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Key features
Link State-protocol: #each router adv about the state of his links / interface's subnet
Hierarchical Area ( group of routers),  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 Header





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)

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


DR or Designated Router

DR function in multi-access network ( VPLS)

OSPF DR Election

 OSPF DR election is non-preemptive

election takes place by exchange of Hello packets

1- set the new priorities and
2- trigger re-election by clearing the adjacency
3- convergence time depend on number of routes

Priority: Highest preferred ( Link )

In case of a tie, the router with highest router-id is elected the DR.


Packet Type

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 Flow & Neighbor Adjacency
LSA or Link State A

Link state type 1: Router

Link state type 2: NetworkBcst interfaces
Link state type 3: SummaryCreated by ABR
Link state type 4: ABSRsummaryCreated by ABR ( created from LSA1? or LSA5? or LSA7??)
Link state type 5: External route

From ASBR ( any to OSPF )

E1 includes – internal cost to ASBR added to external cost,

E2 does not include – internal cost. it is same as external cost.


Link state type 7: External in NSSA 

Adjacency State


Down

init

2Way

Exstart

issue with MTU

negotiate: the Master / Slave ( define the seq number) and in charge of  dB sync

master: highest RID


Exchange

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