What is the function of the OSPF Designated Router (DR)?

On multi-access networks like Ethernet, where many routers can communicate directly, OSPF elects a Designated Router (DR) and a Backup Designated Router (BDR). The primary function of the DR is to serve as a central point for exchanging OSPF information. Instead of every router forming an adjacency with every other router (an n-squared problem), each router forms an adjacency only with the DR and BDR. Routers send their Link-State Updates (LSUs) to the DR, which then floods the updates to all other routers on the segment. This significantly reduces the number of adjacencies and the volume of redundant OSPF traffic.

A. To perform OSPF path calculations: Every OSPF router independently runs the Shortest Path First (SPF) algorithm to calculate its own routing table based on the LSDB.

C. To directly forward OSPF traffic to external networks: This is the function of an Autonomous System Boundary Router (ASBR), which connects the OSPF domain to other routing domains.

D. To advertise OSPF routes to other routers in the area: While the DR does originate a Type 2 Network LSA, its primary function is not advertising in general (all routers do that) but to optimize the flooding process.

1. Moy

J. (1998). RFC 2328: OSPF Version 2. The Internet Society. Section 7

"The Designated Router". The section states

"The Designated Router optimizes the flooding procedure on a broadcast network."

2. Juniper Networks Documentation. (2023). OSPF Designated Router Overview. "On broadcast networks... electing a designated router for the network reduces the amount of routing protocol traffic."