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The tree structure already crossed the switch by regeneration, but the topology-change information did not: a received TC flag was ignored and a TCN only acknowledged, so bridges behind this one kept stale entries until normal aging. A TC flag in a received BPDU now flushes the other non-edge ports once and arms the transmit window our BPDUs already copy the flag from, refreshed to hello+1 seconds by every further flagged frame so it ends one hello after the neighbour stops, without shortening the longer window a local change arms. A TCN is acknowledged as before and then treated like a local change on that port.
155 lines
6.2 KiB
Markdown
155 lines
6.2 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. In the blocking
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state a port forwards nothing except frames sent by the CPU, and nothing it
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receives reaches the CPU.
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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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