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

good afternoon, quick question

AFAIK frame relay is not included in CCNP any more. nor in the past CCNP routing or new CCNP encore. Is this lesson included as matter of better understanding? Wouldn’t it be better to upate this lesson within a DMVPN network in stead of frame realy?

Thanks!!
Jorge

Hello Jorge

You’re absolutely right that Frame-Relay is no longer part of any Cisco certification exams. However, the various OSPF types are (3.2.b in the official Cisco exam topics list.). The point to point OSPF network type can only be examined using a Frame-Relay topology. DMVPN does not use this network type for OSPF.

Also keep in mind this very important statement that Cisco always makes, which is found in the exam description of all of their certifications, including the link I shared above:

The following topics are general guidelines for the content likely to be included on the exam. However, other related topics may also appear on any specific delivery of the exam.

This essentially gives Cisco the right to add whatever other related topics they see fit, and for OSPF network types, this does seem to point to Frame-Relay.

Having said that, configurations of Frame-Relay itself is not expected on any exam, but configurations of OSPF on a Frame-Relay topology does seem to be fair game.

I hope this has been helpful!

Laz

May I know what’s the difference between OSPF Point-to-Multipoint over Frame-Relay VS OSPF Broadcast over Frame-Relay?
Both network types are broadcasting via OSPF by network IP MAST AREA, aren’t they?

I tried to understand the differences but the configurations look almost similar except the declaration of ip ospf network and one with priority 0.

Also, the OSPF non-broadcast VS OSPF point-to-multipoint looks similar too as both of them are required to designate the neighbors.

Hello Po

At the beginning of each of these lessons, Rene explains how OSPF “perceives” the network and consequently behaves. For OSPF Point-to-Multipoint Network Type:

  • There is automatic neighbor discovery so there is no need to configure OSPF neighbors manually.
  • There is no need for DR/BDR elections since OSPF sees the network as a collection of point-to-point links.
  • A single IP subnet is used in the topology.
  • In order for this to work, frame-relay must be configured to process broadcasts, which means the configuration must use the broadcast keyword.

For OSPF Broadcast over Frame Relay, the only difference is that DR/BDR elections take place because OSPF perceives the network as a single broadcast domain, and not as a collection of individual point to point links.

They are very similar. The difference is indeed that for OSPF Point-to-Multipoint Network Type, priority is set to 0 so that no DR/BDR elections take place. The ip ospf network command exists in all configurations since this is the command with which particular networks are advertised.

Non-broadcast does indeed require the neighbors to be configured, but point to multipoint does not.

For details of each OSPF network type and its characteristics and differences, take a look at this post:

I hope this has been helpful!

Laz

Hi Laz,

Just to quote “Point to point and point to multipoint OSPF network types can only be examined using a Frame-Relay topology. DMVPN does not use these network types for OSPF.”

If you have a DMVPN Tunnel, OSPF will detect this tunnel as a Point-to-Point Network Type. Will it not?

Hello Joseph

When OSPF is configured with the networks of the tunnel interfaces, yes the default will be point-to-point network type. However, as seen from this section of the DMVPN Phase 1 OSPF lesson below, the point to point topology will not work for any phase of DMVPN.


This is because by definition, DMVPN uses multipoint interfaces, so the hub router will expect one single neighbor.

However, I stand corrected in my statement about point to multipoint OSPF for DMVPN, as it can be used in such a topology, but only for Phase 1 and Phase 3, but not Phase 2. More on this below:

I have since corrected my statement in the above post.

I hope this has been helpful!

Laz

Perfect, I get it now, Thanks Laz,

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