Introduction to OSPF

The Open Shortest Path First (OSPF) is an Interior Gateway Protocol used to distribute routing information within a single Autonomous System.

• It is open standard (RFC2328)
• It is a Link-state protocol
• Administrative Distance is 110
• Uses cost as metric
• Fast convergence
• VLSM support
• Scalable to large networks
• Stub capabilities
• Authentication (None, Clear-text, MD5)
• Reliability (Sends only hello messages to maintain neighbour relationship)
• Route tagging capabilities
• Uses multicast for routing updates.

Multicast address used by all                      224.0.0.5

Multicast address used by DR/BDR          224.0.0.6

Timer Intervals

These are the values of the OSPF timers:

• Hello—Interval time in seconds that a router sends an OSPF hello packet.
• Dead—Time in seconds to wait before declaring a neighbour dead. By default, the dead timer interval is four times the hello timer interval.
• Wait—Timer interval that causes the interface to exit out of the wait period and select a DR on the network. This timer is always equal to the dead timer interval.
• Retransmit—Time to wait before retransmitting a database description (DBD) packet when it has not been acknowledged.

Default Timers:

On Broadcast and P2P links

• Hello 10 seconds
• Dead 40 seconds

On NBMA links

• Hello 30 seconds
• Dead 120 seconds