CCNP ROUTE 2.0: OSPF Neighbor Adjacency

Steps to form an adjacency with a router configured with OSPF

Determine own Router ID

The router will use the OSPF router-id in the following order

  • Router-id command
  • Highest IP address on a loopback interface
  • Highest IP address on any other configured (active) interface

Add interfaces to the link state database

…either by using the network command or enabling on the interface. Once enabled the interfaces sends hellos and advertises the network.

Sends a hello message on chosen interfaces

Contains information such as:     Router ID, Hello/Dead Timers, Network Mask, Area ID, Authentication Password

Hellos sent every:

Once every 10 Seconds on Broadcast (Ethernet), P2P Networks
Once every 30 Seconds on NBMA (Frame Relay) Networks

The following must match in order to establish FULL OSPF neighbour adjacency:

  • Area ID
  • Hello/Dead Timer Intervals
  • Authentication Password (if used)
  • Subnet Mask
  • Stub area
  • Duplicate router ID

Receives Hello

Checks hello/dead timer interval
Checks the network masks
Checks area ID
Checks authentication passwords

Send reply hello

Queries the hello packet to determine whether listed as an existing neighbour:

If an existing neighbor – router resets the dead timer
If not, adds as a new neighbour

Determine the “Master” – “Slave” relationship

Determined by highest priority or the router with the highest router-id will break the tie if same priority
Master then sends database description (DBD) packet = summary of the link-state database
Slave sends its DBD packet

DBDS are acknowledged and reviewed

Slave request details (Link State Requests – LSR)
Master sends updates (Link State Updates – LSU)
Master request details (LSR)
Slave sends updates (LSU)

Neighbors are synchronised

Routers in FULL STATE

OSPF Neighbor States

DOWN         No hello packets received from a neighbour
INIT        A Router sent a hello packet to another router but cannot see its own router ID in hello packet
2WAY         Routers can see their own router ID in the hello packets from a neighbor
EXSTART     Master and slave negotiations between router and DR/BDR to start exchange of LSAs.
EXCHANGE    Routers will exchange DBD (database descriptors) packets using sequence numbers, sending LSR and LSU to each other
LOADING     Once LSDB the same on routers
FULL        Routers are fully adjacent with each other.


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