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A port set to admit tagged frames only will never see a BPDU, because delivery rides the forward action and the ingress pipeline drops untagged frames before the L2 lookup. The failure is silent and looks like a dead receive path: the port turns edge after three seconds, the bridge elects itself root, and nothing hints at the ingress setting. Diagnosing exactly that cost most of a day on a live switch, with the neighbour provably transmitting the whole time. stp_setup() now prints one line per affected port, so the hint lands at "stp on" and at every config replay on boot. The check runs in its own loop after the MSTP write: port_ingress_filter_get() reads a register into sfr_data, which the state-building loop above is still using. The port number in the message is physical, matching what the ingress command takes. doc/stp.md explains why this can happen here and not on a normal bridge, where BPDUs are consumed before any VLAN classification. stp.rel stays at DSEG 5 with no OSEG and the image at 10498 bytes of XDATA.
179 lines
7.7 KiB
Markdown
179 lines
7.7 KiB
Markdown
# Spanning Tree (STP / RSTP)
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The switch can take part in a spanning tree (IEEE 802.1D / 802.1w) so that
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redundant links between bridges are blocked instead of forming a loop. The
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implementation elects a root bridge from the BPDUs it receives, promotes ports
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to forwarding once their listen period expires, ages the root out when it goes
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silent, and blocks a port on which it sees its own BPDU.
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It is deliberately simple: there is no proposal/agreement handshake and no full
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port-role machine. What it does do reliably is stop a cabling loop from melting
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the network, and interoperate with neighbouring bridges as a well-behaved
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(if unexciting) participant.
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> **Before you enable it on a switch you reach over the network**: read the
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> [management failsafe](#management-failsafe) section. The management VLAN
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> rides a port that STP can block.
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## Quick start
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```
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stp on # start participating
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stp off # stop, all ports back to forwarding
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```
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Live status is on the Spanning Tree page of the web UI (or `/stp.json`).
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With no other bridge around, the switch elects itself root and every port ends
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up forwarding — you can leave it on safely. Put the settings in the startup
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config to make them survive a reboot:
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```
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stp prio 15
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stp port 1 edge on
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stp on
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```
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## Hardware background
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BPDUs are addressed to `01:80:C2:00:00:00`, a reserved link-local group. The
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ASIC's Reserved-Multicast action for that address decides what happens to the
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frame.
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Forwarding to the CPU port works normally — the 8051 sits behind an ordinary
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port of the internal switch and is an ordinary member of a forwarding mask.
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What does not work is the *trap* action, which is a separate mechanism: its
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destination is an external CPU attached to a physical port
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(`cpuTag_externalCpuPort_set`, `EXT_CPU_CTRL` in the vendor SDK), which these
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boards do not populate. Measured on a SWTGW218AS: with the RMA action set to
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trap, zero frames arrive at the 8051, including with `CPU_PMSK` widened and the
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external-CPU destination pointed at both `0xF` and `9`; with the *forward*
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action plus the L2 entry below, they arrive. The ACL trap behaves the same way,
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measured on the neighbouring reserved group `01:80:C2:00:00:02`: a rule matching
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it intercepts the frames — the LACP receive counters stop advancing while the
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rule is enabled and resume the moment it is disabled — but they never reach the
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8051, with `FWD_INT_TRAP` and with `REDIRECT` aimed at the CPU port alike.
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Delivery therefore uses the *forward* action, constrained to the CPU port
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by a static L2 multicast entry (`port_l2mc_set()`), one per VLAN in use:
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* while STP runs, the entry's member mask is the CPU port only — BPDUs reach
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the CPU and are not flooded to other ports, as a participating bridge
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requires;
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* with STP off, the same entries are retargeted to all ports, restoring the
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transparency an unmanaged switch is expected to have, so a surrounding
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spanning tree can span *through* this device.
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Because delivery rides the forward action, a BPDU is an ordinary frame to the
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ingress pipeline and is subject to the port's acceptable-frame-type setting.
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BPDUs are untagged by definition, so a port configured to admit tagged frames
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only (`ingress <port>t`) will never deliver one: a port left on the default
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auto edge turns edge after three seconds of silence, one with edge switched off
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sits out the full forward delay instead, and either way the bridge elects
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itself root no matter what the neighbour sends. `stp_setup()` prints a warning for every
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STP-enabled port in that state. On a normal bridge this cannot happen, since
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BPDUs are consumed before any VLAN classification; here it is a direct
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consequence of the delivery path above.
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Port states live in `RTL837X_MSTP_STATES (0x5310)`, two bits per port:
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`00` disabled, `01` blocking, `10` learning, `11` forwarding. Note that a port
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held in blocking also drops frames the CPU injects into it, so a blocked port
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cannot transmit BPDUs of its own.
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## Timers
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`stp_timers()` runs at 50 Hz (the main loop idles on the 200 Hz system tick and
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STP is called every fourth pass), which is what `STP_HZ` in `rtl837x_stp.h`
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encodes. All configured values are in seconds:
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| setting | default | range |
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|---|---|---|
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| `stp hello <n>` | 2 | 1–10 |
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| `stp maxage <n>` | 20 | 6–40 |
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| `stp fwd <n>` | 15 | 4–30 |
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| `stp txhold <n>` | 6 | 1–10 |
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A port entering the tree spends `fwd` seconds in blocking before it forwards
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(an edge port skips the wait). Root information is discarded after `maxage`
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seconds without a BPDU, and the switch then reclaims the root role.
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## Bridge settings
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```
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stp prio <0-15> # bridge priority = n * 4096, default 8 (32768)
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stp version rstp|stp # RST BPDUs (default) or legacy Config BPDUs
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stp hello|maxage|fwd|txhold <seconds>
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```
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The bridge with the lowest priority wins the root election; ties are broken by
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the MAC address. If you do not want this switch to become the root of an
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existing network, give it a worse priority than the current root — `stp prio 15`
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(61440) is the usual "never me" value.
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## Per-port settings
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```
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stp port <1-9> on|off # take part in STP, or stay plain forwarding
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stp port <1-9> edge on|off|auto # host-facing port handling (default: auto)
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stp port <1-9> cost <0-200000000> # path cost, 0 = automatic (20000)
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stp port <1-9> prio <0-240> # port priority, steps of 16
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stp port <1-9> guard none|bpdu|root
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stp port <1-9> filter on|off # neither send nor accept BPDUs
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stp port <1-9> p2p auto|on|off
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```
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**edge** — an edge port goes forwarding immediately and does not trigger a
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topology change when it comes and goes; `auto` promotes a port to edge after
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three seconds without a BPDU, and demotes it as soon as one arrives. Use
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`edge on` for ports where only hosts are attached.
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**guard** — `bpdu` disables a port as soon as a BPDU arrives on it (a host port
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should never see one); `root` keeps a port from ever becoming the path to the
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root, which protects an existing topology from a newly attached bridge that
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claims a better priority.
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**filter** — the port neither sends nor accepts BPDUs. Useful when the device
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on the far side reacts badly to them (some unmanaged switches with loop
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prevention cut the link) but you still want STP on the rest of the ports.
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## Management failsafe
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Enabling STP on a switch you administer over the network is a genuine risk: the
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management VLAN rides a port that STP may put into blocking, and once that
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happens the way back is a power cycle.
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The firmware therefore runs a commit-confirm watchdog. While STP is enabled,
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any HTTP request re-arms a countdown; if management stays silent for
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`stp failsafe <seconds>` (default 180, 0 disables it), STP disables itself and
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restores forwarding. Keeping the web UI open on the Spanning Tree page is
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enough to hold it off, since the page polls for status.
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```
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stp failsafe 180 # seconds of silence before STP gives up (0 = never)
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```
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The status page shows whether the failsafe has tripped since STP was last
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enabled.
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## Status
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The Spanning Tree page shows the elected root (priority and MAC), the path cost
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to it, the root port, the topology-change counter and, per port, the live state
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read from the ASIC together with the configured options. The same data is
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available as JSON:
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```
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GET /stp.json
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```
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## Limitations
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* One spanning-tree instance; no MSTP, no per-VLAN trees.
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* No proposal/agreement handshake — an RST-capable neighbour will still
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converge, but through the timers rather than the fast transition.
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* Port roles are approximated: the root port and designated ports are
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distinguished, alternate/backup are not.
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* A port in blocking cannot transmit, so a blocked port stops announcing
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itself; recovery relies on the listen timer rather than on a neighbour's
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agreement.
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