RTLPlayground/doc/stp.md

Spanning Tree (STP / RSTP)

The switch can take part in a spanning tree (IEEE 802.1D / 802.1w) so that redundant links between bridges are blocked instead of forming a loop. The implementation elects a root bridge from the BPDUs it receives, promotes ports to forwarding once their listen period expires, ages the root out when it goes silent, and blocks a port on which it sees its own BPDU.

It is deliberately simple: there is no proposal/agreement handshake and no full port-role machine. What it does do reliably is stop a cabling loop from melting the network, and interoperate with neighbouring bridges as a well-behaved (if unexciting) participant.

Before you enable it on a switch you reach over the network: read the management failsafe section. The management VLAN rides a port that STP can block.

Quick start

stp on                  # start participating
stp off                 # stop, all ports back to forwarding

Live status is on the Spanning Tree page of the web UI (or /stp.json).

With no other bridge around, the switch elects itself root and every port ends up forwarding — you can leave it on safely. Put the settings in the startup config to make them survive a reboot:

stp prio 15
stp port 1 edge on
stp on

Hardware background

BPDUs are addressed to 01:80:C2:00:00:00, a reserved link-local group. The ASIC's Reserved-Multicast action for that address decides what happens to the frame.

Forwarding to the CPU port works normally — the 8051 sits behind an ordinary port of the internal switch and is an ordinary member of a forwarding mask. What does not work is the trap action, which is a separate mechanism: its destination is an external CPU attached to a physical port (cpuTag_externalCpuPort_set, EXT_CPU_CTRL in the vendor SDK), which these boards do not populate. Measured on a SWTGW218AS: with the RMA action set to trap, zero frames arrive at the 8051, including with CPU_PMSK widened and the external-CPU destination pointed at both 0xF and 9; with the forward action plus the L2 entry below, they arrive. The ACL trap behaves the same way, measured on the neighbouring reserved group 01:80:C2:00:00:02: a rule matching it intercepts the frames — the LACP receive counters stop advancing while the rule is enabled and resume the moment it is disabled — but they never reach the 8051, with FWD_INT_TRAP and with REDIRECT aimed at the CPU port alike.

Delivery therefore uses the forward action, constrained to the CPU port by a static L2 multicast entry (port_l2mc_set()), one per VLAN in use:

• while STP runs, the entry's member mask is the CPU port only — BPDUs reach the CPU and are not flooded to other ports, as a participating bridge requires;
• with STP off, the same entries are retargeted to all ports, restoring the transparency an unmanaged switch is expected to have, so a surrounding spanning tree can span through this device.

Because delivery rides the forward action, a BPDU is an ordinary frame to the ingress pipeline and is subject to the port's acceptable-frame-type setting. BPDUs are untagged by definition, so a port configured to admit tagged frames only (ingress <port>t) will never deliver one: a port left on the default auto edge turns edge after three seconds of silence, one with edge switched off sits out the full forward delay instead, and either way the bridge elects itself root no matter what the neighbour sends. stp_setup() prints a warning for every STP-enabled port in that state. On a normal bridge this cannot happen, since BPDUs are consumed before any VLAN classification; here it is a direct consequence of the delivery path above.

Port states live in RTL837X_MSTP_STATES (0x5310), two bits per port: 00 disabled, 01 blocking, 10 learning, 11 forwarding. Note that a port held in blocking also drops frames the CPU injects into it, so a blocked port cannot transmit BPDUs of its own.

Timers

stp_timers() runs at 50 Hz (the main loop idles on the 200 Hz system tick and STP is called every fourth pass), which is what STP_HZ in rtl837x_stp.h encodes. All configured values are in seconds:

setting	default	range
stp hello <n>	2	1–10
stp maxage <n>	20	6–40
stp fwd <n>	15	4–30
stp txhold <n>	6	1–10

A port entering the tree spends fwd seconds in blocking before it forwards (an edge port skips the wait). Root information is discarded after maxage seconds without a BPDU, and the switch then reclaims the root role.

Bridge settings

stp prio <0-15>         # bridge priority = n * 4096, default 8 (32768)
stp version rstp|stp    # RST BPDUs (default) or legacy Config BPDUs
stp hello|maxage|fwd|txhold <seconds>

The bridge with the lowest priority wins the root election; ties are broken by the MAC address. If you do not want this switch to become the root of an existing network, give it a worse priority than the current root — stp prio 15 (61440) is the usual "never me" value.

Per-port settings

stp port <1-9> on|off              # take part in STP, or stay plain forwarding
stp port <1-9> edge on|off|auto    # host-facing port handling (default: auto)
stp port <1-9> cost <0-200000000>  # path cost, 0 = automatic (20000)
stp port <1-9> prio <0-240>        # port priority, steps of 16
stp port <1-9> guard none|bpdu|root
stp port <1-9> filter on|off       # neither send nor accept BPDUs
stp port <1-9> p2p auto|on|off

edge — an edge port goes forwarding immediately and does not trigger a topology change when it comes and goes; auto promotes a port to edge after three seconds without a BPDU, and demotes it as soon as one arrives. Use edge on for ports where only hosts are attached.

guard — bpdu disables a port as soon as a BPDU arrives on it (a host port should never see one); root keeps a port from ever becoming the path to the root, which protects an existing topology from a newly attached bridge that claims a better priority.

filter — the port neither sends nor accepts BPDUs. Useful when the device on the far side reacts badly to them (some unmanaged switches with loop prevention cut the link) but you still want STP on the rest of the ports.

Management failsafe

Enabling STP on a switch you administer over the network is a genuine risk: the management VLAN rides a port that STP may put into blocking, and once that happens the way back is a power cycle.

The firmware therefore runs a commit-confirm watchdog. While STP is enabled, any HTTP request re-arms a countdown; if management stays silent for stp failsafe <seconds> (default 180, 0 disables it), STP disables itself and restores forwarding. Keeping the web UI open on the Spanning Tree page is enough to hold it off, since the page polls for status.

stp failsafe 180        # seconds of silence before STP gives up (0 = never)

The status page shows whether the failsafe has tripped since STP was last enabled.

Status

The Spanning Tree page shows the elected root (priority and MAC), the path cost to it, the root port, the topology-change counter and, per port, the live state read from the ASIC together with the configured options. The same data is available as JSON:

GET /stp.json

Limitations

• One spanning-tree instance; no MSTP, no per-VLAN trees.
• No proposal/agreement handshake — an RST-capable neighbour will still converge, but through the timers rather than the fast transition.
• Port roles are approximated: the root port and designated ports are distinguished, alternate/backup are not.
• A port in blocking cannot transmit, so a blocked port stops announcing itself; recovery relies on the listen timer rather than on a neighbour's agreement.