Add PVST test suite for T0 topologies

[bpar9]

Description of PR

Summary:
This PR adds a comprehensive PVST (Per-VLAN Spanning Tree) test suite designed for T0 topologies with two_vlan_a and four_vlan_a configurations. The test suite includes 23 test cases covering basic functionality, protection features, topology changes, configuration persistence, and scale testing.

Reviewer should start with: tests/stp/test_pvst_t0.py - the main test file containing the PvstHelper class, PTF traffic helpers, and all test cases.

Link to Devin run: https://app.devin.ai/sessions/eccdda2912b14aca8c2a51252206d115
Requested by: bhavani@cisco.com (GitHub: @bpar9)

Updates since last revision

Added proper PTF traffic generation and verification based on patterns from tests/vlan/test_vlan.py and tests/fdb/test_fdb.py:

• New PTF imports: ptf.testutils, ptf.packet as scapy, ptf.mask.Mask
• New helper functions:
  • build_eth_packet() - L2 traffic packet construction
  • build_bpdu_packet() - IEEE 802.1D BPDU packet for BPDU Guard tests
  • get_vlan_ptf_ports() - VLAN to PTF port mapping using config_facts/minigraph
  • get_all_vlan_ptf_mapping() - All VLAN port mappings
  • verify_vlan_isolation() - VLAN isolation verification
  • verify_mac_learning() - MAC learning verification
  • send_and_verify_vlan_traffic() - Traffic send/verify helper
• Updated test cases with actual PTF traffic: TC-007 (L2 forwarding), TC-008 (MAC learning), TC-009 (BPDU Guard), TC-010 (Root Guard), TC-011 (PortFast), TC-013 (Link Flap), TC-015 (Traffic Path)
• Updated pvst_setup fixture to include cfg_facts, mg_facts, vlan_to_ptf

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?

To provide automated test coverage for PVST functionality in SONiC, enabling validation of per-VLAN spanning tree instances, root bridge election, protection features (BPDU Guard, Root Guard, PortFast), and configuration persistence across reboots.

How did you do it?

Created a new tests/stp/ test module with:

• conftest.py: Fixtures for VLAN configuration and STP mode selection
• test_pvst_t0.py: 23 test cases organized into 9 test classes:
  • TestPvstBasicFunctionality (5 tests): PVST enable, root bridge election, timer configuration
  • TestPvstPortStateAndTraffic (3 tests): Port state verification, L2 forwarding, MAC learning
  • TestPvstProtectionFeatures (3 tests): BPDU Guard, Root Guard, PortFast
  • TestPvstTopologyChange (2 tests): TCN handling, link flap stability
  • TestPvstT1Uplinks (2 tests): Port-Channel uplinks, traffic path
  • TestPvstConfigPersistence (3 tests): Save/reload, cold/warm reboot
  • TestPvstStatisticsAndDebugging (2 tests): BPDU statistics, clear statistics
  • TestPvstNegative (2 tests): Invalid priority, non-existent VLAN
  • TestPvstScale (1 test): Maximum VLAN instances

How did you verify/test it?

• Passed flake8 lint checks
• Note: Tests have not been executed against a live SONiC device. Reviewers should verify:
  • CLI command syntax matches actual SONiC STP implementation
  • Regex patterns in PvstHelper match actual show spanning_tree output format
  • BPDU packet format in build_bpdu_packet() triggers BPDU Guard correctly
  • PTF port mapping logic works with actual topology

Any platform specific information?

No platform-specific requirements. Tests use standard SONiC STP CLI commands.

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

• T0 topology with two_vlan_a configuration (Vlan100, Vlan200)
• T0 topology with four_vlan_a configuration (Vlan1000-4000)

Documentation

Test cases are based on the PVST Test Plan for T0 Topologies. Each test method includes docstrings with test case IDs (TC-PVST-T0-001 through TC-PVST-T0-023).

Human Review Checklist

• Verify config spanning_tree CLI syntax matches actual SONiC implementation
• Review regex patterns in PvstHelper.get_bridge_id(), get_root_id(), get_port_state(), get_topology_change_count() match actual output format
• Verify reboot() function call with localhost=None is correct for cold/warm reboot tests
• Verify build_bpdu_packet() BPDU format triggers BPDU Guard on SONiC (uses IEEE 802.1D standard format with LLC header)
• Verify get_vlan_ptf_ports() correctly maps VLAN members to PTF port indices using config_facts and minigraph_ptf_indices
• Verify verify_vlan_isolation() correctly tests VLAN flooding and isolation behavior

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