[PR test blocker] PR test failed on kvmtest-multi-asic-t1-lag due to IPv6 BGP sessions are down

[bingwang-ms]

Description

Pipeline link https://dev.azure.com/mssonic/build/_build/results?buildId=731680&view=logs&j=5b6294d6-6ce0-5548-1140-3e07ff8c9891&t=45744f08-d589-5160-2ea8-044dd459feb8&s=819a35c0-daf8-51e9-2cda-8d277a544776

Log from vtestbed addPostRoute.log

Current findings

1. The issue can be reproed consistently after config reload on multi-ASIC.
2. Only IPv6 BGP sessions are impacted
3. It's confirmed to be a routing issue
4. Suspected to be caused by Added change not to create ECMP Group in SAI and program the route if none of ECMP members are active/link-up sonic-swss#3394. Raised another PR to verify [Don't merge] Revert "[submodule][202405] Update submodule sonic-swss to the latest HEAD automatically" #21293

Steps to reproduce the issue:

1. Run config reload on a multi-asic platform
2. Check IPv6 BGP sessions.

Describe the results you received:

Test should pass.

Describe the results you expected:

Test failed.

Output of show version:

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Output of show techsupport:

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Additional information you deem important (e.g. issue happens only occasionally):