IS-IS PDU for Retransmission

B

The Intermediate System to Intermediate System (IS-IS) protocol is a link-state routing protocol used in large enterprise networks and Internet Service Provider (ISP) networks. In IS-IS, routers exchange link-state information in the form of Link State Protocol Data Units (LSPs). These LSPs contain information about the state of the network, including the routers and their links.

When a router receives an LSP from a neighbor, it stores the information in its Link State Database (LSDB) and updates its routing table accordingly. However, sometimes an LSP may be lost during transmission, or a router may miss an LSP due to a network partition or other issue. In such cases, the router needs to request a retransmission of the missing LSP.

To request a retransmission of a missing LSP, the router sends a special type of PDU called a Partial Sequence Number PDU (PSNP). The PSNP contains a list of the LSPs that the router has already received and acknowledges. The PSNP also contains a list of the missing LSPs that the router is requesting from its neighbor.

Therefore, the correct answer to the question is B. the partial sequence number PDU. The PSNP is used to request retransmission of a missing LSP in IS-IS.

Option A, the complete sequence number PDU, is not used to request retransmission of missing LSPs. It is used to acknowledge receipt of all LSPs in a particular flooding scope.

Option C, a hello PDU with a request PDU TLV specifying the missing LSP, is not a valid IS-IS PDU. Hello PDUs are used to establish and maintain neighbor relationships between routers, but they do not include a request PDU TLV.

Option D, a hello PDU with a refresh PDU TLV requesting all LSPs, is also not a valid IS-IS PDU. Hello PDUs can include a refresh PDU TLV, but it is used to request that the neighbor send all of its LSPs, not just the missing ones.