bgpd: clear several parameters in subgroup_announce_check()

[enkechen-panw]

Clear tag, weight and distance in the outbound attribute. Also clear MED, aigp, and local-preference when appropriate.

This helps avoid unnecessary attributes in the outbound attribute table, and also helps reduce duplicate updates to neighbors.

[greptile-apps]

Greptile Summary

This PR clears local-only and protocol-specific BGP attributes in outbound announcements to optimize the attribute table and reduce duplicate updates to neighbors.

Key changes:

• Clears attr->med value (not just flag) when removing MED for EBGP peers
• Restructures AIGP handling to explicitly clear AIGP when transmission is not allowed (previously left attribute in place)
• Adds cleanup at end of function to clear local-only attributes (tag, weight, distance) for all peers
• Clears local_pref for EBGP peers after route-map processing

Timing: All attribute clearing happens after the outbound route-map is applied (lines 2841-2846), allowing route-map match clauses like "match tag" or "match local-preference" to operate on original values as documented in the added comment.

Confidence Score: 4/5

• Safe to merge with low risk - well-structured attribute cleanup that follows BGP protocol semantics
• The changes correctly clear BGP attributes that should not be transmitted or are local-only. The timing is correct (after route-map processing), and the logic properly differentiates between IBGP/EBGP. The AIGP restructuring adds proper cleanup when transmission is not allowed. Minor confidence deduction for lack of test coverage verification.
• No files require special attention

Important Files Changed

Filename	Overview
bgpd/bgp_route.c	Clears local-only BGP attributes (tag, weight, distance) and conditional attributes (MED, AIGP, local-preference) after route-map processing to reduce attribute table bloat and duplicate updates

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[subgroup_announce_check entry] --> B{EBGP peer?}
    B -->|Yes| C[Clear MED if conditions met<br/>NEW: attr->med = 0]
    B -->|No| D[Skip MED clearing]
    C --> E[Apply outbound route-map<br/>lines 2841-2846]
    D --> E
    E --> F{AIGP exists?}
    F -->|Yes| G{AIGP_TRANSMIT_ALLOWED?}
    F -->|No| I[Continue processing]
    G -->|Yes| H[Adjust AIGP metric if nh_reset]
    G -->|No| J[NEW: Clear AIGP<br/>attr->aigp_metric = 0<br/>bgp_attr_unset]
    H --> I
    J --> I
    I --> K[NEW: Clear local attributes<br/>tag, weight, distance = 0]
    K --> L{EBGP peer?}
    L -->|Yes| M[NEW: Clear local_pref<br/>attr->local_pref = 0]
    L -->|No| N[Keep local_pref]
    M --> O[Return true]
    N --> O

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_{Last reviewed commit: bc5801f}