Home Network Layer OSPF Version 2 OSPF Point-to-Multipoint Network Type is for Frame Relay and NBMA Links

OSPF Point-to-Multipoint Network Type is for Frame Relay and NBMA Links

The OSPF point-to-multipoint network type is commonly used on NBMA networks. This lesson explains what OSPF point-to-multipoint network type is, how it works, how to configure and verify that OSPF network type on Cisco IOS, and how OSPF represents point-to-multipoint links in router LSAs.

Here is the network topology I will be using in this tutorial.

Open the links below to download the initial router configurations. The OSPF router ID of each router is n.n.n.n, where n is the router’s number.

Router R1 Router R2 Router R3 Router R4

To learn how Frame Relay works, read this post about the OSPF non-broadcast network type. You may also be interested in reading this guide about the OSPF broadcast network type.

What is OSPF Point-to-Multipoint Network Type?

A point-to-multipoint network is a medium that consists of a collection of point-to-point connections. For instance, a partial mesh Frame Relay network topology, like the one in the figure above, is considered a point-to-multipoint network. Additionally, non-broadcast multi-access (NBMA) networks are also called point-to-multipoint networks.

The OSPF point-to-multipoint network type is one of four OSPF network types, it is an operating mode in which OSPF assumes the router has many point-to-point connections through the same interface. Cisco’s implementation of this network type offers two versions; one supports broadcasts while the other does not.

The OSPF point-to-multipoint network type is not enabled by default on any router interface type. The Hello and Dead intervals for the broadcast version are 10 seconds and 40 seconds, while they are 30 seconds and 120 seconds for the non-broadcast version of that network type.

The OSPF point-to-multipoint network type best fits partial mesh network topologies like the one used in this guide. It does not make sense to implement that network type in a broadcast network or full mesh network topology because that would make OSPD nodes waste lots of CPU cycles and network bandwidth by establishing/maintaining unnecessary OSPF adjacencies and flooding LSAs excessively.

How Do OSPF Nodes Build Up Neighbor Relationships on Point-to-Multipoint Links?

OSPF Point-to-Multipoint Network Type with Broadcast Support

When the broadcast OSPF point-to-multipoint network type is enabled, Hello packets got created and sent every 10 seconds, the dead interval is 40 seconds, there is no OSPF DR/BDR election, the router tries to build full OSPF neighbor relationship with all detected routers over the network, and the DR and BDR fields in the Hello packets are set to 0.0.0.0, as illustrated in this figure.

Moreover, OSPF Hello packets are sent to multicast IP address 224.0.0.5, and they include the list of active neighbors, as shown in the figure.

OSPF Point-to-Multipoint Network Type Without Broadcast Support

When the non-broadcast OSPF point-to-multipoint network type is enabled, Hello packets got created and sent every 40 seconds, the dead interval is 120 seconds, there is no DR/BDR election, the router tries to build full OSPF neighbor relationship with all configured routers with the neighbor command, and the DR and BDR fields in the Hello packets are set to 0.0.0.0, as illustrated in this figure.

Moreover, OSPF Hello packets are sent to unicast IP addresses, the neighbor IP addresses, and they include the list of active neighbors, as shown in the figure.

Establishing OSPF Adjacencies over Point-to-Multipoint Networks

Similar to the OSPF point-to-point network type, on a point-to-multipoint data link, every two routers A and B follow these steps to form an OSPF neighbor relationship:

Step 1. A and B find each other via Hello packets.

Step 2. A and B make sure that they are communicating bidirectionally; that is, they receive messages from each other in response to Hello messages. By including the neighbor’s router ID next to its discovery, they can accomplish this goal.

Step 3. A and B choose to be adjacent and pick the master and initial value of the DD sequence number, which is used to organize DBD packets.

Step 4. A and B share their LSA databases using Database Description (DBD) packets. Each OSPF neighbor sends a sequence of DBD packets to the other router.

Step 5. Each router requests the necessary LSAs to update its LSDB.

Step 6. Once the LSDBs of both routers have been synchronized, each OSPF neighbor is declared fully adjacent and reaches the full OSPF neighbor state.

Step 7. A and B maintain neighbor adjacencies through Hello packets.

How Does OSPF Represent Links with The OSPF Point-to-Multipoint Network Type?

When it comes to a point-to-multipoint link, OSPF generates router LSA entries based on the number of neighbors on that link. One router entry for each OSPF neighbor. The router specifies the neighbor’s OSPF cost and router ID, and the IP address of the interface on which the neighbor is detected. For example, router R1 has three active neighbor relationships, thus it produces three router LSA entries for them, as illustrated in this example.

    Link connected to: another Router (point-to-point)
     (Link ID) Neighboring Router ID: 10.0.4.4
     (Link Data) Router Interface address: 10.0.0.1
      Number of MTID metrics: 0
       TOS 0 Metrics: 64

    Link connected to: another Router (point-to-point)
     (Link ID) Neighboring Router ID: 10.0.3.3
     (Link Data) Router Interface address: 10.0.0.1
      Number of MTID metrics: 0
       TOS 0 Metrics: 64

    Link connected to: another Router (point-to-point)
     (Link ID) Neighboring Router ID: 10.0.2.2
     (Link Data) Router Interface address: 10.0.0.1
      Number of MTID metrics: 0
       TOS 0 Metrics: 64

Additionally, the router creates another router LSA entry to describe the current interface. OSPF advertises the interface’s IP address with a cost of 0 and the /32 subnet mask instead of the current interface’s subnet mask because OSPF considers the network as a collection of devices instead of a whole subnet/network.

This example illustrates a router LSA’s entry that shows how OSPF describes an interface configured with OSPF point-to-multipoint network type.

    Link connected to: a Stub Network
     (Link ID) Network/subnet number: 10.0.0.1
     (Link Data) Network Mask: 255.255.255.255
      Number of MTID metrics: 0
       TOS 0 Metrics: 0

How to Configure The OSPF Point-to-Multipoint Network Type on Cisco IOS?

To change the default OSPF network type of a particular interface to broadcast point-to-multipoint, use the ip ospf network point-to-multipoint command in interface configuration mode. This example sets the network type of interface Serial1/0 to broadcast point-to-multipoint.

R4(config)# interface serial 1/0
R4(config-if)# ip ospf network point-to-multipoint

To set the OSPF network type of an interface to non-broadcast point-to-multipoint, use the ip ospf network point-to-multipoint non-broadcast command in interface configuration mode. This example sets the network type of interface Serial1/0 to non-broadcast point-to-multipoint.

R4(config)# interface serial 1/0
R4(config-if)# ip ospf network point-to-multipoint non-broadcast

To verify the OSPF network type, use the show ip ospf interface command in enable mode, as shown in this example.

R4# show ip ospf interface serial 1/0
Serial1/0 is up, line protocol is up 
  Internet Address 10.0.0.4/24, Area 0 
  Process ID 1, Router ID 10.0.4.4, Network Type POINT_TO_MULTIPOINT, Cost: 64
  Topology-MTID    Cost    Disabled    Shutdown      Topology Name
        0           64        no          no            Base
  Transmit Delay is 1 sec, State POINT_TO_MULTIPOINT
  Timer intervals configured, Hello 30, Dead 120, Wait 120, Retransmit 5
    oob-resync timeout 120
    Hello due in 00:00:10
  Supports Link-local Signaling (LLS)
  Cisco NSF helper support enabled
  IETF NSF helper support enabled
  Index 1/1, flood queue length 0
  Next 0x0(0)/0x0(0)
  Last flood scan length is 1, maximum is 1
  Last flood scan time is 0 msec, maximum is 0 msec
  Neighbor Count is 1, Adjacent neighbor count is 1 
    Adjacent with neighbor 10.0.1.1
  Suppress hello for 0 neighbor(s)

Related Lessons to OSPF Point-to-Multipoint Network Type

Conclusion

I hope this blog post helps you learn something.
Now I’d like to turn it over to you:
What did you like about this tutorial?
Or maybe you have an excellent idea that you think I need to add.
Either way, let me know by leaving a comment below right now.

Mohamed Ouamer
Mohamed Ouamer is a computer science teacher and a self-published author. He taught networking technologies and programming for more than fifteen years. While he loves to share knowledge and write, Mohamed's best passions include spending time with his family, visiting his parents, and learning new things.
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