OSPF Point-to-Multipoint Non-Broadcast Network Type over Frame-Relay

i just dont understand why broadcasting would be disabled? isnt it just a pseudo broadcast for discovering neighbors? I mean does it congest the network or something?

Hi David,

Broadcasting is a very inefficient method for communication. Imagine you had 200 routers and only 10 of them would talk OSPF. When you send a broadcast it means everyone receives it…whether you want it or not. It’s a waste of bandwidth and resources…

Rene

Hi Rene

I thought we could disable/enable broadcast - on a frameRelay multipoint interface - per PVC by using the Frame-Relay map ip “IP address” “PVC No.” [broadcast] command. So, If I have 200 routers only 10 speak OSPF why Wouldn’t I disable broadcast on the 190 PVCs and enable it on only 190 routers? The only downside of this is having to entre so many lines of mapping. My point is it is technically possible.

Hoss

Technically it’s possible :slight_smile:

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 do but if you understand the logic, it will be easier to remember :slight_smile:

Configuring DLCIs doesn’t have anything to do with OSPF or its network types but when you configure frame-relay, you always need DLCIs…maybe you get one from the frame-relay switch or you have to do it yourself.

Rene

1 Like

Hi, 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 Miguel,

I’m doing great, thanks for asking! Hope you as well.

The neighbor command is used to statically configure a neighbor but it doesn’t activate OSPF on an interface. Without the network command, OSPF will never send hello packets on the interface and the routers won’t become neighbors…with or without the neighbor command.

Rene

Rene,

Just an observation, but I think one thing that sets your site apart from others is that you separate lessons like point to point and point to multi-point. Rather than trying to explain them in one lesson going back and forth between each configuration through out the explanation.

This way is very clear, very nicely done!

Chris

Hi Chris,

Thanks for your kind words!

I think it’s important to break things down in smaller pieces, if you have to jump from one config to another then things get confusing. It’s easier to start with a blank slate and then try each network type one-by-one :slight_smile:

Rene

hi rene, awesome lessons, i now able to understand clearly the network types. Also notice here that the hello timer just went up to 30seconds.

Hi John,

Glad to hear you like it. OSPF will change its timers based on its OSPF network type :slight_smile:

Rene

Hi Rene,

You do not configure OSPF neighbors in spoke routers so how can they advertise networks to hub router since the network is non-broadcast ??

Hi Hussein,

If you configure the neighbor command on the hub then the spoke routers will receive it and a neighbor adjacency will be formed.

Rene

Thanks Rene,
Now everything is clear to me

Hi Rene,

Nice article.

But OSPF network types are still a confusing topic after going through your articles. Is it possible to simplify things with a single post where you can update the differences and purpose of different network types. ?

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

Thank You Rene, Your tutorial on Broadcast, Non-broadcast, Multi point are Brilliant.

Now I am able to Discern.

 

so I recreated this topology in GNS3 to see WireShark capture of OSPF messages, I could not get it to work because you did not specify “no shutdown” commands for s0/0 in the configuration section of your post.

Hi Rene,

On Multi-point Frame relay is there a reason that a single subnet is used over multiple subnet sub-interfaces. The typical Frame relay point-to-point it seems that its a defacto standard to use different subnets with sub-interfaces and with multi-point its typical that the same subnet is used and no sub-interfaces. WHat is the reasoning for this standard. It seems to me, that either method could be used for multi-point both sub-interfaces(different subnets) and single subnets (no sub-interfaces). I am missing why one method is used or preferred over the other???

Thanks
Mike