When using IS-IS in native mode for routing in a dual-stack IPv4/IPv6 environment, which of the following statements is FALSE?

The statement that the SPF algorithm needs to be run four times on an L1/L2 router is false for the standard, single-topology implementation of dual-stack IS-IS. In this mode, a single topology is used for both IPv4 and IPv6. An L1/L2 router runs the SPF algorithm twice: once for the Level 1 area and once for the Level 2 backbone. The resulting Shortest Path Tree (SPT) from each calculation is used for forwarding both IPv4 and IPv6 traffic. Running four SPFs (L1-IPv4, L1-IPv6, L2-IPv4, L2-IPv6) is characteristic of a multi-topology IS-IS configuration, which is a separate mode and not the only or default way to run dual-stack IS-IS.

A. In single-topology IS-IS, the fundamental assumption is that the topology is congruent for both protocols. If a link is not dual-stack capable, black-holing can occur.
B. This is the definition of single-topology IS-IS for dual-stack environments. One SPF calculation creates a single SPT for both address families.
D. Traffic black-holing is a known risk if a link on the computed SPT supports only one of the two address families (e.g., it is IPv4-only).

1. Nokia SR OS Router Configuration Guide, Release 21.10.R1, "IS-IS for IPv6". The documentation describes the default single-topology mode where one SPF is run per level for both IPv4 and IPv6, and the optional multi-topology mode which runs separate SPFs.

2. RFC 5308, "Routing IPv6 with IS-IS", Section 2. This RFC introduces the TLVs for IPv6 routing. It allows for a single SPF computation for both address families by including both IPv4 and IPv6 reachability information in LSPs, which is the basis for the single-topology model.

3. RFC 5120, "M-ISIS: Multi Topology (MT) Routing in IS-IS". This document explicitly defines the multi-topology extension where separate SPFs are run for different topologies (e.g., IPv4 and IPv6), confirming this is a specific feature, not the default behavior.