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A reserved-multicast trap goes to the port named in EXT_CPU_CTRL, which names no port out of reset, so trapped frames were dropped at ingress. trap_init() points it at the CPU port once at startup, and trap_rma_set() changes only the action field of an RMA register. A trapped frame arrives without the VLAN tag a forwarded one gets, so TRAP_RX_BODY() gives the offset of the payload for both forms. STP now traps its address while it runs instead of steering it with a static L2 entry per VLAN, so turning it on no longer walks the VLAN table and a pvid change no longer needs stp off and on. With STP off the flooding entries are restored as before. A trapped BPDU passes the tagged-only ingress filter that drops a forwarded one, so the stp_setup() warning about such ports goes.
188 lines
8.0 KiB
Markdown
188 lines
8.0 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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While STP runs, the action is *trap*. A trap delivers to the port named in
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`EXT_CPU_CTRL` (0x6724), whose reset value 0xf names no port, and with that
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value the trapped frame is dropped at ingress. `trap_init()` sets it to 9, the
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port the 8051 sits behind, once at startup. The trap needs no per-VLAN L2
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entries, so it does not depend on which VLANs exist or on the PVIDs, and turning
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STP on no longer walks the whole VLAN table. BPDUs reach the CPU on a port that
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STP holds in blocking with either action: measured on a SWTGW218AS by sending
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BPDUs into a blocked port with forward and with trap, both ports stopped being
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edge and waited out the forward delay. The chip exempts this address from the
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ingress state check; the slow-protocols address used by LACP is not exempt.
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A trapped frame reaches the CPU in the format it came in with: unlike a
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forwarded one it has no VLAN tag inserted after the RTL tag, and the RTL tag
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carries the trap reason. `stp_in()` reads the BPDU from the offset
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`TRAP_RX_BODY()` returns, which works for both.
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With STP off, the action goes back to *forward* and a static L2 multicast
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entry per VLAN in use (`port_l2mc_set()`) targets 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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The port's acceptable-frame-type filter treats the two actions differently.
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BPDUs are untagged, so a port set to admit tagged frames only
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(`ingress <port>t`) drops a forwarded BPDU before it reaches the CPU, while a
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trapped one passes. Both were measured on a SWTGW218AS with `ingress 3t`:
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LACPDUs, delivered by forward, stopped arriving on port 3, and BPDUs sent into
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the same port, delivered by trap, kept arriving. A port that admits tagged
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frames only therefore takes part in STP.
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## Link aggregation
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A LAG (Link Aggregation Group) is handled like an additional port by the STP
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protocol, with its own timers and states. The switch devices support up to 4
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LAGs, which are shown alongside the physical ports in the STP status. A group
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carries its own path cost, priority, edge and guard settings, and gets its own
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row on the Spanning Tree page.
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```
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stp lag 1 cost 10000 # the group decides, not its members
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stp lag 1 edge off
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```
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LAGs are configured via the `lag` command. Once a port is a member of a LAG it
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can no longer be configured individually for STP, so `stp port <n>` on a member
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tells you which group to configure instead. Membership is re-read from the
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aggregation registers once a second, so a group changing under LACP is picked
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up without any coordination between the two.
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The switch hardware does not handle STP state for a LAG as a whole. State
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changes for the members have to be done by the firmware, updating every member
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port, which is possible using a single register write. BPDUs go out through the
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lowest member and carry the group's own port id. Losing one member of a live
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LAG is not a topology change; the logical port only goes down with its last
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link.
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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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