/* OSPF Neighbour States */

DOWN        No Hello received. Initial state or after timeout.

INIT        Hello received from neighbour, but our Router-ID
            not yet in their neighbour list.

2-WAY       Our Router-ID seen in neighbour's Hello.
            Bidirectional communication confirmed.
            On broadcast networks: DR/BDR election happens here.
            Non-DR/BDR neighbours stop here (2-Way with DR/BDR → Full).

EXSTART     Master/Slave election via DBD packets.
            Higher Router-ID becomes Master, controls sequence numbers.

EXCHANGE    Routers exchange Database Description (DBD) packets —
            summaries of their LSDB (LSA headers only, not full LSAs).

LOADING     Router sends LSR (Link State Request) for LSAs it's missing.
            Neighbour sends LSU (Link State Update) with the requested LSAs.

FULL        LSDBs are synchronised. Routing table can be computed.
            This is the healthy operational state.

/* Hello packet fields that must match for neighbour formation */
Area ID:             must be identical
Authentication:      must match (none/simple/MD5)
Hello interval:      must match (default 10s point-to-point, 10s broadcast)
Dead interval:       must match (default 40s = 4 × hello)
Subnet mask:         must match (point-to-point links exempt)
Stub area flag:      must match

/* Troubleshooting stuck neighbours */
show ip ospf neighbor           # current state
show ip ospf neighbor detail    # full detail including timers
debug ip ospf adj               # live adjacency events

/* Stuck in EXSTART: MTU mismatch — one side has jumbo frames, other doesn't */
# DBD packets use the interface MTU — if mismatched, packets get fragmented
# Fix: ip ospf mtu-ignore  (or fix the MTU)

/* Dijkstra's algorithm — simplified */
Input:  LSDB (directed graph of routers and links with costs)
Output: Shortest Path Tree (SPT) rooted at this router

1. Mark all routers with distance = ∞, except self = 0.
2. Put all routers in a priority queue (tentative set), keyed by distance.
3. While priority queue not empty:
   a. Extract router R with minimum distance from queue
   b. Mark R as "confirmed" (add to SPT)
   c. For each neighbour N of R (from R's Router LSA):
      new_dist = dist(R) + cost(R→N)
      if new_dist < dist(N):
          dist(N) = new_dist
          predecessor(N) = R
          Update N's priority in queue
4. SPT complete — predecessor array gives next-hop for each destination.

/* Worked example */
Network:  R1─(1)─R2─(1)─R4
           └─(10)─────────R3─(1)─R4

Computing SPT from R1:
Confirmed: {R1=0}
Tentative: {R2=1, R3=10}

Extract R2 (cost 1):
  Confirmed: {R1=0, R2=1}
  R4 via R2: cost = 1+1 = 2 → add R4=2
  Tentative: {R3=10, R4=2}

Extract R4 (cost 2):
  Confirmed: {R1=0, R2=1, R4=2}
  R3 via R4: cost = 2+1 = 3 → update R3=3 (was 10!)
  Tentative: {R3=3}

Extract R3 (cost 3):
  Confirmed: {R1=0, R2=1, R4=2, R3=3}

Result:
  R4 → via R2 (cost 2) — NOT via R3 (cost 11)
  R3 → via R2→R4 (cost 3) — NOT direct (cost 10)

/* Linux FRR: configure OSPFv3 */
vtysh
  router ospf6
    ospf6 router-id 1.1.1.1   # must set manually (no IPv4 to borrow)
    interface eth0 area 0.0.0.0

  interface eth0
    ipv6 ospf6 area 0.0.0.0

show ipv6 ospf6 neighbor
show ipv6 ospf6 database
show ipv6 ospf6 route

/* Verify OSPFv3 uses link-local source addresses */
# Hellos sent from fe80::... not global unicast
# Link-local = no router will forward these beyond the segment