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Add STP loop guard support#88

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salahadell:adding-loop-guard-feature
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Add STP loop guard support#88
salahadell wants to merge 1 commit into
sonic-net:masterfrom
salahadell:adding-loop-guard-feature

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@salahadell

@salahadell salahadell commented Apr 26, 2026 •
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Summary

Add STP Loop Guard support to sonic-stp and wire it through manager, runtime state handling, and APP_DB sync.

Motivation

When expected BPDUs are lost on non-designated ports, the port should not transition unsafely toward forwarding, potentially creating Layer-2 loops, and broadcast storms.
Loop Guard keeps the port in loop-inconsistent state until recovery conditions are met.

Changes

  • Added loop-guard config field in IPC/config path.
  • Added loop-guard mask and runtime state in STP global/port data.
  • Added loop-guard activation on message-age expiry for eligible non-designated ports.
  • Added BPDU-based recovery logic to clear loop-inconsistent state.
  • Added APP_DB sync for loop-guard active status.
  • Added debug visibility for loop-guard config/state.
  • Added disable-path cleanup so stale loop-guard state is cleared and STP re-evaluation runs immediately.

Behavior

  • With loop guard enabled, BPDU loss on protected non-designated ports enters LOOP-INC.
  • Receiving valid BPDU clears loop-guard active state and normal STP processing continues.
  • Disabling loop guard while port is LOOP-INC clears stale state and triggers immediate STP state re-evaluation.

Files Updated

  • include/stp.h
  • include/stp_ipc.h
  • include/stp_dbsync.h
  • stp/stp.c
  • stp/stp_mgr.c
  • stp/stp_util.c
  • stp/stp_data.c
  • stp/stp_debug.c
  • stpsync/stp_sync.cpp

Validation

  • Interop test with SONiC DUT connected to a Cisco switch and SONiC peers.
  • Enabled loop guard on SONiC DUT test interface.
  • On Cisco side, enabled BPDU filter to stop BPDU transmission.
  • Observed SONiC port moved to LOOP-INC when BPDUs stopped.
  • After BPDU transmission resumed, SONiC port recovered through normal STP behavior.
  • Confirmed loop-guard state updates were reflected in APP_DB.

Related PRs

@salahadell
adding loop guard feature
cc44fff 
Signed-off-by: Salah Adel <salah-adel2011@hotmail.com>
@mssonicbld

mssonicbld commented Apr 26, 2026

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/azp run

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azure-pipelines Bot commented Apr 26, 2026

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This was referenced Apr 26, 2026
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@salahadell

salahadell commented May 18, 2026

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Hello, @divyachandralekha @wajahatrazi @lihuay Could you please review and merge this PR? It adds STP loop guard support and is a dependency for sonic-swss#4517 [already merged] and sonic-utilities#4495 [under review], Thanks

@wajahatrazi

wajahatrazi commented May 18, 2026

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@salahadell Hi, thanks for mentioning, will review and reply. Thanks

@salahadell

salahadell commented Jun 4, 2026

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@salahadell Hi, thanks for mentioning, will review and reply. Thanks

Hi @wajahatrazi, just following up on your earlier message, happy to hear any feedback or address questions whenever you get a chance. Let me know if there's anything I can do to help move this along. Thanks again!

@salahadell

salahadell commented Jul 3, 2026

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@salahadell Hi, thanks for mentioning, will review and reply. Thanks

@wajahatrazi Hello, hope you are good, any updates?

qianqian-33
qianqian-33 reviewed Jul 10, 2026
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Comment thread stp/stp_mgr.c
if (pmsg->enabled)
{
stpmgr_config_root_protect(port_id, pmsg->root_guard);
stpmgr_config_loop_protect(port_id, pmsg->loop_guard);

@qianqian-33 qianqian-33 Jul 10, 2026

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This PR currently implements only PVST. Could we consider implementing MSTP support as well?

wajahatrazi
wajahatrazi reviewed Jul 10, 2026
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Comment thread stp/stp.c
stp_port_class = GET_STP_PORT_CLASS(stp_class, port_number);
stp_port_class->self_loop = false;

/* Loop Guard: if configured on a non-designated port, enter loop-inconsistent

@wajahatrazi wajahatrazi Jul 10, 2026

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What if a port has both loop_guard_active AND root_protect_timer active simultaneously? How does that being handled?

wajahatrazi
wajahatrazi reviewed Jul 10, 2026
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Comment thread stp/stp_util.c
return;
}

// Loop Guard: if port is in loop-inconsistent state and receives a BPDU, recover

@wajahatrazi wajahatrazi Jul 10, 2026

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What BPDU qualifies for recovery? Any BPDU?

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@wajahatrazi wajahatrazi wajahatrazi left review comments
@qianqian-33 qianqian-33 qianqian-33 left review comments
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4 participants

@salahadell @mssonicbld @wajahatrazi @qianqian-33