Which statement is true about IS-IS PDUs?
A.
CSNPs are exchanged only during initial adjacency establishment when a complete LSDB is being built.
B.
PSNPs contain header information for all link-state PDUs in the IS-IS database.
C.
Both the DIS and the backup DIS broadcast CSNPs over a broadcast link.
D.
Separate CSNP and PSNP types exist for Level 1 and Level 2 systems.
Explanation:
Protocol Data Units
IS-IS routers use protocol data units (PDUs) to exchange information. Each protocol data unit (PDU) shares a common header.IS-IS Hello PDU
IS-IS hello PDUs establish adjacencies with other routers and have three different formats: one for point-to-point hello packets, one for Level1 broadcast links, and one for Level2 broadcast links. Level1 routers must share the same area address to form an adjacency, while Level2 routers do not have this limitation. The request for adjacency is encoded in the Circuit type field of the PDU.
Hello PDUs have a preset length assigned to them. The IS-IS router does not resize any PDU to match the maximum transmission unit (MTU) on a router interface. Each interface supports the maximum IS-IS PDU of 1492bytes, and hello PDUs are padded to meet the maximum value. When the hello is sent to a neighboring router, the connecting interface supports the maximum PDU size.Link-State PDU
A link-state PDU (LSP) contains information about each router in the network and the connected interfaces. Also included is metric and IS-IS neighbor information. Each LSP must be refreshed periodically on the network and is acknowledged by information within a sequence number packet.
On point-to-point links, each LSP is acknowledged by a partial sequence number PDU (PSNP), but on broadcast links, a complete sequence number PDU (CSNP) is sent out over the network. Any router that finds newer LSP information in the CSNP then purges the out-of-date entry and updates the link-state database.
LSPs support variable-length subnet mask addressing.Complete Sequence Number PDU
The complete sequence number PDU (CSNP) lists all the link-state PDUs (LSPs) in the link-state database of the local router. Contained within the CSNP is an LSP identifier, a lifetime, a sequence number, and a checksum for each entry in the database. Periodically, a CSNP is sent on both broadcast and point-to-point links to maintain a correct database. Also, the advertisement of CSNPs occurs when an adjacency is formed with another router. Like IS-IS hello PDUs, CSNPs come in two types: Level1 and Level2.
When a device receives a CSNP, it checks the database entries again its own local link-state database. If it detects missing information, the device requests specific LSP details using a partial sequence number PDU (PSNP).Partial Sequence Number PDU
A partial sequence number PDU (PSNP) is used by an IS-IS router to request LSP information from a neighboring router. A PSNP can also explicitly acknowledge the receipt of an LSP on a point-to-point link. On a broadcast link, a CSNP is used as implicit knowledge. Like hello PDUs and CSNPs, the PSNP also has two types: Level 1 and Level 2.When a device compares a CSNP to its local database and determines that an LSP is missing, the router issues a PSNP for the missing LSP, which is returned in a link-state PDU from the router sending the CSNP. The received LSP is then stored in the local database, and an acknowledgement is sent back to the originating router.