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OSPF Broadcast Network

The beauty of this network type is the ability to multicast.  Take the network below. If each router were to use unicast in the hello protocol, each one would have to send 3 lots of hellos every 10 seconds in order to establish & maintain adjacencies. Instead multicast allows the router to send 1 hello to do the same job.  This best explained in the packet capture beneath.

I’ve taken a packet capture on R3’s fa0/0 interface. The result is shown below. You can see that each router is only sending one hello, and it’s destined to 224.0.0.5. The highlighted line, which is a packet sent from 10.0.0.3 (R3),  shows that R3 has active neighbors for routers 1.1.1.1, 2.2.2.2, and 4.4.4.4.  So when any of those routers receive the hello, they are aware that R3 knows about them. For example, R1 can see that R3 knows about him because R3 lists 1.1.1.1 as an active neighbor. This shows how just 1 multicast packet can be used instead of 3 unicast hellos.

The DR’s Responsibility

Whenever a router needs to send a Link State Update (LSU), they send it to the DR & BDR (224.0.0.6). The DR then floods this to all neighbors. So on R3 if I just add a route into OSPF, we will see this in action with a packet capture:

R3(config)#int Loopback3
R3(config-if)#ip address 99.99.99.99 255.255.255.255
R3(config-if)#ip ospf network point-to-point
R3(config-if)#exit
R3(config)#router ospf 1
R3(config-router)#network 99.99.99.99 0.0.0.0 area 0

The packet capture that was performed during this configuration change is shown below:

As you can see the update was sent from R3 (10.0.0.3) to the DR only (224.0.0.6). The DR (10.0.0.4 – R4) then sent the update to the AllSPFRouters address (224.0.0.5), and each router replied back to the DR with an LS Acknowledgement. So as long as each router is synchronized with the DR/BDR on the subnet, then they will be able to obtain all the information about the broadcast network.

If a new router joined the network, we could then expect a network LSA to be sent from the DR to the new router, so that it has the most up to date information. To demonstrate this, I cleared the OSPF process on R3. The packet capture can be seen below:

So as you can see, the DR sent all the information (the attached routers) in just one update. If you wanted to see the type 2 network LSA in the OSPF database, you can find it using the following command #sh ip ospf database network [IP Address of the DR]

R2#sh ip ospf database network 10.0.0.4

            OSPF Router with ID (2.2.2.2) (Process ID 1)

                Net Link States (Area 0)

  Routing Bit Set on this LSA
  LS age: 567
  Options: (No TOS-capability, DC)
  LS Type: Network Links
  Link State ID: 10.0.0.4 (address of Designated Router)
  Advertising Router: 4.4.4.4
  LS Seq Number: 80000004
  Checksum: 0xBC2C
  Length: 40
  Network Mask: /24
        Attached Router: 4.4.4.4
        Attached Router: 1.1.1.1
        Attached Router: 2.2.2.2
        Attached Router: 3.3.3.3

 

The OSPF Broadcast Network Characteristics

– OSPF hellos are sent to the AllSPFRouters multicast address – 224.0.0.5

– Default timers are Hello 10 secs, Dead 40 secs

– DR/BDR election occurs

 

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