bgpd: Fix nht to properly notice a change

[donaldsharp]

commit: 8dcd0a6 broke path handling in some rare cases. This can be especially seen in bgp evpn processing when a path transitions from !valid to valid. The bgp_evpn_rt5_addpath code is failing occassionally because of this change. Modify the code to intentionally also call bgp_process when the old_path_valid is not the same as the new path being valid.

[greptile-apps]

Greptile Summary

This PR fixes a regression introduced by commit 8dcd0a6b9cdbfac47eea2cf2100badf7744dbf98 in bgpd/bgp_nht.c. The evaluate_paths() function already correctly detected when a path transitioned between valid and invalid states (updating flags, calling aggregate and EVPN/VPN helpers, and firing the bgp_nht_path_update hook), but the subsequent call to bgp_process() — which triggers bestpath re-evaluation — was only gated on BGP_NEXTHOP_METRIC_CHANGED or BGP_NEXTHOP_CHANGED. When neither of those change flags was set but validity still changed (e.g. a path going from !valid to valid during BGP EVPN RT5 addpath processing), bgp_process() was never invoked and the downstream state became inconsistent.

Changes:

• In bgpd/bgp_nht.c, the condition guarding bgp_process() now also checks old_path_valid != bnc_is_valid_nexthop, ensuring re-evaluation is always triggered on any validity transition.

Confidence Score: 5/5

• This PR is safe to merge — it is a targeted one-liner bug fix with clear, well-understood intent.
• The change is minimal and surgical: a single additional OR condition added to the bgp_process() guard. The condition (old_path_valid != bnc_is_valid_nexthop) is already evaluated identically just two lines above for the hook call, so there is no risk of computing it differently. The fix restores symmetry between the hook invocation and the bgp_process invocation, which is the logical correct behavior. No new variables or data structures are introduced.
• No files require special attention.

Important Files Changed

Filename	Overview
bgpd/bgp_nht.c	Adds old_path_valid != bnc_is_valid_nexthop as a condition to trigger bgp_process(), ensuring path re-evaluation occurs whenever nexthop validity changes even if metric/nexthop content flags are not set.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[evaluate_paths: iterate paths on bnc] --> B[Record old_path_valid\nfrom BGP_PATH_VALID flag]
    B --> C{old_path_valid\n!= bnc_is_valid_nexthop?}
    C -- Yes --> D[Update BGP_PATH_VALID flag\nincrement/decrement aggregates\nEVPN import/unimport\nVPN leak update/withdraw]
    D --> E[hook_call bgp_nht_path_update]
    C -- No --> F[Skip flag update]
    E --> G{Trigger bgp_process?}
    F --> G
    G -- "old_path_valid != bnc_is_valid_nexthop\nOR BGP_NEXTHOP_METRIC_CHANGED\nOR BGP_NEXTHOP_CHANGED" --> H[bgp_process\nre-evaluate bestpath]
    G -- None of the above --> I[Skip bgp_process]

    style D fill:#d4edda
    style H fill:#d4edda
    style G fill:#fff3cd

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_{Last reviewed commit: ae614f4}

[ton31337]

@Mergifyio backport dev/10.6

[mergify]

backport dev/10.6

✅ Backports have been created

Details

• #20998 bgpd: Fix nht to properly notice a change (backport #20986) has been created for branch dev/10.6

[riw777]

looks good