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The note said a blocked port drops frames the CPU injects into it and so cannot send BPDUs of its own. Hardware says otherwise, and it matters, because that sentence is the reason one would go looking for a way to let control frames out of a blocked port when there is nothing to fix there. Held a two port group in blocking and watched from the neighbour. Our BPDUs kept leaving it, 27 of them with a largest gap of 2.00 s, which is the hello interval with nothing missed. Pings across the same port stopped dead for 11.63 s in one unbroken gap, so data really is held. In the same window the neighbour sent 74 frames with a largest gap of 1.04 s while our receive counter for them moved by 2, and the port stayed in its trunk throughout, so nothing in the aggregation code was discarding them.
181 lines
7.4 KiB
Markdown
181 lines
7.4 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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> **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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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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## 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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## 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. Enabling STP, by hand or
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from the startup config, arms a one-shot window of `stp failsafe <seconds>`
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(default 180). One HTTP request inside the window confirms that management
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survived the new tree and disarms the watchdog until the next enable; a window
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with no management activity disables STP and restores forwarding. After the
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confirmation STP runs unsupervised, so a quiet network no longer loses its
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tree to three minutes of nobody looking at the web UI.
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```
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stp failsafe 180 # length of the armed window after enabling (0 = never armed)
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```
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Any later event that newly takes a port out of forwarding arms the window
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again: a port rejoining via `stp port <n> on`, root guard firing, the loop
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latch. If management traffic keeps flowing past the new block, the very next
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request confirms and disarms; if the block cut it, the silent window restores
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forwarding as above. A stable network with nothing newly blocked never re-arms.
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A command executed on the serial console also confirms, on the grounds that an
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operator with out-of-band access does not need the automatic restore; the
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command that enabled STP does not count, only activity after it.
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A headless switch that nobody confirms over HTTP should set `stp failsafe 0`,
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otherwise a reboot with STP in the startup config disables it again three
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minutes later. Setting a new value while STP runs arms a fresh window.
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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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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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