Which IPv4 multicast address is used for all OSPF Designated Routers and Backup Designated Routers?

The correct address for all OSPF Designated Routers (DR) and Backup Designated Routers (BDR) is 224.0.0.6. This multicast address is specifically allocated for OSPF to allow routers on a multi-access network, such as Ethernet, to communicate efficiently with one another. By using the 224.0.0.6 multicast address, OSPF enables DR and BDR routers to receive updates and control messages without needing to send them to every individual router in the network. This optimizes network traffic and reduces unnecessary load, as only the DR and BDR will respond to routing updates on behalf of the other routers in that segment.

In contrast, other multicast addresses like 224.0.0.5 are used for all OSPF routers, which is different from the specific roles of the DR and BDR. Meanwhile, addresses such as 224.0.0.9 and 224.0.0.10 are designated for other routing protocols or specific multicast applications, and thus do not apply to OSPF's functionalities. Understanding the specific roles of these multicast addresses is essential for network configuration and troubleshooting within OSPF environments.