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Key features
Link State-protocol: #each router adv about the state of his links / interface's subnet
Hierarchical ,  Area type: backbone, std , Stub, NSSA
All routers share the same database when in the same Area
Route redistribution ( from/to other protocol ) + summarization 
OSPF-TE:  carry BW,

Dijkstra / SPF Algorithm
1- place the source router at the root of the tree
2- calculate the cost ( Ref BW/link BW) to the destination
3- pick the one with the lowest cost
4- if same cost lowest RID ( def= system Mac@)

OSPF-TE: build a separate tree with all other constrate: BW, ...

OSPF Frame
| LL |  IP header ( protocol id= 89 ) | OSPF packet type | LT | 
  14B          20B                         20B + Data       

Link Layer / Link Trailer 

OSPF Header:  
       Ver , Packet Type/Length , RID/AreaID, Checksum, Auth type/key
Checksum: complement(1>0 and 0>1) 16bit at the time, + all together

Multicast @ ( + unicast)

L3 Multicast address:
        All OSPF speaker: 224.0.0.5
        All DR/BDR: 224.0.0.6
L2 Multicast address:   
       01-00-5E-00-00-05 & 01-00-5E-00-00-06

Unicast in case of repeat: LSR could be drop,

OSPF Packet Type
1- Hello
2- Database Descriptor
3- Link State Request
4- Link State Update
5- Acknowledge  ( All LS messages are acknowledge )

OSPF Packet Type 1: Hello
Network mask:     ( the src IP is in the IP@ header! )
hello Interval: 10sec         Router Dead interval: 40sec
Options:
Router Priority:
( Bcst link:  DRID and BDRID )
Neighbor:  IP@         ( Bcst list of Neighbor)

Image Added

https://lpmazariegos.com/2015/12/28/ospf-hello-packets/


OSPF Packet Type 2: Database Descriptor

OSPF Packet Type 3: Link State Request

OSPF Packet Type 4: Link State Update

OSPF Packet Type 5: Acknowledge

OSPF Packet FlowImage Added