Add EVPN-VXLAN testplan with Single and Multi-Homing testcases for 4x4 spine-leaf topology

[bpar9]

Description of PR

Summary:
This PR adds a comprehensive EVPN-VXLAN test plan document for a 4x4 spine-leaf topology using Cisco 8101-32FH-O switches with Keysight Ixia traffic generation. The test plan validates both single-homed and multi-homed endpoint behavior in the EVPN-VXLAN fabric.

The test plan includes 32 test cases organized into 6 categories:

• Basic EVPN-VXLAN Functionality Tests (4 test cases: 1-4)
• Single Homing Tests (8 test cases: 5-12)
• Multi-Homing Tests (10 test cases: 13-22)
• High Availability Tests (3 test cases: 23-25)
• Scale Testing (3 test cases: 26-28)
• Multi-Encapsulation Tests (4 test cases: 29-32)

Reviewer should start with the Single Homing Tests section (Test Cases 5-12) and Multi-Homing Tests section (Test Cases 13-22) as these are the main additions.

Updates since last revision

• Updated to use eBGP EVPN sessions between leaf switches via spine nodes (pure eBGP design):
  • Each leaf has unique AS number (65001-65004)
  • All spines share same AS number (65100)
  • EVPN routes transit through spine switches
  • No route reflectors needed
• Updated topology diagrams to show eBGP AS numbers
• Updated configuration examples for eBGP EVPN with as-path multipath-relax and ebgp-multihop
• Added eBGP EVPN route flow diagram showing leaf-spine-leaf path

Previous updates

• Added Multi-Homing Tests section (Test Cases 13-22) covering EVPN-MH scenarios
• Added Multi-Homing Topology diagram showing dual-homed endpoints with Ethernet Segments
• Updated packet validation to use spytest framework instead of 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 document test cases for validating both single-homed and multi-homed endpoint behavior in EVPN-VXLAN fabrics using eBGP for both underlay and overlay routing.

How did you do it?

Created a new test plan document following the existing sonic-mgmt testplan format. Key sections include:

Single Homing Tests:

• MAC learning and EVPN Type-2 route advertisement (ESI=0)
• ARP/ND resolution across VXLAN fabric
• Traffic forwarding between single-homed endpoints
• Leaf switch failure behavior (expected traffic blackhole)
• VLAN to VNI mapping and Inter-VNI routing (L3VNI)
• BUM traffic handling with ingress replication
• MAC move detection

Multi-Homing Tests:

• EVPN Type-1 (EAD) and Type-4 (ES) route advertisement via eBGP
• Designated Forwarder election per VLAN
• Active-active load balancing (aliasing)
• Split-horizon filtering for BUM traffic
• MAC synchronization across Ethernet Segment members
• Fast failover with BFD (<200ms)
• LAG member link failure handling

How did you verify/test it?

This is a documentation-only change (test plan). No code changes to verify.

Any platform specific information?

Test plan is designed for Cisco-8101-32FH-O switches with 400G interfaces using eBGP EVPN for both underlay and overlay routing between leaf switches via spine nodes.

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

4x4 spine-leaf topology with Keysight Ixia traffic generation. Supports both single-homed and multi-homed (LAG) endpoint configurations.

Documentation

New test plan document added at: docs/testplan/EVPN-VXLAN-Spine-Leaf-Testplan.md

Human Review Checklist

• Verify Single Homing test cases (5-12) cover all required single-homing scenarios
• Verify Multi-Homing test cases (13-22) cover all required EVPN-MH scenarios
• Verify topology diagrams (single-homing and multi-homing) match intended testbed configuration
• Verify eBGP EVPN configuration examples are correct for SONiC (AS numbers, multipath-relax, ebgp-multihop)
• Confirm pure eBGP design (no route reflectors) is the intended architecture
• Confirm spytest framework is appropriate for packet validation in this context
• Confirm hardware assumptions (Cisco-8101-32FH-O, Keysight Ixia) are correct
• Check if this should integrate with any existing EVPN-VXLAN documentation

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Link to Devin run: https://cisco-demo.devinenterprise.com/sessions/4e9976eabafa40d88f527ec2412f7e7d
Requested by: @bpar9

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