The following are frequently used terms for IS-IS:
area
An area is a routing sub-domain which maintains detailed routing information about its own internal composition, and also maintains routing information which allows it to reach other routing sub-domains. Areas correspond to the Level 1 sub-domain.
end system
End systems send NPDUs to other systems and receive NPDUs from other systems, but do not relay NPDUs. This International Standard does not specify any additional end system functions beyond those supplied by ISO 8473 and ISO 9542.
neighbor
A neighbor is an adjacent system reachable by traversing a single sub-network by a PDU.
adjacency
An adjacency is a portion of the local routing information which pertains to the reachability of a single neighboring end or intermediate system over a single circuit. Adjacencies are used as input to the decision process to form paths through the routing domain. A separate adjacency is created for each neighbor on a circuit and for each level of routing (Level 1 and Level 2) on a broadcast circuit.
circuit
A circuit is the subset of the local routing information base pertinent to a single local Subnetwork Point of Attachments (SNPAs).
link
A link is the communication path between two neighbors. A link is up when communication is possible between the two SNPAs.
designated IS
A designated IS is the intermediate system on a LAN which is designated to perform additional duties. In particular, the designated IS generates link-state PDUs on behalf of the LAN, treating the LAN as a pseudonode.
pseudonode
Where a broadcast sub-network has n connected intermediate systems, the broadcast sub-network is considered to be a pseudonode. The pseudonode has links to each of the n intermediate systems and each of the ISs has a single link to the pseudonode (rather than n-1 links to each of the other intermediate systems). Link-state PDUs are generated on behalf of the pseudonode by the designated IS.
broadcast sub-network
A broadcast sub-network is a multi-access subnetwork that supports the capability of addressing a group of attached systems with a single PDU.
general topology sub-network
A general topology sub-network is a topology that is modeled as a set of point-to-point links, each of which connects two systems. There are several generic types of general topology subnetworks, multipoint links, permanent point-to-point links, dynamic and static point-to-point links.
routing sub-domain
A routing sub-domain consists of a set of intermediate systems and end systems located within the same routing domain.
level 2 sub-domain
A level 2 sub-domain is the set of all Level 2 intermediate systems in a routing domain.