OSPF Network Types

mattsthe2

I'm reading the Cisco Press BSCI section on OSPF Network types and im having trouble understanding when you would use the various types like point to point, NBMA etc.

Is this usually for Frame-Relay, what about MPLS. And does the ISP usually dictate which one to use.

Thanks,

Matt

Paul Boz

It literally depends on the WAN technology you're using. Obviously you use point to point on point to point links.

The typical NBMA networks are Frame Relay, ATM and X.25 networks. Typically on WAN links broadcasts are not sent so you need special concessions for protocols that rely on broadcasts (OSPF, for example). Without special topology considerations OSPF wouldn't be able to run on WAN links.

MPLS is different because it's a tagging method that can carry ethernet, ATM, etc. You can run OSPF in broadcast mode fine over MPLS VPNs or tunnels.

nice343

frame-relay=NBMA or can be maunally configured as Point-to-multipoint
serial = point-to-point
Ethernet = broadcast

mattsthe2

its intersting you say that. I have two comments in regard to the two posts.

In my last company we had MPLS and we ran OSPF as our LAN protocol and BGP out to the MPLS with OPSF Restribution. At my new company same MPLS provider however we are advertising OSPF out to the provider, no BGP. The provider and us use Area 0 but they set there side to Priority 0. I just thought that was a little weird and didnt know it was possible to use OSPF in an MPLS network in this manner.

I found this interesting also because internet for our company goes out of our corp office. We dont inject a default route to our remote offices. So i believe our MPLS provider must know the difference somehow between our office traffic and default traffic.

Anyway my other question was when you know when to use NBMA or Point-to-multipoint in a frame-relay network?

GT-Rob

non-broadcast is on by default on frame relay links. You can enable it on the interface if broadcasts are allowed over the link.

Paul Boz

mattsthe2 wrote:

Anyway my other question was when you know when to use NBMA or Point-to-multipoint in a frame-relay network?

That's a part of knowing OSPF. The topology and wan/lan technologies dictate what OSPF mode you will run.

mattsthe2 wrote:

The provider and us use Area 0 but they set there side to Priority 0. I just thought that was a little weird and didnt know it was possible to use OSPF in an MPLS network in this manner.

I found this interesting also because internet for our company goes out of our corp office. We dont inject a default route to our remote offices. So i believe our MPLS provider must know the difference somehow between our office traffic and default traffic.

Sounds like you guys are using a MPLS VPN service.

mattsthe2

Sounds like you guys are using a MPLS VPN service.

Yes Paul we are...so i guess thats quite normal then. thanks.

kpjungle

I am reading the same thing. Having seen some CBT Nuggets videos about OSPF, it seems like the different modes have subtle differences.

There are the two RFC's and the 3 cisco "enhancements".
As far as I can see, there is no one-to-one mapping between your topology and the mode you will run. For example, the RFC nbma mode and the Cisco broadcast mode. The difference is whether you specify neighbors or not (to support non-broadcast VC's).

With both of them, a DR/BDR selection is performed, therefor you must "rig" the election, to make sure the DR is one that has connectivity to all other routers.

point-to-multipoint is the same, but no DR/BDR selection occurs.

One major difference I found out watching the CBT videos, is that the nbma and broadcast modes, really do act like a lan, in that some routes you wont be able to reach (without frame-relay maps), from every other router.

In point-to-multipoint, this is taken care of by the OSPF protocol. It will not create frame-relay maps, but it will route around it. (because it treats all the links as point to point links).