[action] [PR:22676] Fix test issue for test_ipv6_bgp_scale.py

[mssonicbld]

Summary: Fix test issue for test_ipv6_bgp_scale.py
Fixes # (issue)

1. Fix test issue in calculate_downtime() function. Please see the detail info as below

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2. Print actual downtime in the logs

• Before change, when case fail, we do not know the actual downtime value, it is inconvenient

Failed: Dataplane downtime is too high, threshold is 2.0 seconds

• After change, we can see the actual downtime value in the error message

Failed: Dataplane downtime is too high: actual 5.5822 seconds, threshold is 2.0 seconds

Type of change

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

Back port request

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

Approach

What is the motivation for this PR?

The calculate_downtime() function in test_ipv6_bgp_scale.py uses [:-1] on the mask_rx_cnt dict values to exclude the backplane port from the RX total:

rx_total = sum(list(ptf_dp.mask_rx_cnt[masked_exp_pkt].values())[:-1]) # Exclude the backplane

This is incorrect because [:-1] removes the last entry by dict insertion order, not the backplane port. The dict insertion order depends on which port first receives a matching packet, which is non-deterministic and varies between runs.

In topologies without a backplane (e.g., t1-isolated-*), mask_rx_cnt contains only legitimate data port entries and no backplane entry at all. The [:-1] unconditionally removes a valid egress port's counter, causing false packet loss to be reported.

For example, in a test_bgp_admin_flap[1] failure:

• mask_rx_cnt had exactly 32 entries, all corresponding to BGP neighbor ports (no backplane)
• [:-1] excluded port (0, 176) (Ethernet176 / ARISTA113T1) which received 1019 packets
• This caused missing_pkt_cnt = 1019 and downtime = 0.4185s, exceeding the 0.2s threshold
• In reality, all 31,990 sent packets were received (TX total == full RX sum), meaning zero actual packet loss

The bug also makes the test flaky — depending on which port happens to be last in the dict, the "missing" count changes, causing random pass/fail results.

How did you do it?

Replaced the fragile [:-1] approach with explicit backplane port identification using ptf.config['port_map'].

When PTF initializes, backplane ports are registered in ptf.config['port_map'] with interface name "backplane" (see ptfadapter/__init__.py get_ifaces_map()). The new helper _get_backplane_ports() queries this config to find backplane port keys, and calculate_downtime() excludes only those specific ports from the RX sum.

• In topologies with backplane (e.g., ciscovs-7nodes): backplane ports are correctly identified and excluded
• In topologies without backplane: _get_backplane_ports() returns an empty set, no ports are excluded

How did you verify/test it?

Run regression test, pass

Any platform specific information?

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

Documentation

[mssonicbld]

Original PR: #22676

[mssonicbld]

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