pimd: MLAG: skip pim_register_join on non-DR

[hnattamaisub]

Rootcause and fix:
In mlag+pim, packets are software forwarded because the mroute is stuck in pimreg state in non-DR node.
Align with RFC:avoid REG_JOIN on the mlag node that lost DR election. Gate pim_register_join() with pim_upstream_could_register() on the WRVIFWHOLE connected-source path.

[greptile-apps]

Greptile Summary

This PR fixes a multicast software-forwarding regression in MLAG+PIM deployments where non-DR nodes get stuck in pimreg state because pim_register_join was being called unconditionally on connected-source paths. The fix adds pim_upstream_could_register(up) guards — which internally checks PIM_I_am_DR — to the two remaining unguarded pim_register_join call sites, aligning with RFC behaviour.

• pim_mroute_msg_wholepkt (CAN_BE_LHR / pimreg-as-FHR path, line 497): adds the DR guard so that a non-DR MLAG peer receiving a WHOLEPKT on pimreg no longer adds the register tunnel to the OIL.
• pim_mroute_msg_wrvifwhole (connected-source FHR path, line 849): same guard prevents the WRVIFWHOLE handler from initiating PIM registration on a non-DR node.
• All other pim_register_join call sites in pim_upstream.c and pim_mroute_msg_nocache are already correctly gated (either via pim_upstream_could_register or an early-exit DR check), so the fix is complete.

Confidence Score: 5/5

Safe to merge — a minimal two-line change in each affected path, with no modification to data structures, timers, or state-machine transitions.

The change guards exactly the two pim_register_join call sites that lacked a DR check in the MLAG context. All other call sites in the file and in pim_upstream.c are already gated by pim_upstream_could_register or an equivalent early-exit, so the fix is complete.

No files require special attention.

Important Files Changed

Filename	Overview
pimd/pim_mroute.c	Adds pim_upstream_could_register(up) guard to pim_register_join in both pim_mroute_msg_wholepkt (CAN_BE_LHR/pimreg FHR path) and pim_mroute_msg_wrvifwhole (connected-source FHR path), preventing non-DR MLAG nodes from entering pimreg state.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[Multicast packet arrives] --> B{WRVIFWHOLE or WHOLEPKT?}
    B -->|WRVIFWHOLE| C[pim_mroute_msg_wrvifwhole]
    B -->|WHOLEPKT on pimreg| D[pim_mroute_msg_wholepkt]
    C --> E{pim_if_connected_to_source?}
    E -->|Yes| F[pim_upstream_add with FHR flag]
    F --> G{pim_is_group_filtered AND pim_upstream_could_register?}
    G -->|DR node: both pass| H[pim_register_join]
    G -->|non-DR node: could_register=0| I[Skip pim_register_join]
    D --> K{Exact S,G upstream found?}
    K -->|No, but *,G with CAN_BE_LHR| L{src locally connected?}
    L -->|Yes| M[pim_upstream_add with FHR flag]
    M --> N{pim_is_group_filtered AND pim_upstream_could_register?}
    N -->|DR node| O[pim_register_join]
    N -->|non-DR node| P[Skip pim_register_join]

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_{Reviews (4): Last reviewed commit: "pimd: MLAG: skip pim_register_join on no..." | Re-trigger Greptile}

[hnattamaisub]

ci:rerun

[Jafaral]

@greptile review

[Jafaral]

@Mergifyio backport stable/10.6 stable/10.5

[mergify]

backport stable/10.6 stable/10.5

✅ Backports have been created

Details

• #21933 pimd: MLAG: skip pim_register_join on non-DR (backport #21920) has been created for branch stable/10.6
• #21934 pimd: MLAG: skip pim_register_join on non-DR (backport #21920) has been created for branch stable/10.5

[Jafaral]

Do we need to do the same here?

[hnattamaisub]

ci:rerun

[Jafaral]

@greptile review

[Jafaral]

@Mergifyio backport stable/10.4

[mergify]

backport stable/10.4

✅ Backports have been created

Details

• #21935 pimd: MLAG: skip pim_register_join on non-DR (backport #21920) has been created for branch stable/10.4