OSPF over NBMA Topology Modes of Operation

grechy

Hey All!

I understand the differences between multi-point - point-point ect. I found this site helped me out
Route My World! » Blog Archive » BSCI: OSPF Network Types (part 2)

However I am still struggling on when is the best time to use each technology. The above link mentions

Point-to-Multipoint Nonbroadcast

• Neighbors must be configured manually
• Does not require a DR or BDR
• This mode should be used (instead of the RFC-compliant point-to-multipoint mode) if multicast and broadcast are not enabled on the VC.
  • That is because the router cannot dynamically discover its neighboring routers using the multicast hello packets

But you can simply add the broadcast command after your frame relay map statement. Is this more to do with what the ISP allows?

wave

A few things that helped me decide when to use each type are:

-Do you need a DR? Point-to-ANYTHING does not require a DR. If you need a DR then you're going to choose either nonbroadcast or broadcast mode. Do you want to configure your neighbors manually and have unicast communication between them? If yes, then you choose nonbroadcast. But if you want to **** and allow for more flexibility incase an RID changes or something, and you don't want to configure neighbors manually...Then you choose broadcast.

-If you have a DR it must have connectivity to every router. So if one of your routers isn't connecting back to the DR directly then you're going to choose point-to-point or point-to-multipoint. If you have a hub and spoke topology then you're probably going to choose point-to-multipoint which uses just one IP subnet. Again you have the choice of nonbroadcast mode which requires manual neighbor configuration and it will use unicast to communicate with neighbors.

If you only have a small network or you want to move away from the single IP subnet between your routers, you might choose point-to-point mode to connet all of your routers, more configuration is required and you'll have more routes in your routing table. This will work with a partial-mesh topology (as would multipoint modes).

So it all comes down to wether you need a DR and whether you want unicast or multicast communication which affects neighbor discovery.

If you're using one of the nonbroadcast modes you're going to be talking to your neighbors using unicast which will make your design more static. Sure, you can limit the broadcast traffic by only applying the broadcast keyword to your frame relay map statements that point from your hub to your spoke routers but this gives you more flexibility with design

Ultimately these modes are tools for design, so you always have some control over which direction you can go in.

Ardeen Packeer has some great blog posts on when to use each of the network types. I just tried to pull them up and his site seems to be down. I was reading his posts just a few days ago so hopefully the outage is just temporary: This website is currently unavailable. ...You can pull it up from google cache it's called: Tutorial: OSPF Network Types and Frame Relay Part 1

grechy

Cheers! Great post