Link-State Request

C

With IS-IS, a partial sequence number PDU (PSNP) performs a similar function as a Link State Request packet does in OSPF, which is to request link-state data.

In a Junos network, the Interior Gateway Protocol (IGP) uses the link-state protocol (LSP) to communicate and maintain routing information between routers. The LSP is a type of packet that contains information about the state of the router's interfaces and the state of its connections to other routers in the network.

In order to maintain accurate routing information, Junos devices periodically exchange LSPs with their neighboring routers. However, sometimes a device may need to request up-to-date link-state information from a specific neighbor. To do this, Junos devices use a special type of LSP called a Partial Sequence Number Packet (PSNP).

PSNPs are used to request specific LSPs from a neighbor. When a router receives a PSNP, it responds with another special type of LSP called a Complete Sequence Number Packet (CSNP). CSNPs contain a list of all the LSPs that the router currently has in its database. The requesting router can then use this information to determine which LSPs it needs to request from its neighbor.

So, the answer to the question is option C: PSNP. Junos devices use PSNPs to request up-to-date link-state information from their neighboring routers.