Exact meaning of "broadcast network" in OSPF

[ShahriarBasiri]

Dear Community,

According to rfc2328(OSPFv2), broadcast network is "Networks supporting many (more than two) attached routers,
together with the capability to address a single physical message to all of the attached routers (broadcast)". In the definition, it also states that "An ethernet is an example of a broadcast network". This statement seem really ambiguous.

Q1) Ethernet is an underlying protocol for network layer. Does it mean we have broadcast network whenever ethernet protocol is used? Even if two routers are directly connected via an ethernet cable?

Q2) According to examples across the web, if two routers are directly connected with an ethernet cable, this is a point-to-point network (which totally conforms to rfc point-to-point definition). but if there is a switch between routers, that makes a broadcast network. how it is concluded from rfc definition.

Any help is really appreciated,

Best Regards.

[MHM Cisco World]

if routers connect via SW "SW use ethernet which use broadcast for all port" 

the routers config OSPF -> here the broadcast network between each router not P2P.

if one router send update it will broadcast for all routers connect to this SW.

[Giuseppe Larosa · Accepted solution]

Hello @ShahriarBasiri ,

the key point here is the sentence:

>> capability to address a single physical message to all of the attached routers

This capability depends on the underlaying OSI L2 technology.

Ethernet supports broadcast and multicast as well a single ethernet frame can reach all hosts in a VLAN.

In an NBMA network to reach N neighbors the OSPF process has to send N copies of the packet one to each neighbor this happens for example with the dear old Frame-Relay where a point to point PVC exists between the router and each of its neighbors.

Coming to your questions:

Q1) Ethernet is an underlying protocol for network layer. Does it mean we have broadcast network whenever ethernet protocol is used? Even if two routers are directly connected via an ethernet cable?

Yes by default the OSPF network type is broadcast even if there are only two routers .

This can be overriden using ip ospf network point-to-point in interface config on both routers.

It is considered best practive now to avoid DR/BDR election when only two devices are involved

Q2) According to examples across the web, if two routers are directly connected with an ethernet cable, this is a point-to-point network (which totally conforms to rfc point-to-point definition). but if there is a switch between routers, that makes a broadcast network. how it is concluded from rfc definition.

I do not agree the routers cannot detect the LAN switch in the middle it can be present or not. The difference is that in case of interface failure a direct cable causes the other router to immediately detect the failure, in case of a swich in the middile the expiration of the OSPF dead interval tells that the other router has failed or OSPF has been disabled on other router side.

The IP OSPF network type follows the type of interface : it is broadcast on LAN interfaces it is point to point on serial interfaces or POS interfaces it is NBMA on Frame-Relay or ATM main interfaces (note the last two can have point to point subinterfaces)

Hope to help

Giuseppe

[ShahriarBasiri]

Hello @Giuseppe Larosa, 

Thanks for your explanation, 

So, OSPF elects DR/BDR even if there are only two routers connected by ethernet protocol, which is not necessary in this case. 

why it doesn't discover network type from hello packet responses? Every router can broadcast hello packets in layer2 and from number of responses , it can find whether this is a broadcast or point-to-point network. In this manner, even if there are just two routers connected by one switch, it is considered as point-to-point.

[Giuseppe Larosa · Accepted solution]

Hello @ShahriarBasiri ,

in an ethernet segment there is the potential for adding more OSPF neighbors in a later time if using a switch in the middle.

If OSPF would move to point to point by itself the addition of a third router would cause the rebuilding of all OSPF adjacencies because the network type should be changed to broadcast and a DR/BDR election should occur. This would cause a great impact.

With the current behaviour when a third router is added to the ethernet segment (if the subnet mask allows for this) it has just to build OSPF adjacencies with the DR and BDR the existing two routers are not impacted.

So it is left to the network engineer the responsability to decide if an ethernet segment has to be considered point to point or not.

It is a design and administrative decision.

Hope to help

Giuseppe

[ShahriarBasiri]

Dear Giuseppe,

Thanks for your great explanation.