Which IPv4 multicast address is assigned to all OSPF routers?

The multicast address assigned to all OSPF routers is 224.0.0.5. This address is designated for OSPF routers in a multicast environment, allowing OSPF packets to be sent to all OSPF routers that are part of the same network segment without requiring a direct unicast to each individual router.

When an OSPF router sends routing updates, it uses this multicast address so that all OSPF routers can listen for and process these updates simultaneously, enhancing network efficiency and reducing unnecessary network traffic. This behavior is critical for dynamic routing protocols like OSPF to ensure that all routers in an OSPF area are synchronized with the current network topology.

The other multicast addresses listed serve different purposes. For instance, 224.0.0.1 targets all hosts on a local network, 224.0.0.10 is used for OSPF designated routers, and 224.0.0.6 is specifically for OSPF backup designated routers. Each of these addresses is important in its own context but does not apply directly to all OSPF routers in the same way that 224.0.0.5 does.