The IS-IS Shortest Path First (SPF) or Dijkstra algorithm calculates the shortest path from the local router to all other destinations. The algorithm maintains a 'tree' database for nodes with a confirmed shortest path and a 'candidate' database for nodes whose paths are being evaluated. The process iteratively selects the lowest-cost node from the candidate database and moves it to the tree database. Subsequently, it examines the neighbors of this newly added node (using the Link-State Database or LSDB) and adds them to the candidate database for future evaluation. Option B, despite its slightly imprecise wording, correctly describes this core iterative step of adding a neighbor of a node in the tree to the candidate list.

A. According to specific Juniper reference implementations, the algorithm is seeded by placing the local router directly into the tree database, not the candidate database.

C. Tuples are moved from the candidate database to the tree database, not the other way around. The LSDB is the input for the calculation and is not modified by it.

D. The algorithm terminates when the candidate database is empty, signifying that all reachable nodes have been processed and moved to the tree database.

1. Southwick

P.

Reynolds

H. (2010). Junos Enterprise Routing

2nd Edition. O'Reilly Media

Inc. Chapter 6

"The IS-IS Engine

" p. 258.

Quote: "The algorithm is seeded by adding the local router to the tree database with a cost of 0. The algorithm then examines the LSPs of the node just added to the tree and adds all of its neighbors to the candidate database... The algorithm then selects the entry in the candidate database with the lowest cost and moves it to the tree database. This process is repeated until the candidate database is empty." This source directly refutes option A and supports the logic of option B.

2. Juniper Networks. (2009). OSPF and IS-IS: Choosing an IGP [White paper].

Reference: Page 5 describes the SPF algorithm maintaining two lists: TENT (candidate list) and PATHS (tree list). It states

"TENT is the list of candidate nodes that are not yet on the SPF tree but are adjacent to it. PATHS is the list of nodes that are already on the SPF tree." This confirms the terminology used in the explanation.

3. Moy

J. (1991). RFC 1195: Use of OSI IS-IS for routing in TCP/IP and dual environments. IETF. Section 4.2.3

"The Decision Process."

Reference: This RFC provides the authoritative description of the SPF algorithm for IS-IS. Steps 3 through 5 detail the process of examining neighbors of a node just added to the tree (PATHS) and adding/updating them in the tentative list (TENT/candidate)

which aligns with the logic in option B.