Cisco Enterprise Network Core Technologies (ENCOR) Practice Exam

The correct multicast address used by OSPF (Open Shortest Path First) to update routers in a designated router setup is 224.0.0.5. This address is specifically designed for OSPF routing protocol messages sent to all OSPF routers on a local network segment.

OSPF operates using two primary multicast addresses: 224.0.0.5 and 224.0.0.6. The address 224.0.0.6 is used to reach OSPF designated routers (DRs) and backup designated routers (BDRs) specifically, but it is not employed for all OSPF router communications. Instead, 224.0.0.5 is used to send OSPF routing updates and hello packets to all OSPF routers, including those that may not have been designated yet. This distinction is important for maintaining efficient and effective communication within OSPF networks.

Understanding this helps clarify OSPF operations and how routing updates are managed within a network, which is crucial for efficient network routing and topology management.