Nokia IS-IS Routing Protocol key terms

A router that forwards packets between networks in an IS-IS routing domain. In the protocol's terminology, it is any node that performs Layer 3 forwarding rather than a host.

A host or non-forwarding node attached to an IS-IS network. It does not run the routing protocol itself but may be discovered by adjacent routers and reported in their link-state advertisements.

A companion protocol to IS-IS in which end systems announce themselves to routers and routers announce themselves to hosts, allowing automatic neighbor discovery without manual host configuration.

A link-state interior gateway protocol that uses a two-level hierarchy and runs directly over Layer 2, making it independent of any Layer 3 addressing protocol.

The packet a router originates to describe its own adjacencies and connected prefixes. LSPs are flooded throughout an area or level and stored in every router's link-state database.

The collection of all LSPs received for a level. Each router runs the shortest-path algorithm over this database to build its routing table.

A control packet carrying summaries of LSP identifiers and sequence numbers. Complete SNPs list every LSP in the database, while partial SNPs list a subset to request or acknowledge updates.

A summary of all LSPs in a router's database for a level, sent periodically on broadcast links and on demand on point-to-point links to synchronize neighbors.

A packet used to request missing LSPs or to acknowledge receipt of an LSP. It carries only the subset of LSP identifiers relevant to the exchange.

A periodic packet sent on an interface to discover neighbors, negotiate adjacency parameters, and detect failures. Different hello formats exist for point-to-point and broadcast media.

The specific hello packet type exchanged between intermediate systems. It carries the system ID, area address, and parameters such as the hold time and supported protocols.

A relationship formed between two routers on a common link after they exchange hellos and agree on parameters. The adjacency must be up before link-state information is exchanged.

The mechanism where each router confirms it has seen the other's hello, ensuring the adjacency is bidirectional before it is declared up. It prevents one-way link problems from corrupting routing.

A countdown value advertised in hellos. If no new hello arrives before it expires, the neighbor is declared down and the adjacency is removed.

A router that maintains adjacencies and link-state information only within a single area. It reaches destinations outside its area through the nearest Level 2 router.

A router that forms adjacencies in the backbone level and carries inter-area reachability. It does not normally maintain Level 1 adjacencies.

A router that participates in both levels, maintaining a Level 1 database for its own area and a Level 2 database for the backbone. It typically advertises itself as the default route into its area.

A group of routers sharing the same area address and forming a Level 1 flooding domain. Routing within an area is flat, while inter-area routes pass through Level 2.

A variable-length identifier assigned to a router that indicates its area membership. Neighbors must share at least one area address to form a Level 1 adjacency.

A fixed-length identifier that uniquely names a router within the routing domain. It is combined with the area address to form the router's network entity title.

The address of the router itself, formed from the area address, system ID, and a selector byte of zero. It identifies the device for the routing protocol rather than a specific interface.

The addressing format used by IS-IS, consisting of an authority and format identifier, area address, system ID, and selector. It is independent of IP addressing.

The router elected on a broadcast or non-broadcast multi-access link to originate the pseudonode LSP and manage database synchronization for that link.

A virtual node created by the designated intermediate system to represent a multi-access link in the topology. It lets the link be modeled as a hub with each attached router as a spoke.

The link-state PDU originated by the designated intermediate system on behalf of a multi-access link. It lists all routers attached to that link and is identified by the DIS system ID plus a circuit identifier.

A multi-access medium such as Ethernet on which many routers can communicate. IS-IS elects a designated intermediate system and models the segment as a pseudonode.

A link with exactly two endpoints, such as a serial or tunnel interface. No designated intermediate system is elected and adjacencies form directly between the two routers.

The Dijkstra algorithm run over the link-state database to compute the shortest path tree. IS-IS runs separate SPF computations for Level 1 and Level 2.

An optimization that recomputes only affected routes when a change does not alter the topology, such as a prefix appearing or disappearing. It avoids a full shortest-path run.

A cost value associated with a link or prefix that SPF sums to choose the best path. Wider metrics allow larger values and finer granularity than the original narrow format.

The original metric encoding with a small maximum value per link and a limited total path cost. It constrains designs that need fine-grained or high-cost link values.

An extended metric encoding that supports much larger link and path values, enabling granular cost assignment and compatibility with traffic engineering extensions.

A flag in a Level 1 router's LSP indicating that it has a path out of the area. Level 1 routers use it to select a default route toward a Level 2-capable router.

A flag a router sets in its LSP to signal that it should not be used as a transit node. Other routers still reach directly connected prefixes but route around the overloaded device.

A field in each LSP counting down the seconds until the LSP expires. The originating router refreshes it before it reaches zero; expiry triggers removal from the database.

A field in an LSP that increases with each new version, letting routers determine which copy is newest. A higher sequence number always supersedes a lower one.

The reliable process of propagating LSPs to all routers in a level. Each router forwards new LSPs out all interfaces except the one they arrived on, with acknowledgments ensuring delivery.

The exchange that brings neighbors' link-state databases into agreement after an adjacency forms, using complete and partial sequence number PDUs to identify and request missing LSPs.

A mechanism that validates hello, sequence, and link-state packets using a password or keyed digest, preventing unauthorized routers from injecting false routing information.

Extensions that distribute link attributes such as bandwidth and administrative groups so label-switched paths can be computed with constraints beyond the basic shortest-path metric.