[action] [PR:21979] Keep One Port Up in BGP Scale Test to Preserve PTF Reachability and Accurate Downtime

[mssonicbld]

Description of PR

Summary:
In IPv6 BGP scale flap testing, when all_flap is enabled we must keep one neighbor stable avoid dataplane downtime miscalculation. This PR updates the target selection logic to flap “all minus one” neighbors by removing the randomly chosen injection neighbor from the flapping neighbor list, and updates the log message accordingly.
Fixes # (issue)

Type of change

• Bug fix
• Testbed and Framework(new/improvement)
• New Test case
• Skipped for non-supported platforms
• [ x ] Test case improvement

Back port request

• 202205
• 202305
• 202311
• 202405
• 202411
• 202505
• 202511

Approach

What is the motivation for this PR?

The flap test requires a dedicated “injection neighbor” to remain up for route injection/verification. Previously, with all_flap=True, the injection neighbor could also be included in the flapping set, causing inconsistent/incorrect behavior.

How did you do it?

In tests/bgp/test_ipv6_bgp_scale.py (_select_targets_to_flap), when all_flap is enabled:

• Copy the neighbor list (list(bgp_neighbors)) to avoid aliasing.
• Randomly pick the injection neighbor.
• Remove the injection neighbor from flapping_neighbors so we flap all other neighbors.
• Update the log message to reflect “All - 1 neighbors”.

How did you verify/test it?

Ran the tests on Arista 7060X6 and Mellanox SN5640

Any platform specific information?

Supported testbed topology if it's a new test case?

Documentation

[mssonicbld]

Original PR: #21979

[mssonicbld]

/azp run

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