Network Sales and Services Handbook (Cisco Press Networking Technology)

OSPF's rapid convergence is due to its use of areas. Areas are the compartmentalization of an internetwork (autonomous systems) into smaller pieces; a collection of networked end systems, routers, and transmission facilities.

OSPF area numbers are 32 bits in length. Area IDs range from 1 to 4,294,967,295 (the theoretical maximum number of OSPF supported areas).

OSPF Area Router Types

Based on area membership, there are three types of routers within an OSPF network, as illustrated in Figure F-1:

• Internal routers All router interfaces are defined in the same area, but not Area 0 (Backbone Area).

• Area border routers These interconnect the backbone and its area members.

• Backbone routers At least one defined interface belongs to Area 0 (Backbone Area).

OSPF Routing Types

OSPF supports four different types of routing:

• Default If an explicit route cannot be found for a given IP network or subnetwork, the router will forward the packet to the destination specified in the default route.

• Inter-area This exchanges data between different areas. All inter-area routing must traverse through Area 0. Nonzero OSPF areas are not permitted to communicate directly with each other.

• Intra-area Routing is self-contained and is limited to the routers that are internal to a single area.

• External routes When different ASes exchange routing information, the routes they exchange are referred to as external routes.

OSPF Area Types

The area types, listed here, determine which LSAs the area will receive:

• Stub Area Does not accept external LSAs. LSA Type 5s are rejected. Can accept route summaries. Stub areas carry a default route, intra-area routes, and inter-area routes, but they do not carry external routes. Stub areas are recommended for areas that have only one area border router, and they are often useful in areas with multiple area border routers.

• Totally Stubby Areas Will not accept LSAs with external routes or summaries.

• Internal Routers Exchange LSAs 1 and LSAs 2. They share the same routing databases, and all interfaces are within the same area.

• Backbone Routers (BBR) Exchange LSAs 1 and LSAs 2. Share at least one interface in Area 0.

• Area Border Router (ABR) Share an interface with another OSPF area. This router keeps a database for each area.

• Autonomous System Border Router (ASBR) Have at least one interface in a non-OSPF network; uses LSA 5s to distribute this routing information into the OSPF network.

OSPF Packets

OSPF uses five different packet types. Each is designed to support a different specific network function:

• Hello packets (Type 1) Used to establish and maintain relationships, or adjacencies, between neighboring nodes.

• Database description packets (Type 2) Exchanged between two OSPF routers as they initialize an adjacency. This is used to describe the contents of an OSPF router's link-state database.

• Link-state request packets (Type 3) Used to request specific pieces of a neighboring router's link-state database.

• Link-state update packets (Type 4) Used to transport LSAs to neighboring nodes. There are 11 types of LSAs:

  - LSA1: Router Links LSA Sends information about the router's links.

  - LSA2: Network Link LSA Sent by the DR to all routers in the AS, containing a list of routers in the segment.

  - LSA3: Summary Link LSA Sent by ASBRs, containing a list of networks available outside the area.

  - LSA4: Summary Link LSA Sent by ASBRs, containing a list of networks available outside the area.

  - LSA5: External Link LSA Sent by ASBRs, containing a list of external network routes.

  - LSA6: Group Membership LSA Part of MOSPF (Multicast OSPF), which routes multicast packets. As of this writing, Cisco does not support MOSPF.

  - LSA7: NSSA External LSA Originated by ASBRs in not-so-stubby areas (NSSAs). LSA7s operate in the same fashion as LSA5s, except that LSA7s are limited to NSSAs.

  - LSA8: External Attributes LSA

  - LSA9: Opaque LSA (link-local scope)

  - LSA10: Opaque LSA (area-local scope)

  - LSA11: Opaque LSA (AS scope)

• LSA types 8, 9, 10, and 11 have been proposed but have not been implemented.

• Link-state acknowledgement packets (Type 5) OSPF features a reliable distribution of LSA packets. This reliable distribution means that packet receipt must be acknowledged; otherwise, source nodes would have no mechanism to determine actual receipt of the LSA.