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Most Commonly Used OSPF terms:
Link state database
The data structure held by an ospf router for the purpose of storing topology data.
Shortest path first(SPF)
The name of the algorithm OSPF uses to analyze the LSDB. The analysis determines the best(Lowest cost) route for each prefix/length. You might also hear people refer to this as Dijkstra's algorithm.
Link state Update
The name of the OSPF packet that holds the detailed topology information, specifically LSAs. LSA's are sent within the LSU.
Link state advertisement(LSA)
The name of a class of OSPF data structures that hold topology information. LSAs are helf in memory in the LSDB(Topology Table) and communicated over the network in LSU(Link State Updates) messages.
Area
A Contiguous grouping of routers and router interfaces. Routers in an area strive to learn all topology information about the area, but they do not learn topology information about areas to which they do not connect. An ABR will send summarize information about other areas to devices in other areas.
Area border router (ABR)
A router that has interfaces connected to at least two different OSPF areas, including the backbone area. ABRs hold topology data for each area, and calculate routes for each area, and advertise about those routes between areas.
Backbone router
Any router that has at least one interface connected to the backone area. Most of the time the backbone area is AREA 0. In order for one area to speak to another area they most go through the backbone area.
Internal routers
A router that has interface connected to only one area, making the router completely internal to that one area.
Designated Router(DR).
On multiaccess data links like LANS, an OSPF router elected by the routers on that data link to perform special functions. These functions include the generation of LSAs representing the subnet, and playing a key roll in the database exchange process.
Backup Designated Router(BDR)
A router on a multiaccess data link that monitors the DR and becomes prepared to take over for the DR, should the DR fail. All devices within the same VLAN/Subnet establish a full adjacency with the DR and BDR.
Most Commonly Used OSPF commands:
show ip ospf interface brief
lists the interfaces on which OSPF is enabled (based on the network commands and area command in interface mode); it omits passive interfaces.
show ip protocols
Lists the contents of the network configuration commands for each routing process, and list of enabled but passive interfaces.
show ip ospf neighbors
lists known neighbors, including neighbor state; does not list neighbors for which some mismatched parameter is preventing a valid OSPF neighbor relationship. You could have two devices listed under this command that have a mismatched mtu size.
show ip ospf database
Lists all LSAs for all connected areas. (Type 1-7)
show ip route
Lists the contents of the IP routing table, listing OSPF-learned routes with a code of O on the left side of the output.
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