Nokia 4A0-113 OSPF key terms

A self-contained data unit flooded by routers that describes one piece of the OSPF topology, such as a router's links, a network segment, or an external prefix. Each LSA has a type, age, sequence number, and advertising router.

The collection of all LSAs a router has accepted for an area. Every router in the same area holds an identical LSDB, which is the input to the SPF calculation.

An LSA generated by every OSPF router for each area it belongs to. It lists the router's interface links with their costs, states whether the router is an ABR or ASBR, and floods only within its area.

An LSA originated by the Designated Router on a broadcast or NBMA segment. It lists all routers attached to that segment and is flooded only inside the area containing the segment.

An inter-area prefix advertisement originated by an Area Border Router. It tells other areas about networks reachable in the source area, and it does not carry topology detail, only prefix and cost.

An LSA generated by an Area Border Router to tell routers in other areas how to reach an Autonomous System Boundary Router. It advertises a route to the ASBR itself, not to external prefixes.

An LSA flooded throughout the OSPF domain that describes a prefix redistributed from outside OSPF. It carries a forwarding address, an external metric, and an E1 or E2 metric type flag.

An LSA used inside a Not-So-Stubby Area to carry external prefix information. The NSSA's Area Border Router translates it into a Type 5 LSA before flooding it into the backbone.

The periodic message exchange that discovers neighbors, negotiates parameters, and proves bidirectional reachability. Hello packets carry the router ID, area ID, hello and dead intervals, priority, DR, BDR, and neighbor list.

The timer a router waits without receiving a Hello from a neighbor before declaring it down. By default it is four times the Hello interval, and a mismatch prevents adjacency formation.

The time between consecutive Hello packets sent on an interface. It must match on both ends of a link for a neighbor relationship to progress, and it varies by network type.

The sequence of states an adjacency passes through: Down, Attempt, Init, 2-Way, ExStart, Exchange, Loading, and Full. Reaching Full means the two routers have synchronized link-state databases.

A packet exchanged during ExStart and Exchange that carries headers of the sender's LSAs. It lets each router learn which LSAs the neighbor holds so it can request missing ones.

A packet sent during the Loading state to ask a neighbor for specific LSAs identified by type, link-state ID, and advertising router. The neighbor answers with Link State Update packets.

A packet that carries one or more complete LSAs, used for both flooding new information and answering Link State Request packets. It is acknowledged by Link State Acknowledgment packets.

A packet that confirms receipt of flooded LSAs, making OSPF flooding reliable. Acknowledgments can be sent explicitly or implied by echoing the LSA back to the sender.

The router elected on a multi-access segment to originate the network LSA and manage adjacency and flooding there. All other routers on the segment form full adjacencies only with the DR and BDR.

The router elected second on a multi-access segment. It forms full adjacencies like the DR, receives all flooding, and takes over the DR role without re-forming adjacencies if the DR fails.

An 8-bit interface value used in DR and BDR election on multi-access segments. The highest priority wins, ties are broken by highest router ID, and a priority of zero makes a router ineligible.

An OSPF interface mode for multi-access media such as Ethernet, where DR and BDR election occurs, Hellos are multicast, and neighbors are discovered dynamically. It produces a Type 2 network LSA for the segment.

An OSPF interface mode for links with exactly two routers, such as PPP or HDLC. No DR election happens, Hellos are sent to 224.0.0.5, and the adjacency goes directly to Full.

An OSPF interface mode that treats a partially meshed or hub-and-spoke medium as a set of point-to-point links. It avoids DR election and represents each neighbor as a host route in the router LSA.

An OSPF interface mode for non-broadcast multi-access media such as Frame Relay, where DR election still applies but neighbors must be statically configured because multicast and broadcast are not supported.

A 32-bit value that uniquely identifies an OSPF router in the domain, usually the highest loopback address or an explicitly configured address. It must be unique and stable, and changes require an OSPF restart.

A logical grouping of OSPF interfaces and networks that share one link-state database. Areas limit flooding and SPF scope, and all non-backbone areas must connect to the backbone area 0.

Area 0, the transit core through which all inter-area traffic must pass. Every other area must attach to it, either directly or through a virtual link.

A router with interfaces in two or more areas, at least one being the backbone. It maintains a separate link-state database per area and originates Type 3 and Type 4 summary LSAs between them.

A router that redistributes routes from another protocol or a static source into OSPF. It originates Type 5 external LSAs, or Type 7 inside an NSSA, and is reachable in other areas via a Type 4 LSA.

The Dijkstra algorithm run on the link-state database to build a shortest-path tree rooted at the local router. It computes the lowest-cost path to every prefix in the area.

The metric assigned to an OSPF-enabled interface, by default reference bandwidth divided by interface bandwidth. The sum of costs along a path determines the preferred route.

The numerator used to derive default OSPF interface cost, conventionally 100 Mbps but configurable on Nokia SR OS to account for links faster than 100 Mbps. Raising it gives higher-resolution costs on high-speed interfaces.

Two ways to cost redistributed routes. Type 1 adds the external metric to the internal path cost, so the closest ASBR wins; Type 2 keeps the external metric only, so the lowest external metric wins regardless of internal distance.

An area that blocks Type 5 external LSAs and instead receives a default route from its Area Border Router. It reduces database and CPU load on routers that do not need external detail.

A Cisco-style extension of a stub area that also blocks Type 3 summary LSAs, leaving only intra-area routes and a default route. Nokia SR OS implements the equivalent behavior with a stub area plus no summaries.

A stub-like area that can still import external routes, carried as Type 7 LSAs and translated to Type 5 by the Area Border Router. It is useful when an edge area must redistribute but should not receive external LSAs.

A logical point-to-point adjacency carried through a transit area that connects a detached area or partitions of the backbone to area 0. It requires a common area and full adjacency between the endpoints.

The aggregation of multiple contiguous prefixes into one advertisement, configured on an Area Border Router for inter-area ranges or on an ASBR for external ranges. It shrinks LSDB size and hides instability.

A mechanism that validates Hello and LSA-carrying packets using a shared key, configured as simple plaintext or MD5/ HMAC-based message digest. All routers on a link must use the same type and key.

The shared secret used to authenticate OSPF packets on an interface or virtual link. Mismatched keys cause Hello packets to be discarded, so the adjacency never leaves the Down or Init state.

An independent OSPF process on a Nokia SR OS router, identified by a numeric instance ID and bound to a VPRN or the base router. Each instance has its own router ID, areas, and link-state database.

The reliable hop-by-hop propagation of LSAs to all routers in an area, triggered by new, changed, or refreshed LSAs. It uses Link State Update packets and acknowledgments to guarantee delivery.

The ExStart exchange in which routers compare router IDs to decide which side controls the Database Description sequence numbers. The higher router ID becomes master and sets the initial sequence.