Router Link-State PDUs (LSPs)

BCE

Link-state PDUs are sent as a response to a link-state request with a PSNP and during the formation of an IS-IS neighbor relationship. They are also sent as a triggered event when network changes occur.

Link-State PDUs (LSPs) are used by routers in a link-state routing protocol to advertise information about the state of the network to other routers. Here are the three circumstances under which a router sends LSPs:

1. LSPs are sent when there is a network change Whenever there is a change in the network topology, such as a link going down or coming up, or a new router being added to the network, the router sends out an LSP to inform the other routers in the network of the change. This is important to ensure that all routers have a consistent view of the network topology.

2. LSPs are sent during adjacency formation When two routers form an adjacency, they exchange LSPs to synchronize their network topology databases. This allows them to establish a common view of the network topology and calculate their routes accordingly.

3. LSPs are sent in response to PSNP messages When a router receives a Partial Sequence Number PDU (PSNP) from another router, it compares the information in the PSNP to its own database of LSPs. If there are any missing LSPs, the router sends out LSPs to the other router to fill in the gaps in its database. This helps to ensure that both routers have a complete and up-to-date view of the network topology.

In contrast, option A is not correct because LSPs are not sent periodically every 9 seconds in most link-state routing protocols. However, some protocols like IS-IS use a fixed interval to send LSPs, but it is usually longer than 9 seconds. Option D is also incorrect because LSPs are not sent in response to CSNP messages; rather, CSNPs are used to acknowledge receipt of LSPs.