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OSPF

Link State overview:#router enters the network#sends hello messages on its operational links to determine if some other link state router is operating on the interfaces as well#if so, it forms adjacency with it.

#The adjacency is used to exchange summary link state database info.#summary link state information is not the complete link state database information#If the Link state databases arent the same, they send to each other the link states dbase information to synchronze

> Form neighbors (use Hello packet)> Form adjacencies (Use Database description packet)

- adjacency requires tat @ router advertise its local database information.

# Each LS router uses Dijkstra algorithm to process the database information tofind the shortest path to each destination.# The essence is to maintain a consistent database by every LS router. This ensures that the network is loop free.

OSPF packet header fields:1. Version - set at 22. Type - hello packet, Database descriptor, link state request, link state update, link state acknowledgment? 1-5 resp.3. Packet length

4. Router ID - of the advertising rtr5. Area ID - area ID assigned to the interface used to send the OSPF packet6. Checksum7. Authentication type - Null, Simple Password, MD58. Authentication - authentication data to verify packet's integrity

Hello packet:Fields:> Network mask - of advertising ntw interface> Hello interval - value as requested by the advertising rtr - default = 10 s> Options - used to advertise capabilities by the local rtr> Router priority - priority of local rtr (for DR and BDR elections)

> Router dead interval - dead interval requested by the advrtising rtr- default= 40 s> Designated router - Interface address of current DR. 0.0.0.0 is displayed whenit is not yet elected

> BDR - Interface address of current BDR. 0.0.0.0 is displayed when it is not yet elected> Neighbor - router ID of all neighbors

Database Description packet:The DD packet (type code 2) summarizes the local database by sending LSA headersto the remote router. The remote router analyzes

these headers to determine whether it lacks any information within its own copy

of the LSD.Fields:Interface MTU, Options, Flags, DD sequence Number and LSA Headers

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Link-state request packetFields: Link-state Type, Link-state ID, Advertising routerNB: Link-state ID - This field encodes information specific to the LSA. @ different type of advertisement places different infor here.

Link-state Update packet:

Fields: Number of LSAs, Link-state Advertisements

Link-state acknowledgment packet:Fields: LSA Headers

Forming adjacencies:

Down > starting stateInit state > Ospf rtr receives a hello packet but local rtr ID is not listed in the received neighbor fieldAttempt > Only valid 4 NBMA networks. Hello packet hasnt been received

from the neighbor and that the local rtr is going to senda unicast hello packet to that neighbor within the specifiedhello interval period.2-way > local rtr has received a hello packet with its own rtr ID in the neighbor field, bidirectionalcommn has been established and the peers are now ospf neighborsExStart > The local rtr and the neighbor establish which rtr is in charge of dbase synchronization. The one with higher rtr ID is the one.Exchange > They exchange DD packetsloading > local rtr requires complete LSA info so it sends LS request packetsFull > Fully functional OSPF adjacency.

Troubleshooting adjacency formation: