Nokia NRS II Composite (OSPF) key terms

The basic unit of data OSPF routers flood to describe the topology. Each LSA carries a type, an advertising router ID, a sequence number, an age, and a type-specific payload such as router links or network prefixes.

The per-area collection of all LSAs a router has accepted as current. Every router in the same area holds an identical LSDB, and the SPF algorithm runs over this database rather than over raw neighbor information.

The router elected on a broadcast or NBMA segment to represent the segment in the topology. It originates the network LSA and is the only router that forms full adjacencies with all other routers on that segment.

The runner-up in the DR election, which also forms full adjacencies on the segment and takes over the DR role without reflooding if the DR fails. It does not originate the network LSA while the DR is alive.

The periodic type 1 packet that discovers and keeps neighbors alive. Its fields, including area ID, hello and dead intervals, network mask, and option bits, must match before two routers will form an adjacency.

The timer, normally four times the hello interval, after which a silent neighbor is declared down. On Nokia SR OS it is derived from the configured hello interval unless explicitly overridden.

A type 2 packet carrying a summary of LSA headers so neighbors can compare databases. The master sends first, the slave responds, and the exchange moves the adjacency from ExStart to Exchange.

A type 3 packet listing the LSAs a router needs after comparing DBD summaries. The neighbor answers with Link State Update packets, completing database synchronization.

A type 4 packet that carries one or more complete LSAs, either in response to a Link State Request or as a flood to neighbors when an LSA changes. It is acknowledged by Link State Acknowledgment packets.

A type 5 packet that confirms receipt of flooded LSAs. It can acknowledge several LSAs at once and is what lets a sender remove entries from its retransmission list.

The neighbor state in which master/slave roles are negotiated using initial DBD packets with sequence numbers. The router with the higher router ID normally becomes master.

The final neighbor state, reached when both routers hold identical LSDBs for the area. Only in Full state does the adjacency contribute to SPF and to forwarding.

The LSA each router originates for its own links within an area. It lists interface types, link IDs, and costs, and is flooded unchanged throughout the area.

The LSA originated by the DR of a broadcast or NBMA segment. It lists the routers attached to the segment and lets SPF model the segment as a single pseudonode.

An inter-area prefix advertisement generated by an area border router. It tells other areas which networks exist behind the ABR, but carries no topology detail beyond the prefix and cost.

An LSA that carries a redistributed prefix from outside the OSPF domain, with a forwarding address and an E1 or E2 metric. It is flooded across the whole domain except into stub and NSSA areas.

The LSA a not-so-stubby area uses to carry external prefixes internally. The NSSA ABR translates it into a type 5 LSA when advertising out to the backbone.

A logical grouping of OSPF interfaces that shares a single LSDB. Areas limit flooding scope and SPF churn, and all non-backbone areas must connect to area 0 through an ABR or a virtual link.

A router with interfaces in at least two areas, one of which is normally the backbone. It originates type 3 summaries for each area and filters or aggregates prefixes between areas.

A router that redistributes routes from other protocols or static sources into OSPF. It originates type 5 LSAs and must be reachable by all other routers for external routes to be usable.

The shortest-path-first calculation that builds a loop-free tree rooted at the local router from the LSDB. Running it yields the best cost to every reachable prefix in the area.

A recomputation triggered by a change that affects only leaves of the SPF tree, such as a type 3 or type 5 LSA. It avoids rebuilding the full topology tree and shortens convergence time.

A per-interface value, by default reference bandwidth divided by link bandwidth, that SPF sums along a path. Lower total cost wins, and equal totals allow load sharing.

The numerator used to derive default interface cost, 100 Gb/s in classic OSPF and configurable on Nokia SR OS. Raising it is necessary on links faster than the reference so costs stay distinct.

The installation of several next hops for one prefix when their total costs are equal. It spreads traffic across parallel links and is a standard SR OS feature for both unicast and LFA paths.

A logical point-to-point adjacency, carried inside a transit area, that connects an area to the backbone through a non-backbone router. It repairs a partitioned area 0 or attaches a distant area.

An area that blocks type 5 external LSAs and reaches outside networks through a default route injected by its ABR. It reduces LSDB size on low-end routers at the edge of the domain.

A Cisco-style extension, supported on Nokia SR OS, that blocks both type 5 and type 3 LSAs except a single default. Only intra-area routes and the default remain in the LSDB.

An area that blocks type 5 LSAs but allows external prefixes to be carried internally as type 7 LSAs. It suits sites that must redistribute a few routes without becoming a full transit area.

The import of routes from another protocol, static, or direct source into OSPF as external LSAs. It requires an ASBR, a metric and metric type choice, and usually a route policy to prevent loops.

The mechanism that verifies hello and LSA exchanges using a shared key. OSPFv2 supports null, simple password, MD5, and SHA keyed digests, configured per interface or area on SR OS.

A keyed digest method in which each OSPF packet carries a key ID and a hash rather than the key itself. It protects against spoofed hellos and forged LSAs on a segment.

A setting that applies one authentication type and key to every interface in an area, avoiding per-interface repetition. Interface-level settings normally override the area default.

A precomputed backup next hop whose path to the destination does not pass through the protected router. It gives sub-second repair for link and node failures without waiting for SPF.

An LFA variant that tunnels repair traffic to a remote node, typically using an MPLS label, when no directly connected loop-free neighbor exists. It extends coverage in ring and sparse topologies.

A segment-routing-based fast reroute method that computes a post-convergence path and encodes it as a label stack. It provides full coverage regardless of topology shape.

The process and time taken for all routers to agree on a new topology after a change. It includes failure detection, flooding, SPF, and route installation, and is shortened by LFA and tuning timers.

A capability that lets a router restart its control plane while neighbors keep forwarding, avoiding a full adjacency reset. Helpers hold routes until the restarting router resynchronizes its LSDB.

A 32-bit identifier, usually the highest loopback address or an explicit setting, that uniquely names a router in the LSDB. Duplicate router IDs break adjacency formation and must be found during troubleshooting.

The Nokia SR OS command that lists neighbors, their state, priority, dead timer, and interface. It is the first check when an adjacency fails to reach Full.