LSP Flooding and Synchronization
What the exam tests
Flooding questions ask how LSPs reach every router reliably, what CSNP and PSNP do, and why mesh groups exist.
Core rules
- Flooding is reliable: an LSP is sent to all adjacencies except the one it arrived on, and it is acknowledged.
- On point-to-point links, PSNP acknowledges LSPs and requests missing ones.
- On broadcast links, the DIS sends CSNP summaries; routers that detect missing LSPs send PSNP requests.
- A point-to-point link also exchanges CSNP at adjacency establishment to synchronize databases.
- LSP retransmission timers resend unacknowledged LSPs.
- Mesh groups reduce flooding on highly meshed broadcast or point-to-point groups by limiting which interfaces an LSP is flooded out of.
Worked example
R1 and R2 are point-to-point neighbors. R1 floods a new LSP to R2 and starts a retransmit timer. R2 installs the LSP and returns a PSNP listing the LSP ID. If the PSNP is lost, R1 resends at the retransmit interval; R2 sees the duplicate and re-acknowledges. On a broadcast LAN, the DIS's CSNP performs the same synchronization role for all attached routers.
Traps
Assuming CSNP is only used on broadcast links; forgetting that PSNP is both acknowledgement and request; misconfiguring mesh groups so LSPs never reach part of the topology; MTU drops silently breaking flooding.