When IS-IS runs SPF, every lowest-cost path to a destination is eligible for installation. Because all links in the diagram carry the same metric, the SPF calculation seen from R3 produces four next-hops toward R5 that have an identical total cost. The ECMP parameter on every router has been set to 4, so SR OS will keep up to four equal-cost routes. Consequently, R3’s routing table will list four entries (one per next-hop) for prefix 192.168.3.0/24.

A. 1 – would be true only if ECMP were disabled or if a single lowest-cost path existed.

B. 2 – requires only two equal-cost next-hops, but SPF finds four.

C. 3 – fewer than the permitted ECMP value and fewer than the number of equal-cost paths actually present.

1. Nokia SR OS 22.2.R1 Routing Protocols Guide

IS-IS Equal-Cost Multipath section

p. 521–522: “The router installs up to the configured maximum-paths (default 1

user-configurable) for every destination that has multiple next-hops of identical metric.”

2. Nokia SR OS 21.10.R3 System Management Guide

Command “maximum-paths ”

p. 143: “Sets the number of ECMP routes that IS-IS can place in the RTM; value of 4 allows four parallel routes.”

3. IETF RFC 1195

Section 7.2.7: “A router may install multiple equal-cost paths; selection among them is an implementation matter.”