A router receives an OSPF database description packet from a neighbor that references an LSA, which is already in the router's LSDB. The sequence number in the DBD packet is lower than the sequence number of the LSA in the LSDB. What action does the router take?

During the OSPF database exchange process (Exchange state), routers send Database Description (DBD) packets to summarize their Link-State Databases (LSDBs). When a router receives a DBD packet containing a header for an LSA with a sequence number lower than the one in its own LSDB, it concludes that its local copy is more recent. To synchronize the neighbor, the router will immediately send its newer LSA to that neighbor. This is accomplished by sending a Link-State Update (LSU) packet containing the full, more recent LSA.

B. The router sends a link-state acknowledgement to its neighbor. An LSAck is sent to acknowledge the receipt of an LSU, not in response to an older LSA header in a DBD packet.
C. The router updates the information in its LSDB with the information received from its neighbor. The router will not overwrite its newer LSA (higher sequence number) with older information from a neighbor.
D. The router sends a link-state request to its neighbor. A Link-State Request (LSR) is sent only when the neighbor's DBD indicates it has a newer LSA than the one in the local LSDB.
E. The router takes no action. The router must take action because it has detected that its neighbor's LSDB is out of date, and synchronization is required.

1. Moy, J. (1998). RFC 2328: OSPF Version 2. The Internet Society.

Section 10.3, "The Exchange State": This section explicitly states the procedure: "If the database LSA is more recent, the router sends its LSA to the neighbor in a Link State Update packet." This directly supports the correct answer.

2. Nokia (2022). 7450 ESS, 7750 SR, 7950 XRS, and VSR Unicast Routing Protocols Guide, Release 22.10.R1.

Chapter: OSPF, Section: OSPF Protocol: The guide confirms that the Nokia OSPF implementation is compliant with RFC 2328. The behavior described in the RFC, including the database exchange process, is the standard implemented behavior on Nokia routers. The process of comparing LSA headers in DBD packets and sending updates for more recent local LSAs is a fundamental part of this compliance.