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A blocked port does pass a received BPDU up to the CPU. The evidence is in logicog's capture of a looped pair: the port the loop check had already blocked kept reporting a BPDU age of zero seconds across dumps taken more than a forward delay apart, and that counter is only cleared in stp_in(). That is also what makes the loop latch work, since the port that stays forwarding has to go on hearing the blocked one. The tag flag comment said the source address is not learned on the egress port. doc/CpuPort.md defines it as not learning the source address from the frame at all, which is the narrower claim to make. The dangling heading above it described a field that is documented there too.
156 lines
6.3 KiB
Markdown
156 lines
6.3 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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STP can be enabled and controlled via the web interface or the command line,
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as follows:
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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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and on the serial console via `stp status`.
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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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The *trap* action does not deliver to it. Its destination is an external CPU
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attached to a physical port (`cpuTag_externalCpuPort_set`, `EXT_CPU_CTRL` in
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the vendor SDK), which these boards do not populate. The ACL trap and
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redirect actions do not deliver to the 8051 either.
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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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A BPDU delivered this way is an ordinary frame to the port's ingress logic
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and passes through its acceptable-frame-type filter. BPDUs are untagged, so a
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port set to admit tagged frames only (`ingress <port>t`) never delivers one
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to the CPU. `stp_setup()` prints a warning for every STP-enabled port in that
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state.
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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. A port in
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blocking forwards nothing between ports, but it still sends what the CPU
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hands it and still passes a received BPDU up to the CPU, which is what lets
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loop detection go on working on a port it has already blocked.
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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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## Topology changes
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A change on a local non-edge port (the link coming or going, a port promoted
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to forwarding) flushes the addresses learned on it and sets the TC flag in
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our BPDUs for `maxage + fwd` seconds. A TC flag received in a BPDU is passed
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on: the switch flushes the other non-edge ports once and keeps the flag in
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its own BPDUs until one hello after the last flagged frame, so the
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notification crosses the switch instead of dying at it. A legacy TCN is
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acknowledged with TCA and then treated like a local change.
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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 forwards immediately and does not trigger a
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topology change when its link comes and goes; `auto` promotes a port to edge
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after 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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## 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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The `stp status` command prints the same view on the serial console.
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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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* Topology changes propagate away from the root only: nothing is announced
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on the root port (no TCN and no BPDUs at all), so bridges upstream rely on
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their own detection.
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