An IS-IS router on a broadcast medium has detected its LSDB is missing an LS PDU.
Which action will the router take?
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A. B. C. D.C
A Partial SNP (PSNP) is similar to CSNP except that it describes only some LSPs rather than the entire database. On a P2P network, PSNP is used to acknowledge received LSPs. On a LAN network, PSNP is used to request missing or latest LSPs.
Incorrect Answers:
A: IS-IS Sequence Number PDU (SNP)
SNPs are used to maintain IS-IS LSDB by describing some or all of the LSPs in the database. The DIS periodically multicast Complete SNP (CSNP) to describe all the LSPs in the Pseudonode database.
https://sites.google.com/site/amitsciscozone/home/is-is/is-is-packetsIn IS-IS, Link State PDUs (LSPs) are used to share routing information between routers in the same area. When an IS-IS router on a broadcast medium detects that its Link State Database (LSDB) is missing an LS PDU, it needs to take action to retrieve the missing information.
The options given in the question are:
A. The router will send a CSNP to the router that sent it a PSNP with missing PDUs. B. The router will send a link-state request packet to its closest Level 1/Level2 router. C. The router will send a PSNP to the router that sent it a CSNP with missing PDUs. D. The router will send a link-state request packet to its DIS router.
Let's examine each option in detail:
A. The router will send a CSNP to the router that sent it a PSNP with missing PDUs.
In IS-IS, the Complete Sequence Number Packet (CSNP) is used to synchronize LSDBs between routers. If a router receives a Partial Sequence Number Packet (PSNP) from a neighbor with missing LSPs, it will respond with a CSNP that lists all the LSPs in its database. However, this option does not address the issue of the missing LSP on the local router's LSDB.
B. The router will send a link-state request packet to its closest Level 1/Level2 router.
When a router needs to retrieve missing LSPs, it sends a Link State Request (LSR) packet to a neighbor that is known to have the missing LSP. The neighbor can either respond with the missing LSP or send back a Link State Update (LSU) packet that contains the missing LSP. In this option, the router sends an LSR to its closest Level 1 or Level 2 router, which could potentially have the missing LSP.
C. The router will send a PSNP to the router that sent it a CSNP with missing PDUs.
As explained earlier, a PSNP is used to request missing LSPs from a neighbor router. If a router receives a CSNP with missing LSPs, it can respond with a PSNP to request those missing LSPs. However, this option does not address the issue of the missing LSP on the local router's LSDB.
D. The router will send a link-state request packet to its DIS router.
The Designated Intermediate System (DIS) is responsible for maintaining the IS-IS topology on a broadcast medium. When a router needs to retrieve missing LSPs, it can send an LSR to the DIS, which is likely to have a complete copy of the LSDB. The DIS can then respond with the missing LSP or send back an LSU packet that contains the missing LSP.
Therefore, option D is the most appropriate answer in this case as it directs the router to send an LSR to the DIS router, which is responsible for maintaining the IS-IS topology on the broadcast medium, and is therefore most likely to have a complete copy of the LSDB.