Add testplan and test implementation for BGP Local-Preference feature

[bpar9]

Description of PR

Summary:
This PR adds a comprehensive test plan document and pytest implementation for validating the BGP Local-Preference feature in SONiC. BGP Local-Preference is a well-known path attribute used to influence outbound traffic routing decisions within an AS, where higher values are preferred (default is 100).

Reviewer start point: Begin with the test plan document to understand the test coverage, then review the test implementation for correctness.

Files added:

• docs/testplan/BGP-Local-Preference-Test-Plan.md - Test plan documenting 21 test cases
• tests/bgp/test_bgp_local_preference.py - Pytest implementation with 14 test functions

Link to Devin run: https://cisco-demo.devinenterprise.com/sessions/71c0f42835ce4a9586a92d30b6615c2c
Requested by: @bpar9

Updates since last revision

• Added test cases Add Klish AAA CLI validation tests for VS KVM testing #11-Add inline L3VNI traffic to non-mac_only MAC move tests #16 covering multiple paths, tie-breaking, and interaction with AS-Path/MED/Weight
• Added test cases Add L3VNI DCI MAC move test cases (L3VNI_dci:59,60,62,63) #17-Review TC47/TC49: align test_base_dci_l3vni_external_connectivity with testplan #21 for route reflector scenarios (testplan only)
• Extended topology support to include t0 and t0-multi-asic
• Implemented 6 new test functions for test cases Add Klish AAA CLI validation tests for VS KVM testing #11-Add inline L3VNI traffic to non-mac_only MAC move tests #16

Type of change

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

Back port request

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

Approach

What is the motivation for this PR?

To provide automated test coverage for the BGP Local-Preference feature, validating that SONiC correctly handles local-preference configuration via route-maps and applies it to BGP path selection.

How did you do it?

• Created test plan document with 21 detailed test cases covering:
  • Default values, route-map configuration, boundary values, IPv6 support, persistence (Add PVST test suite for T0 topologies #1-Add BGP local-preference test cases for T1 topology #10)
  • Multiple paths, tie-breaking, interaction with other BGP attributes (Add Klish AAA CLI validation tests for VS KVM testing #11-Add inline L3VNI traffic to non-mac_only MAC move tests #16)
  • Route reflector scenarios (Add L3VNI DCI MAC move test cases (L3VNI_dci:59,60,62,63) #17-Review TC47/TC49: align test_base_dci_l3vni_external_connectivity with testplan #21)
• Implemented pytest test file with 14 test functions and helper functions for route-map configuration via vtysh
• Used existing sonic-mgmt patterns (fixtures, assertions, multi-ASIC support)

How did you verify/test it?

• Code passes flake8 linting (79 character line limit)
• Follows existing BGP test patterns in sonic-mgmt

Items for reviewer attention:

1. Route reflector test cases (Add L3VNI DCI MAC move test cases (L3VNI_dci:59,60,62,63) #17-Review TC47/TC49: align test_base_dci_l3vni_external_connectivity with testplan #21) are documented in testplan but not yet implemented in the test file
2. test_local_preference_persistence_after_bgp_restart assumes vtysh route-map config persists after BGP restart - please verify this matches expected SONiC/FRR behavior
3. Tests rely on existing routes from neighbors; if no routes exist, tests skip rather than fail
4. Multi-path tests (Add Klish AAA CLI validation tests for VS KVM testing #11-Add L3VNI cross-DC test cases (L3VNI_dci:1-2, 6-30, 39-43, 91, 101) + 6 trigger classes #12) require multiple BGP neighbors - tests skip if fewer than required neighbors are available

Any platform specific information?

None - tests use vtysh for route-map configuration which is platform-agnostic.

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

• t0
• t0-multi-asic
• t1
• t1-multi-asic

Documentation

Test plan document added at docs/testplan/BGP-Local-Preference-Test-Plan.md

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