OSPF is a #Layer 3 #classless #Link-State Routing Protocol developed by Internet Engineering Task Force (IETF)# where its metric is an arbitrary value called cost (defined by RFC 2328). In Cisco, the cost calculated using bandwidth. It is secure, scalable and provides fast Convergence#. It supports both IPv4 and IPv6. The implementation of OSPF varies from vendor to vendor.
There are five types of link-state packets that could be sent by an OSPF Router:
- Hello, which is used to establish (discover neighbour) and maintain adjacency, and also involved in electing DR and BDR
- DBD (Database Description), which is an abbreviated list of the sender’s link-state database
- LSR (Link-State Request), which is used by routers to request more information about any entry in DBD
- LSU (Link-State Update), which is used for OSPF routing updates
- LSAck (Link-State Acknowledgement), which is used to confirm receipt of the LSU
Note: Two routers need to have the same hello interval (default 10 – 30 seconds), dead interval (the period the router will wait to receive Hello packet before declaring the neighbour down, default 40 – 120 seconds) and network type in order to establish adjacency.
LSU can be used to send out one of 11 types of Link-State Advertisement (LSA) to neighbouring routers:
- Router LSAs (type 1)
- Network LSAs (type 2)
- Summary LSAs (type 3 or 4)
- Autonomous System (AS) External LSAs (type 5)
- Multicast OSPF LSAs (type 6)
- Defined for Not-So-Strubby Areas (type 7)
- External Attributes LSA for Border Gateway Protocol version 4 (BGPv4) (type 8)
- Opaque LSAs (type 9, 10, 11)
There will be an election of Designated Router (DR) and Backup Designated Router (BDR) within an area. DR will be the main router to updates all other OSPF routers, whereas BDR is the backup if DR fails. It is to reduce the amount of OSPF traffic on multiaccess networks.
In OSPFv3, it will advertise not only the IPv6# link-local address but also the global address.
See Cisco OSPF Router Configuration# for Cisco specific router configuration.