TL&DR: Third time is (supposedly) a charm. Not in this case.

It’s a bit hard to understand why one would launch an enterprise data-center-grade network operating system without candidate/active configuration in the 21st century, but that’s what Cisco and Arista did with Nexus OS and EOS. Arista fixed that omission with configuration sessions more than a decade ago¹. It took Cisco a bit longer.

The first attempt was the so-called two-stage configuration commit, which is functionally almost identical to the Cisco IOS XR “candidate” configuration:

• The candidate configuration is really a scratchpad of syntax-checked configuration commands.
• The commit command merges the commands on the scratchpad with the running configuration.

However:

• I couldn’t find an equivalent of the show configuration merge command that would tell me what the running configuration will be after the commit command.
• The two-stage configuration works (according to Nexus OS documentation) only in interactive CLI mode, not over NX-API or NETCONF
• There can be a single active two-stage configuration session at any time.

TL&DR: mostly useless.

But wait, Cisco introduced candidate configuration in Nexus OS release 10.6(3)F (a decade after Arista EOS²):

• It’s a real candidate configuration
• You can make changes to a snapshot of the running configuration and view them with show candidate-config diff or view the final configuration with show candidate-config
• You can commit or abort the configuration process, and even exit the candidate configuration and resume at a later time (even though getting back is a bit tricky).

Looks like exactly what we need, right? Well, it is, until you try to use IS-IS. For whatever mysterious reason, the candidate configuration does not work with all Nexus OS features. If you try to use it on a device running IS-IS, you get an error message³:

dut# conf candidate
Error: Current running-config has commands which are not supported by candidate session.
Please check the 'show candidate-config last-unsupported' and remove the unsupported configs from running-config.

These are the commands that the Nexus OS release 10.6(4) [Maintenance Release] cannot process as part of its candidate configuration:

dut# show candidate-config last-unsupported
feature isis
interface Ethernet1/1 ; isis network point-to-point
interface Ethernet1/1 ; ip router isis Gandalf
interface Ethernet1/2 ; isis network point-to-point
interface Ethernet1/2 ; ip router isis Gandalf
interface loopback0 ; ip router isis Gandalf
router isis Gandalf
router isis Gandalf ; net 49.0042.0101.0000.0042.00
router isis Gandalf ; is-type level-1
router isis Gandalf ; log-adjacency-changes
router isis Gandalf ; address-family ipv4 unicast

To recap: Cisco launched a device configuration feature that supports OSPF but not IS-IS in the mid-2020s. The 1980s called and want that stolen mindset back.