OSPF Neighbor State
Below are the states.
1. Down State – No
hello packet has been received from neighbor but
hello packets can still be sent to the neighbor. In this state routers prepare
themselves for neighbor process. If the
neighbors are fully adjacent and they stop receiving hello from each other
within dead interval or if the manually configured neighbor is being removed from
configuration then the neighbor state changes from full to down.
2. Attempt State – Used
in Non-broadcast multi-access environment such as frame-relay where neighbors
are manually configured. OSPF does use
Attempt instead of Init State. Unicast hello is used in this state because
router does not have to discover them dynamically.
3. Init State – Hello
Packet received but router ID is not included in hello packet. If OSPF hello
parameters such as timer values, do not match, OSPF routers will never progress
beyond this state.
4. 2-Way State – Bidirectional
communication has been established between two routers. It means both peers have seen the other’s hello packet. In
this state routers see own router ID in received hello packet’s neighbor field,
In this state a router decides whether
to become full adjacent with neighbor or stick in 2-way state. If the OSPF network
in point to point or point to multipoint then OSPF routers will become full
adjacent with each other. If the network is multi-access or non-broadcast then
a router become fully adjacent with DR and BDR, It stays in the 2-way with
other routers. DR and BDR is elected in
this mode for broadcast and non-broadcast network but BDR is elected first in
this mode.
5. Exstart State – Master
and Slave election between routers and their DR and BDR, Router (Master) with
higher router ID start exchanging Link state information. A router with low priority can be a master and A router
with high priority can be slave. Don’
mix master/slave relationship with DR/BDR/DROTHER relationship because Master
and slave election is based on the router ID not based on the priority so DR
can be slave and DROTHER can be master. Master
and slave election is on a per neighbor basis. Initial sequence number for
adjacency information is also selected.
6. Exchange State – Link
state DBD is exchanged. DBD contains Link
state advertisement only. OSPF routers use sequence number to get accurate
information and Sequence number is incremented by master. Routers decide how
much information needs to be exchanged
7. Loading state – Actual
exchange of link state information. Routers
send link state request packet. Neighbor can provide requested link state information
in link state update. Link state update is reliable packet and it need
acknowledge so routers don’t miss any information.
8. Full State – Finally formed adjacency. Neighbors are fully adjacent with each other. OSPF database is fully synchronized. For Broadcast and NBMA media, routers will achieve the Full State with their DR and BDR router only, while for Point-to-point and Point-to-multipoint networks a router should be in the Full State with every neighboring router
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