non-broadcast
Hub(config-if)#exit
Hub(config)#router ospf 1
Hub(config-router)#network 192.168.123.0 0.0.0.255 area 0
Hub(config-router)#neighbor 192.168.123.2
Hub(config-router)#neighbor 192.168.123.3This is the hub configuration. I changed the network type to point-to-multipoint non-broadcast, advertised the 192.168.123.0 /24 network in OSPF and most important…specified the OSPF neighbors myself. Let’s look at the spoke router configuration:
Hi Renee
“There are a couple of things that you need to be aware of:
No Automatic neighbor discovery so you 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.”
Seems to me I have to commit them to memory.
I have to say them out loud day in and day out like a mantra.
I am asking whether or not there is a rhyme or reason to help me memorize or understand them.
In case of PMNB (point to multipoint nonbroadcast) DLCI’s are not configured?
Hi Lee,
I don’t have a rhyme but there’s some logic to these OSPF network types:
Only broadcast and non-broadcast require a DR/BDR election since OSPF sees these network types as “multi-access”. Everything with “point” in it is seen as a collection of point-to-point links so there is no DR/BDR election.
Everything with “non-broadcast” requires you to configure neighbors since OSPF won’t be able to find them by itself.
Only with point-to-point we use a subnet for each PVC, all other OSPF network types usually have a single IP subnet.
Still some memorization to d
... Continue reading in our forumHi, Rene
I hope you are doing great.
I have one question:
- why are you using the command network in order to auto discover the neighbors If you are statically defining them with the command neighbors ?
Have a great day.
Hi John,
Glad to hear you like it. OSPF will change its timers based on its OSPF network type
Rene
Hi Ajith,
Thanks, this can be pretty confusing. Most of these network types were used for certain types of networks like hub-and-spoke frame relay with or without broadcast capability.
On regular Ethernet interfaces you don’t really have to bother with choosing different OSPF network types. One of the few protocols where you still have to think about this is DMVPN, which also offers a hub and spoke topology like frame-relay.
Rene