Add L3VNI DCI test cases: TC48, TC54, TC55, TC56

[bpar9]

Description of PR

Summary: Adds 4 new cross-DC L3VNI test cases from the DCI Solution Testplan, plus supporting helper functions in vxlan_helper.py.

Test cases added to test_vxlan_dci.py (in TestVxlanDCIBase):

• L3VNI_dci:48 (test_base_dci_l3vni_external_intf_flap): External router interface shut/no-shut flap test. Configures VRF-lite eBGP peering (same as TC47), runs continuous traffic, flaps the ext-facing interface 3× with 15s hold, verifies BGP re-establishment and traffic recovery. Parametrized for IPv4/IPv6.
• L3VNI_dci:54 (test_base_dci_l3vni_line_rate_performance): Runs cross-DC L3VNI traffic at 100% line rate for 1 hour. Verifies 0% loss and max latency ≤ 100µs. Includes periodic core-file health checks.
• L3VNI_dci:55 (test_base_dci_l3vni_convergence_time): Measures convergence time after trigger events. Parametrized over: link_shut (≤50ms), bgp_clear (≤500ms), bgw_reboot (≤30s), process_kill_bgp (≤5s).
• L3VNI_dci:56 (test_base_dci_l3vni_imix_traffic): IMIX traffic pattern (64B weight-7, 512B weight-4, 1518B weight-1) for 5 minutes with per-frame-size latency thresholds.

Helper functions added to vxlan_helper.py:

• snapshot_interface_counters / compare_interface_counters: Capture and diff DUT interface error/drop counters
• compute_convergence_ms: Calculate convergence time from Ixia traffic stats (prefers loss_duration_us, falls back to frames_lost/PPS)
• get_traffic_item_stats: Detailed per-stream stats (tx/rx pkts, PPS, loss%, latency)
• pick_cross_dc_l3_stream: Select a cross-DC L3 stream from tgen handles

Note: TC47 and TC49 (external router IPv4/IPv6 connectivity) already exist as test_base_dci_l3vni_external_connectivity.

Type of change

• 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?

Expand cross-DC L3VNI DCI test coverage to include interface flap resilience and performance/convergence benchmarking.

How did you do it?

Followed existing test patterns in TestVxlanDCIBase (e.g., test_base_dci_l3vni_external_connectivity for TC48 setup, existing traffic verification patterns for performance tests). All tests call report_result(), use st.banner()/st.log() conventions, and perform core-file checks.

How did you verify/test it?

• Python syntax verification (py_compile) passes for both files
• flake8 clean on the new code (existing pre-existing warnings untouched)
• Not executed against a live testbed (requires IXIA + 3-DC topology)

Any platform specific information?

Requires IXIA (IxNetwork) traffic generator and the 3-DC EVPN-VXLAN DCI topology.

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

3-datacenter EVPN-VXLAN DCI fabric: DC1 (4 leafs + 2 spines + 2 BGW spines), DC2 (2 leafs + 2 BGW spines), DC3 (1 leaf + 1 BGW spine), with external router connected to DC1/DC2 leaves.

Documentation

Refer to DCI_Solution_Testplan.xlsx for test case specifications.

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Reviewer Notes — Areas to scrutinize

1. snapshot_interface_counters parsing logic (vxlan_helper.py): The counter parsing searches for header-like strings (RX_ERR, TX_DRP) in data rows — verify this matches actual SONiC show interfaces counters output format on target devices.
2. TC48 code duplication with TC47: The external router discovery and VRF-lite configuration is largely duplicated from test_base_dci_l3vni_external_connectivity. Consider whether a shared setup helper would be better.
3. Convergence thresholds (TC55): link_shut ≤ 50ms is aggressive — verify this is achievable on target hardware/topology.
4. Ixia stat field assumptions: get_traffic_item_stats assumes avg_delay/max_delay are in nanoseconds (divides by 1000 for µs). Confirm this matches the IxNetwork version in use.
5. TC55 uses st.vtysh()/st.config() directly for trigger actions rather than vxlan_obj.config_dut() — intentional for simplicity but worth noting for consistency.

Link to Devin session: https://cisco-demo.devinenterprise.com/sessions/1849ed21f9c04ca1a02f8329683d1afd
Requested by: @bpar9

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