OSPF process in short

1, form adjacency

init: each routers will send out hello packets to ( if network type is broadcast) or unique ip address (if network type is none broadcast);

2-way: router A has received hello packet from B router,  the the hello packet includes its A’s id

extat: select DR/BDR if need, point to point network or point to multipoints networks do not select DR/BDR

exchange: exchanges the LSA

full: adjacency formed

When DR/BDR is presented in the network, all other routers can only form adjacency with DR and BDR, the rest of the routers can not form adjacency with each other, they will stay in 2 way state with each other.  When DR/BDR is not presented in the network ( point to point, or point to mulit points networks), adjacency is formed for each link.

Below is the table describing the process:

ospf adjencency process

2, Each router now has all information, LSA database, it will run dijkstra algorithm to calculate the shortest path towards each network work. That means each router will maintain the whole routing table calculated according to the LSA database. So it is critical for OSPF to guarantee that each router has the same LSA database.

3, OSPF routes will be selected to routing table according to administrative distance value  (110)in Cisco and default preference  (10)according to Juniper.


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