In flooding, we send unicast/multicast frames to all the routers in the network, whereas in broadcasting, we have a broadcast address by which we send packets to all the routers.

In OSPF, "Router disseminates LSPs (link state packets) to all the routers excluding itself and not only to its neighbors."

So why can't we use directed or limited broadcasting here to send these LSPs to all routers? Why is flooding used here instead of broadcasting?


You are confusing the layer-2 flooding with OSPF flooding. The routers in an OSPF area may not all be directly connected, but every router in the area needs to have a full understanding of all the other routers and networks in the area. OSPF floods this information to every other router in the area, even if some of the other routers may not be reachable by layer-2 frames from the flooding router.

Broadcast requires that the routers be on the same network. This is strictly true for Limited Broadcast, and Directed Broadcast should be disabled due to security concerns (see RFC 2644, Changing the Default for Directed Broadcasts in Routers).

  • So routers inside a particular area may not belong to same network ? – Zephyr Oct 19 '17 at 18:49
  • Of course. There can be many networks in the same area, and a router in an area may only be able to get to all the other routers, but one, through another router. Also, understand that not all the links between routers are broadcast networks. Multiple OSPF routers on a broadcast network elect a DR and BDR, and they only exchange routes with those. – Ron Maupin Oct 19 '17 at 18:54
  • I have one basic doubt. A router has many interfaces or ports with different IP addresses. Router device just acts as a connection between two or more networks .So on which network will the router actually belong as it is just connecting 2 different networks? – Zephyr Oct 19 '17 at 19:05
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    A router will route packets between two or more networks. The router belongs to every network in which it has an interface with an address. Routers can even have virtual interfaces, e.g. loopback interfaces, that belong to networks that exist nowhere else. OSPF routers can even belong to different OSPF areas. That is what an ABR does; it belongs to Area 0 (the backbone) and another area, and it routes traffic from the other area to Area 0 in order for traffic to reach other areas. – Ron Maupin Oct 19 '17 at 19:18
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    The router belongs to both networks. That is the purpose of the router. Can you tell me to which road an intersection belongs? The intersection at 1st St. and Avenue A belongs to which of those roads? – Ron Maupin Oct 19 '17 at 19:28

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