The OSPF broadcast network type is generally used on links supporting more than two routers. This guide explains what OSPF broadcast network type is, how it works, how to configure and verify that OSPF network type on Cisco IOS, and how OSPF represents broadcast links using router and network LSAs (LSA Type 1 and LSA Type 2).

Here is the network diagram I will be using in this guide.

The following table list the IP addresses configured on the routers.

Router Interface IP Address
R1 Serial1/0 10.0.123.1/24
Loopback0 10.0.110.1/24
R2 Serial1/0 10.0.123.2/24
FastEthernet 0/0 10.0.245.2/24
Loopback0 10.0.120.2/24
R3 Serial1/0 10.0.123.3/24
Loopback0 10.0.130.3/24
R4 FastEthernet 0/0 10.0.245.4/24
Loopback0 10.0.140.4/24
R5 FastEthernet 0/0 10.0.245.5/24
Loopback0 10.0.150.5/24

On each Serial1/0 interface, we map each DLCI with the corresponding remote IP address using the frame-relay map ip broadcast command in order to let routers communicate with other. The broadcast keyword enables Layer 3 broadcasts for the current DLCI. Layer 3 broadcasts will be explained in the last section of this guide.

Open the links below to download the initial router configurations.

Router R1

Router R2

Router R3

Router R4

Router R5

What is OSPF Broadcast Network Type?

A broadcast network is a medium type that allows communication between more than two network devices, whether they are locally or remotely attached to it. Additionally, the network allows each device to send a single layer 2 frame and get it reaches all devices simultaneously. Ethernet is an example of a broadcast network.

The OSPF broadcast network type is an operating mode in which OSPF assumes the router is connected to a broadcast medium. The router discovers neighbors and participate in the DR/BDR election using multicast Hello packets. By default, OSPF hello messages are sent every 10 seconds on broadcast networks.

Additionally, Ethernet-enabled interfaces, Frame Relay interfaces and multipoint Frame Relay subinterfaces configured to support layer 3 broadcasts, are examples of links on which the OSPF broadcast network type can be enabled. Besides, the default OSPF network type for Ethernet is broadcast, as shown in this example.

Router R5

R5# show ip ospf interface fastEthernet 0/0
FastEthernet0/0 is up, line protocol is up 
  Internet Address 10.0.245.5/24, Area 0 
  Process ID 1, Router ID 5.5.5.5, Network Type BROADCAST, Cost: 1
  Topology-MTID    Cost    Disabled    Shutdown      Topology Name
        0           1         no          no            Base
  Transmit Delay is 1 sec, State DR, Priority 1
  Designated Router (ID) 5.5.5.5, Interface address 10.0.245.5
  Backup Designated router (ID) 4.4.4.4, Interface address 10.0.245.4
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    oob-resync timeout 40
    Hello due in 00:00:05
  Supports Link-local Signaling (LLS)



omitted output

How Do OSPF Routers Establish Neighbor Relationships on Broadcast Links?

When the OSPF broadcast network type is used, Hello packets got created every 10 seconds and sent to multicast IP address 224.0.0.5, the dead interval is 40 seconds, and routers on the broadcast link elect a DR and BDR, which form full OSPF neighbor relationships with all routers.

The DROther routers do not form OSPF adjacencies with each other. Moreover, the DR and BDR fields in the Hello packets include the Designated Router and Backup Designated Router IP addresses, and each OSPF Hello packet includes the neighbor list of the originating router, as illustrated in this figure.

Note that the DR is responsible to produce the network LSA of the broadcast data link, and it receives LSAs from neighbors and refloods them back into the network.

On a broadcast network, OSPF adjacencies go through these steps:

Step 1. Routers select a Designated Router and Backup Designated Router, and OSPF neighbor relationships between all the routers reach the 2-WAY state.

Step 2. The Designated Router decides to establish OSPF adjacencies with all neighbor routers using the process I will explain later in this section.

Step 3. Likewise, the Backup Designated Router decides to build up OSPF adjacencies with all OSPF neighbors. Also, the BDR takes over as the DR if the current DR goes down.

Finally, note that OSPF neighbor relationships between DROther routers get stuck at the 2-WAY neighbor state.

DR and BDR form full OSPF neighbor relationships with every router on the broadcast network the way two routers A and B establish an OSPF adjacency on a point-to-point link. Here is the full process:

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 will be able to 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 Broadcast Networks in Router and Network LSAs?

In general, OSPF generates one route LSA entry for each broadcast interface. If a DR is elected, OSPF marks the interface as a transit network, and it includes the IP address and cost of the interface, and the DR’s IP address, as shown in this example.

Link connected to: a Transit Network 
     (Link ID) Designated Router address: 10.0.245.5
     (Link Data) Router Interface address: 10.0.245.4 
      Number of MTID metrics: 0 
       TOS 0 Metrics: 1   

If no DR exists, OSPF considers the broadcast link as a stub network, includes its cost, subnet IP address, and subnet mask, as illustrated here.

Link connected to: a Stub Network 
     (Link ID) Network/subnet number: 100.10.2.0  
     (Link Data) Network Mask: 255.255.255.0  
      Number of MTID metrics: 0 
       TOS 0 Metrics: 1

For each broadcast link, the corresponding DR originates one network LSA. A network LSA includes the IP address and subnet mask of DR, which helps to calculate the IP address of the corresponding subnet by ANDing those two fields. Also, this LSA lists all OSPF neighbors connected to the current broadcast network, as indicated in this example.

R5# show ip ospf database network 10.0.245.5

            OSPF Router with ID (5.5.5.5) (Process ID 1)

                Net Link States (Area 0)

  Routing Bit Set on this LSA in topology Base with MTID 0
  LS age: 1412
  Options: (No TOS-capability, DC)
  LS Type: Network Links
  Link State ID: 10.0.245.5 (address of Designated Router)
  Advertising Router: 5.5.5.5
  LS Seq Number: 8000000A
  Checksum: 0xD017
  Length: 36
  Network Mask: /24
        Attached Router: 5.5.5.5
        Attached Router: 2.2.2.2
        Attached Router: 4.4.4.4

How to Configure The OSPF Broadcast Network Type on Cisco IOS?

By default, the OSPF broadcast network type is enabled on Ethernet, FastEthernet, GigabitEthernet interfaces, and so on.

To change the default OSPF network type of a specific interface to broadcast, use the ip ospf network broadcast command in interface configuration mode. This example assigns the OSPF broadcast network type to R1’s Serial1/0 interface.

R1(config)# interface serial 1/0
R1(config-if)# ip ospf network broadcast

To verify the OSPF network type, issue the show ip ospf inteface command in enable mode, as shown in the following example.

R1# show ip ospf interface serial 1/0
Serial1/0 is up, line protocol is up 
  Internet Address 10.0.123.1/24, Area 0 
  Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 64
  Topology-MTID    Cost    Disabled    Shutdown      Topology Name
        0           64        no          no            Base
  Transmit Delay is 1 sec, State DR, Priority 1
  Designated Router (ID) 1.1.1.1, Interface address 10.0.123.1
  No backup designated router on this network
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    oob-resync timeout 40
    Hello due in 00:00:04


omitted output



How to Configure Routers to Use The OSPF Broadcast Network Type on Frame Relay?

Frame Relay does not offer native layer 2 broadcasts, unlike Ethernet. In Ethernet, frames with FFFF-FFFF-FFFF as their destination MAC address get delivered to all devices on the same data link. For example, whenever a broadcast frame gets sent in a VLAN or LAN, all devices receive a copy of it.

In Frame Relay, there is no layer 2 address or DLCI that would trigger the network to act like Ethernet networks handle frames sent to FFFF-FFFF-FFFF.

However, you can get similar behavior by enabling layer 3 broadcasts. Let’s explain this.

In Ethernet networks, when an IP packet gets sent to broadcast IP address 255.255.255.255, the IP packet gets encapsulated into a frame, and then sent to FFFF-FFFF-FFFF.

Frame Relay does not work this way. However, when layer 3 broadcast support is enabled, each time the router generates a broadcast or multicast IP packet and has to forward it over a major Frame Relay interface or a multipoint Frame Relay subinterface, the router sends a copy of the IP packet over each DLCI attached to the interface.

This means the router duplicates the broadcast/multicast IP packet proportionally to the number of the interface’s DLCIs. For instance, R1’s Serial1/0 interface has two DLCIs attached to it, therefore, R1 would generate two copies of each broadcast/multicast IP packet that should exit that interface, and send one copy to R2 and the other to R3.

This feature helps configure OSPF on FrameRelay without the need to use the neighbor command. However, it does consume extra CPU cycles because of packet duplication.

In this example, we ping R2 and R3 from R1 using one ping statement.

R1# ping 255.255.255.255 source serial 1/0

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 255.255.255.255, timeout is 2 seconds:
Packet sent with a source address of 10.0.123.1

Reply to request 0 from 10.0.123.2, 124 ms
Reply to request 0 from 10.0.123.3, 160 ms
Reply to request 1 from 10.0.123.2, 72 ms
Reply to request 1 from 10.0.123.3, 104 ms
Reply to request 2 from 10.0.123.3, 52 ms
Reply to request 2 from 10.0.123.2, 84 ms
Reply to request 3 from 10.0.123.3, 136 ms
Reply to request 3 from 10.0.123.2, 164 ms
Reply to request 4 from 10.0.123.2, 84 ms
Reply to request 4 from 10.0.123.3, 116 ms

R1 sent separate ICMP echo requests over each DLCI connected to interface Serial 1/0, as shown in the following Wireshark captures. The first capture indicates that R1 sourced the ICMP request with IP address 10.0.123.1 and forward it using DLCI 102.

This second capture shows that R1 forwards the same ICMP request packet using DLCI 103.

Before continuing, note that routers on the Frame Relay cloud did not build any OSPF adjacency yet because we did configure neighbors manually on each router and the current OSPF network type of their serial 1/0 interfaces is non-broadcast, which does not initiate neighbor relationship negotiation automatically until the neighbor command is configured.

These examples change the OSPF network type of R1, R2, and R3′ serial 1/0 interfaces to broadcast.

Router R1

R1(config)# interface serial 1/0
R1(config-if)# ip ospf network broadcast

Router R2

R2(config)# interface serial 1/0
R2(config-if)# ip ospf network broadcast

Router R3

R3(config)# interface serial 1/0
R3(config-if)# ip ospf network broadcast

At this point, routers would start using multicast Hello packets to discover neighbors and form neighbor relationships. OSPF hellos would get generated and sent like ICMP requests in the last example.

R1 sends two multicast OSPF hellos to R2 and R3, which do the same. This figure states that R1 sent a hello packet over DLCI 102.

This second figure indicates that R1 forwarded another Hello packet over DLCI 103.

The show ip ospf neighbor command output states that R3 and R2 have been elected Designated Router and Backup Designated Router, respectively.

R1# show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
2.2.2.2           1   FULL/BDR        00:00:35    10.0.123.2      Serial1/0
3.3.3.3           1   FULL/DR         00:00:35    10.0.123.3      Serial1/0

To sum up, configuring OSPF on Frame Relay can be done easily by enabling layer 3 broadcasts using the frame-relay map ip broadcast command and instructing the Frame Relay interfaces to use the OSPF broadcast network type.

Related Lessons to OSPF Broadcast Network Type

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.