Ospf Rfc 2328

OSPF RFC 2328
OSPF is an open-standard, classless routing protocol that converges quickly and uses cost as a metric. (Cisco IOS automatically associates cost with bandwidth.) OSPF is a link-state routing protocol and uses Dijkstra’s Shortest Path First (SPF) algorithm to determine its best path to each network. The first responsibility of a link-state router is to create a database that reflectsthe structure of the network.

OSPF routers exchange Hellos with each neighbor, learning Router ID (RID) and cost. Neighbor information is kept in the adjacency database. The router then constructs the appropriate Link State Advertisements (LSA), which include information such as the RIDs of, and cost to, each neighbor. Each router in the routing domain shares its LSAs with all other routers.Each router keeps the complete set of LSAs in a table—the Link State Database (LSDB).
Each router runs the SPF algorithm to compute best paths. It then submits these paths for inclusion in the routing table, or forwarding database.

OSPF routing domains are broken up into areas. Two-level hierarchy to OSPF areas exists; area 0 is designed as a transit area, and other areas should be attacheddirectly to area 0 and only to area 0. The link-state database must be identical for each router in an area.

Dividing an OSPF network into areas does the following:
* Minimizes the number of routing table entries
* Contains LSA flooding to a reasonable area
* Minimizes the impact of a topology change
* Enforces the concept of a hierarchical network design

Following are severaltypes of areas:
* Backbone area: Area 0, which is attached to every other area.
* Regular area: Non-backbone area; its database contains both internal and external routes.
* Stub area: It’s database contains only internal routes and a default route.
* Totally Stubby Area: Cisco proprietary area designation. Its database contains routes only for its own area and a default route.* Not-so-stubby area (NSSA): Its database contains internal routes, routes redistributed from a connected routing process, and optionally a default route.
* Totally NSSA: Cisco proprietary area designation. Its database contains only routes for its own area, routes redistributed from a connected routing process, and a default route.

OSPF defines router roles as well. One router can havemultiple roles:
* An internal router has all interfaces in one area.
* Backbone routers have at least one interface assigned to area 0.
* An Area Border Router (ABR) has interfaces in two or more areas.. ABRs separates LSA flooding areas, can summarize area routes, and can source default routes. They maintain a link-state database for each area to which they are connected.
* AnAutonomous System Boundary Router (ASBR) has interfaces inside and outside the OSPF routing domain.

OSPF Metric
By default, Cisco assigns a cost to each interface that is inversely proportional to 100 Mbps.
Cost= 100 Mbps
Bandwidth
The default formula doesn’t differentiate between interfaces with speeds faster than 100 Mbps, the cost formula can be adjusted using the auto-costcommand under the OSPF routing process. The cost can also be manually assigned under the interface configuration mode with the ip ospf cost command.

Link State Advertisements (LSA)
Each router maintains a database, called the link-state database (LSDB), containing the latest received LSAs. A separate LSDB is maintained for each area connected to the router.

LSA Operation
Each LSA is numberedwith a sequence number, and a timer is run to age out old LSAs. The default timer is 30 minutes. When a LSA is received, it’s compared to the LSDB. If it is new, it is added to the database, and the SPF algorithm is run. If it is from a Router ID that is already in the database, the sequence number is compared, and older LSAs are discarded. If it is a new LSA, it is incorporated in the database,...