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

This topic is to discuss the following lesson:

Hey,

Since we are using a frame relay network and I see that you have not included a broadcast command with the the frame relay command, how do the OSPF neighbors identify each other? As mentioned before without the broadcast command no multicast traffic will pass. Is this behavior because of point to point topology? How will the Hub/Spoke know that it has an OSPF speaking router on the other end, if no multicast traffic is flowing?

Hello Apoorva

RFC 2328 indicates what kind of hello and LSA communication OSPF uses for each type of network. These are summarized below:

  • Point-to-Point (P2P) - Multicast
  • Point-to-Multipoint (P2MP) - Unicast
  • Broadcast - Multicast
  • Non-broadcast multiple access (NBMA) - Unicast
  • Virtual Link - Unicast

This means that the Point to Multipoint communication between OSPF routers is unicast.

Now having said that, when you configure the frame relay maps in your configuration, you will require the broadcast keyword. The frame relay maps are not actually shown in the lesson so this is why you don’t see the broadcast keyword anywhere.

I hope this has been helpful!

Laz

sir can you explain me ?
1> what is point to point ( as my understanding point to point link is serial links ) and also i want to ask one thing how they can do multicast because you have told me in another question which i have asked with you that ethernet link has a multiaccess technology so they can do multicast please explaint it
2> what is point to multipoint link please explain with example i am very confused on it

Hello Harshit

When we speak about point to point and point to multipoint we are primarily talking about the different types of topologies that exist using Frame Relay.

Strictly speaking, a point to point connection is one that is created between two routers using a serial technology, as you correctly state. Frame Relay is such a technology. Now if you connect two routers together using Frame Relay this is a point to point connection. However, you can also create two point to point connections over Frame Relay, where you have a single Hub router connected to two spoke routers like so:


What makes this topology point to point is that each link has a separate subnet. The S0/0 interface of the hub has two subinterfaces, each one on a different subnet, connected to each individual spoke.
Now a point to multipoint scenario is one where all three routers have IP addresses in the same subnet, still using Frame Relay as the technology. Now this seems strange, because we’re using a non-multiaccess technology, how can we have three routers in the same subnet? Well, Frame Relay is able to make this happen by “simulating” multicast within a multiaccess technology. And this lesson describes how you can make OSPF, which requires multicast, to function in an inherently non-multicast non-multiaccess network.

You can find out more about point to point and point to multipoint topologies at the following lessons:



I hope this has been helpful!

Laz

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sir
I want to ask when we are using point to point connection we have to create a subinterface in hub router because we have a single link but why we create a subinterface in spoke routers why not we directly use it without subinterface

Hello Harshit

On the spoke routers it’s not necessary to have a subinterface configured. You can indeed configure the physical interface with Frame-Relay encapsulation and have it function correctly. However, it can be useful to use a subinterface as the number used can help you remember the DLCI you are using. For example, if you’re using a DLCI of 105 on the particular VC on a spoke router, you can configure a subinterface of Serial 0/0.105 so that you can remember that the interface corresponds to DLCI 105. This is strictly a personal preference however, and is not technically necessary.

I hope this has been helpful!

Laz