OSPF Notes

Items contained in the OSPF Hello Packet include

  • Router ID of the originating router
  • Area ID of the originating router interface
  • Address mask of the originating interface
  • Authentication type and authentication information for the originating interface
  • HelloInterval of the originating interface
  • RouterDeadInterval of the originating interface
  • Router priority
  • DR and BDR
  • Five flag bits signifying optional capabilities
  • Router IDs of the originating router’s neighbor. This list contains only routers from which Hellos were heard on the originating interface within the last RouterDeadInterval is the AllSPFRouters multicast IP and is the AllDRRouters multicast IP address.

The OSPF Network Types are Broadcast, Non-Broadcast MultiAccess (NBMA), Point-to-Point, Point-to-Multipoint, and Virtual-Link

Items that must be agreed on during the OSPF neighbor hello process are

  • Hello/dead transmission intervals
  • Area ID number
  • Subnet mask (for multiaccess networks)
  • Stub area flag
  • Authentication type and password

Cisco default OSPF priority is 1. Priority 0 disqualifies a router from being a DR or BDR. Highest OSPF priority becomes the DR.

Standard OSPF LSA Types:

  • Type 1 – Router LSA
  • Type 2 – Network LSA
  • Type 3 – Network Summary LSA
  • Type 4 – ASBR Summary LSA
  • Type 5 – AS External LSA
  • Type 7 – NSSA External LSA

The order of determining the OSPF router-id is as follows: Statically assigned router-id, highest Loopback IP address, highest interface IP address

Inter-area routes are between two different areas, whereas intra-area routers are within a single area.

Router LSA’s describe the states of the router’s links within the area and are flooded only within an area for which that router is a member.

Type 2 (Network) LSA’s are only generated by the Designated Router (DR), and describe the set of routers attached to a particular NBMA or broadcast network.

Type 3 LSA’s are only transmitted to areas where the network or subnet advertised is not found.

Type 4 (ASBR Summary) LSA’s describe a route to a destination outside the OSPF area yet still inside the autonomouse system, that is, an inter-area route. Type 4 LSA’s describe routes to AS boundary routers (ASBRs) and are also generated by ABRs.

Type 5 (AS External) LSA’s describe routes to destinations that are external to the AS, and are not flooded to stub areas.

Type 7 LSA’s are generated by ASBRs and describe routes within a NSSA. A NSSA is a stub network with an ASBR that is connected to an external network. The ASBR advertises type 7 LSA’s about the external network to the stub network, then the ABR tranlates the type 7 to type 5 LSAs into area 0.

Order of OSPF states:

Down -> INIT -> 2WAY -> EXSTART -> EXCHANGE -> Loading -> FULL

5 main types of OSPF packets

  • Hello
  • Database Description (DD or DBD)
  • Link State Request (LSR)
  • Link State Update (LSU)
  • Link State Acknowledgement (LSA)

Default OSPF Hello timers are 10 seconds (broadcast) and 30 seconds (NBMA). The default OSPF dead interval is 4x the hello timer.