OSPF notes

Packet types:
Hello (type1): used to discover neighbors, build and maintain adjacencies
DBD(type2): database description packets. These are a summary of the topological database. Detects MTU mismatches (fragmentation is not allowed). Have bits I (start of DBD), M (more ackets DB), MS(master/slave used to decides who starts sending DBD first)
LSR(type3): link state request. Request specific link state advertisement (LSA) from a neighbor, typically after receiving DBD packets and noticing the local link state database is out of date
LSU(type4): link state updates. Advertises LSAs into the network . Can contain multiple LSAs
LSAck(type5): link state acknowledgements. Acknowledges LSUs

Packet formats:

OSPF packet:
Version | Type | Length | Router ID | Area ID | Cksum | Auth type | Auth data | Data

LSU packet (type 4):
Number of LSAs | LSA header | LSA data | LSA header | LSA data | ...

LSA header:
Age | Options | Link-state Type | Link-state ID | Advertising router | Seq number | Cksum | Length

LSA types:

Router LSA (type 1):
- has area scope
- describes local router
- standards LSA header  plus some extras (only a few showed below):
   - 5 bits set to 0 followed by bits 6,7,8  set for virtual links (V), ASBR (E), ABR (B)
   - number of links (2 bytes)
   - link ID (4 bytes)
   - link data (4 bytes)
   - link type (1 bytes)
   - metric (2 bytes)

 Link-type: Type 1 (P2P). Link ID: Neighbor Router ID. Link data: Local interface IP
 Link-type: Type 2 (Transit). Link ID: DR's interface IP. Link data: Local interface IP [broadcast segment]
 Link-type: Type 3 (Stub). Link ID: Network number. Link data: Subnet mask [passive interfaces, loopback interfaces]
 Link-type: Type 4 (Virtual link). Link ID: Neighbor Router ID. Link data: Local interface IP