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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, |
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| 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, ... |
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| 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 |
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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, |
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| OSPF Packet Type | 1- Hello
2- Database Descriptor
3- Link State Request
4- Link State Update
5- Acknowledge ( All LS messages are acknowledge ) |
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| OSPF Packet Header | |
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| 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/ |
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| OSPF Packet Type 2: Database Descriptor |
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| OSPF Packet Type 3: Link State Request |
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| OSPF Packet Type 4: Link State Update |
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| OSPF Packet Type 5: Acknowledge |
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| OSPF Packet Flow & Neighbor Adjacency |  |
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| Link state type 1: Router |
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| Link state type 2: Network |
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| Link state type 3: Summary |
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| Link state type 4: ABSRsummary |
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| 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. |
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| Link state type 7: External in NSSA |
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