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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.

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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.

You can define a network by the topological configuration or by the capabilities of the underlying (data link) protocol.

From a topology perspective, two routers connected together are a point to point network, simply because there are exactly two devices on the network. A talks only to B, and B talks only to A. It wouldn't matter what the underlying protocol is (Ethernet, WiFi, DSL, etc).

From a protocol perspective, the same network could be considered a broadcast network because of the features of the data link protocol. Some protocols, like Ethernet and WiFi have broadcast capability -- that is, they can address a single message to all hosts on the network. Other protocols like HDLC (now obsolete) can't do that. They are designed to send messages to exactly one host that is on the other end of the wire.

At the time OSPF was developed, there were many more data link layer protocols in use than there are today. OSPF makes the distinction between broadcast, point-to-point, etc, because of how neighbors discover and communicate with each other, and how LSAs are exchanged.

Depending on the capabilities of the underlying data link protocol, OSPF can discover neighbors using broadcast/multicast, otherwise neighbors have to be explicitly configured.

OSPF assumes a broadcast network has more than two neighbors, so it uses the designated router concept to minimize LSA exchanges.

By selecting the network type, OSPF can make the most efficient use of the router resources by choosing the best way to talk to neighbors, minimizing bandwidth and CPU -- two things that were precious when the protocol was developed.

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A broadcast network is what you've got on an Ethernet switch (or multiple connected ones): any node can talk to any other directly. If there are VLANs, each VLAN represents its own broadcast network/domain.

The distinction that RFC makes is the contrast between a broadcast network with point-to-multipoint addressing, a non-broadcast network with P2MP, and a direct point-to-point link between exactly two routers.

Ethernet is an underlying protocol for network layer. Does it mean we have broadcast network whenever ethernet protocol is used?

Yes. When you use an Ethernet cable to connect two routers, it's still a broadcast network, with just two members. The difference is that a broadcast network can address all connected nodes at once.

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).

That's actually a good one. When you connect two routers directly through Ethernet, it may be point-to-point link on the network layer (when a /31 prefix is used). On the data link layer, it's still a broadcast network since that's the way Ethernet works generally. That's what counts for OSPF.

Note the definition for non-broadcast networks in the RFC (emphasis mine):

Networks supporting many (more than two) routers, but having no broadcast capability.

So, it's all about the capabilities here: Ethernet is always broadcast, something like a serial link is a non-broadcast network, and there may even be P2MP networks without broadcast (but I'm at a loss here).

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