Documentation
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Overview ¶
Package stp decodes Spanning Tree Protocol BPDUs per IEEE 802.1D-2004 (STP / RSTP) and IEEE 802.1Q-2014 §13 (MSTP).
Wrap-vs-native judgement
Native. IEEE 802.1D is fully public; the BPDU wire format is a tight bit-packed binary header — Protocol ID + Version + BPDU Type + per-type body (Configuration or Topology Change Notification). RSTP and MSTP extend the Configuration BPDU with additional flags and a Version 1 / Version 3 trailing block. No crypto, no compression, no varints. Operators paste BPDU bytes (after the LLC header strip — DSAP/SSAP 0x42, Control 0x03, sent to the STP-bridge multicast MAC 01:80:C2:00:00:00) from a `tcpdump -X ether dst host 01:80:c2:00:00:00` line, a Wireshark Follow-Frame view, or any STP-emitting switch and get the documented bridge / root / cost / port / timer breakdown.
What this package covers
**3-byte common header**:
Protocol ID (2 bytes BE) — must be 0x0000.
Version (1 byte) — 0 = STP (IEEE 802.1D), 2 = RSTP (IEEE 802.1D-2004), 3 = MSTP (IEEE 802.1Q-2014 §13).
**BPDU Type** (1 byte):
0x00 Configuration BPDU
0x80 Topology Change Notification (TCN) BPDU
0x02 RSTP/MSTP BPDU (carries the extended flags)
**Configuration BPDU body** (35 bytes):
Flags (1 byte) with **8-bit name table** (TC bit 0 / Proposal bit 1 / Port Role bits 2-3 / Learning bit 4 / Forwarding bit 5 / Agreement bit 6 / TC Ack bit 7). Port Role: 0 Unknown / 1 Alternate-or-Backup / 2 Root / 3 Designated.
Root Bridge ID (8 bytes) — 4-bit priority + 12-bit system ID extension + 6-byte MAC. Priority must be a multiple of 4096 per IEEE 802.1D §17.13.7.
Root Path Cost (4 bytes BE).
Bridge ID (8 bytes) — same split as Root Bridge ID.
Port ID (2 bytes BE) — 4-bit Port Priority + 12-bit Port Number.
Message Age (2 bytes BE; units of 1/256 second).
Max Age (2 bytes BE; same units).
Hello Time (2 bytes BE; same units).
Forward Delay (2 bytes BE; same units).
**TCN BPDU body** — empty (the 4-byte common header is the entire frame).
**Bridge ID decoding** — splits the leading 2 bytes into the 4-bit System Priority (multiple of 4096) and the 12-bit System ID Extension (typically the VLAN ID for PVST+ or 0 for classic STP), then formats the trailing 6 bytes as a MAC.
**Timer formatting** — all 4 timer fields are converted from IEEE 1/256-second units to milliseconds for readability.
**MSTP trailer** (Version=3) — the 64-byte MSTI configuration block isn't deeply decoded; it's surfaced as a raw hex blob with a length prefix.
What this package does NOT cover (deliberately out of scope)
LLC header (DSAP/SSAP/Control) — feed the BPDU bytes starting at the Protocol ID field (after the 3-byte LLC strip).
PVST+ / per-VLAN STP — Cisco proprietary uses standard BPDUs with SNAP encapsulation and embeds the VLAN ID in the System ID Extension; the decoder handles the extension but the SNAP wrapper is the operator's responsibility.
Convergence-time simulation — the timers are surfaced; reasoning about how long convergence takes belongs to a higher-level analysis.
MSTP MSTI configuration block beyond the raw hex surface — IEEE 802.1Q §13 layout is documented but deep dissection is a future Spec.
Index ¶
Constants ¶
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Variables ¶
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Functions ¶
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Types ¶
type BridgeID ¶
type BridgeID struct {
Priority int `json:"priority"`
SystemIDExtension int `json:"system_id_extension"`
SystemIDExtensionHex string `json:"system_id_extension_hex"`
MAC string `json:"mac"`
}
BridgeID is the Root Bridge ID and Bridge ID 8-byte structure (priority + system ID extension + MAC). Per IEEE 802.1t the original 16-bit priority field was split into a 4-bit Priority (in multiples of 4096) plus a 12-bit System ID Extension — typically the VLAN ID for PVST+ or 0 for classic STP.
type Config ¶
type Config struct {
Flags int `json:"flags"`
FlagsHex string `json:"flags_hex"`
FlagsDecoded []string `json:"flags_decoded,omitempty"`
PortRole int `json:"port_role"`
PortRoleName string `json:"port_role_name"`
RootBridgeID *BridgeID `json:"root_bridge_id"`
RootPathCost uint32 `json:"root_path_cost"`
BridgeID *BridgeID `json:"bridge_id"`
PortPriority int `json:"port_priority"`
PortNumber int `json:"port_number"`
MessageAgeMs int `json:"message_age_ms"`
MaxAgeMs int `json:"max_age_ms"`
HelloTimeMs int `json:"hello_time_ms"`
ForwardDelayMs int `json:"forward_delay_ms"`
}
Config is the body of Configuration / RSTP / MSTP BPDUs.
type Result ¶
type Result struct {
ProtocolID int `json:"protocol_id"`
Version int `json:"version"`
VersionName string `json:"version_name"`
BPDUType int `json:"bpdu_type"`
BPDUTypeHex string `json:"bpdu_type_hex"`
BPDUTypeName string `json:"bpdu_type_name"`
TotalBytes int `json:"total_bytes"`
Configuration *Config `json:"configuration_bpdu,omitempty"`
TCN *TCN `json:"tcn_bpdu,omitempty"`
MSTITrailerHex string `json:"msti_trailer_hex,omitempty"`
Notes []string `json:"notes,omitempty"`
}
Result is the top-level decoded view.
Source Files