Multicasting Address

[Sulaiman Ejaz]

Once the DR and BDR have been elected, the other routers (known as DRothers) will establish

adjacencies with the DR and BDR only. All routers continue to multicast Hellos to the AllSPFRouters

address 224.0.0.5 so that they can track neighbors, but DRothers multicast update packets to the

AllDRouters address 224.0.0.6. Only the DR and BDR will listen to this address; in turn, the DR will

flood the updates to the DRothers on 224.0.0.5.

                              reference: TCP/IP routing by jhef doyle

I didn't understand this, what these addressess are? I know these are used for multicasting but how can i see these address in routers?

regards,

Sulaiman Ejaz

[kcnajaf]

Hi Sulaiman,

There are two multicast addresses used for ospf routing.

224.0.0.5 used to send/receive hello packets among ospf routers.

If you have a network with DR/BDR the non DR routers send all their updates to 224.0.0.6 and then they get replicated to the others by sending it to 224.0.0.5.DR/BDR is only elected on broadcast/non broadcast network type.

In short these are the address OSPF uses in order to send Hello packets. These are not configurational parameters rather these are standard address used by OSPF protocol.

You could verify which address OSPF is sending the updated using "debug ip ospf events" as below

R1#debug ip ospf events

R1#
*Mar  1 00:28:46.891: OSPF: Send hello to 224.0.0.5 area 0 on FastEthernet0/0 from 132.1.1.1
R1#
*Mar  1 00:28:48.579: OSPF: Rcv hello from 2.2.2.2 area 0 from FastEthernet0/0 132.1.1.2
*Mar  1 00:28:48.583: OSPF: End of hello processing
R1#
*Mar  1 00:28:53.467: OSPF: Rcv hello from 3.3.3.3 area 0 from FastEthernet0/0 132.1.1.3
*Mar  1 00:28:53.471: OSPF: End of hello processing
R1#
*Mar  1 00:28:56.103: OSPF: Send hello to 224.0.0.5 area 0 on FastEthernet0/0 from 132.1.1.1

Hope that helps.

Regards

Najaf

Please rate when applicable or helpful !!!

[Rolf Fischer]

Hi,

some addititional information to Najaf's posting:

If the media of an OSPF interface is not broadcast-capable, OSPF traffic has to be send to unicast destinations, which have to be configured since the router cannot know them.

The advantage of Multicast is that the interface needs to send only 1 packet to reach multiple destinations of a multiaccess-network and there are no neighbor-statement needed; in other words: Automatic neighbor discovery.

Like the multicast addresses of other routing- and control-protocols, OSPF's AllSPFRouters and AllDRouters addresses belong to the Local Network Control Block 224.0.0.0/24:

http://www.iana.org/assignments/multicast-addresses/multicast-addresses.xhtml#multicast-addresses-1

The usage of this addresses is part of the implementation, it depends on the network type if they are used for OSPF communication or unicast instead.

Hope that helps

Rolf