On a broadcast interface, an IS-IS router receives an LSP that is newer than the one on its database. Which of the following statements best describes the actions taken by the router as a consequence?

According to the IS-IS protocol specification for broadcast circuits, when a router receives a Link State PDU (LSP) with a higher sequence number than the version in its own database, it performs three key actions. First, it updates its Link State Database (LSDB) with the newer LSP. Second, it must acknowledge the receipt of the LSP by sending a Partial Sequence Number PDU (PSNP) back to the source. Third, it floods the new LSP to all its neighbors on its other interfaces to ensure the updated information propagates throughout the network. This ensures reliable and rapid convergence.

A. Sending back a copy of its own (older) LSP is the action taken when a router receives an LSP that is older than the one in its database, not newer.
B. This is incomplete. While updating the database and flooding are correct actions, it omits the crucial step of acknowledging the LSP, which is required for reliable flooding.
D. This is also incomplete. It correctly identifies the update and acknowledgment steps but omits the critical action of flooding the new LSP to other neighbors.

1. RFC 1142, OSI IS-IS Intra-domain Routing Protocol, Section 7.3.15.3, "Receiving Link State PDUs". This section explicitly states that upon receiving a newer LSP, the router shall store it, transmit it on other circuits, and acknowledge it. For broadcast circuits, acknowledgement is via a PSNP.

2. Nokia. (2021). 7750 SR OS R21.10.R1 Router Configuration Guide, Section: IS-IS Protocol Details. "When a new LSP is received, the router updates its LSDB, floods the LSP to other neighbors, and sends a PSNP to acknowledge receipt."

3. Ginsberg, L. (1991). The IS-IS Protocol: A Developer's Guide. Addison-Wesley. Chapter 5 discusses the LSP flooding and update mechanism, including the update, acknowledge, and flood sequence.