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4b5bf09c83
The wording read as a claim about the CPU interface in general, which is wrong and misleading: the 8051 sits behind an ordinary port of the internal switch and is an ordinary member of a forwarding mask - which is exactly what this implementation relies on. Say what is actually broken instead: the trap action, a separate mechanism whose destination is an external CPU port these boards do not populate. Record the measurements behind it, including the widened CPU_PMSK and both external-CPU destinations, and add the ACL trap result - a rule matching the group intercepts frames but does not deliver them either, which is a second, independent path to the same conclusion. Reported-by: vDorst
168 lines
6.9 KiB
Markdown
168 lines
6.9 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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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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