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stp: full RSTP and a reworked Spanning Tree page

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* stp: select the root port by cost and block redundant links
* stp: move the protocol to BANK4 and keep the command line in BANK2
* stp: implement the RSTP state machines of 802.1D-2004 clause 17
* stp: counters, more settings and a fuller Spanning Tree page
* stp: make a link aggregation group one port of the RSTP tree
* web: serve the Spanning Tree JSON from BANK4 as well

Kept in BANK1, the larger Spanning Tree page would leave under 1 kB
there for the rest of the web server and every page still to come, so
send_stp() and send_stp_counters() move to httpd/page_stp.c in BANK4
with the protocol, as banked calls, with their own copies of the few output
helpers and of the JSON header, so nothing in BANK4 calls into BANK1
directly or hands it a pointer into its code space. BANK4 takes the
free flash between BANK3 and the web files.

KP_9000_6XHML_X2: BANK1 44336, BANK2 36969, BANK3 11942, BANK4 20965
bytes of 49152.
This commit is contained in:
DrDoof authored and d00f committed 2026-10-27 22:11:24 +01:00
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commit aff6ac0923
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doc/stp.md
+142 -50
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@@ -2,9 +2,11 @@
The switch can take part in a spanning tree (IEEE 802.1D / 802.1w) so that
redundant links between bridges are blocked instead of forming a loop. The
implementation elects a root bridge from the BPDUs it receives, promotes ports
to forwarding once their listen period expires, ages the root out when it goes
silent, and blocks a port on which it sees its own BPDU.
implementation elects a root bridge from the BPDUs it receives, picks the
cheapest path to it as the root port, blocks ports on which a better bridge is
already designated, promotes ports to forwarding once their listen period
expires, drops information a neighbour stopped sending, and blocks a port on
which it sees its own BPDU.
STP can be enabled and controlled via the web interface or the command line,
as follows:
@@ -64,32 +66,6 @@ LACPDUs, delivered by forward, stopped arriving on port 3, and BPDUs sent into
the same port, delivered by trap, kept arriving. A port that admits tagged
frames only therefore takes part in STP.
## Link aggregation
A LAG (Link Aggregation Group) is handled like an additional port by the STP
protocol, with its own timers and states. The switch devices support up to 4
LAGs, which are shown alongside the physical ports in the STP status. A group
carries its own path cost, priority, edge and guard settings, and gets its own
row on the Spanning Tree page.
```
stp lag 1 cost 10000 # the group decides, not its members
stp lag 1 edge off
```
LAGs are configured via the `lag` command. Once a port is a member of a LAG it
can no longer be configured individually for STP, so `stp port <n>` on a member
tells you which group to configure instead. Membership is re-read from the
aggregation registers once a second, so a group changing under LACP is picked
up without any coordination between the two.
The switch hardware does not handle STP state for a LAG as a whole. State
changes for the members have to be done by the firmware, updating every member
port, which is possible using a single register write. BPDUs go out through the
lowest member and carry the group's own port id. Losing one member of a live
LAG is not a topology change; the logical port only goes down with its last
link.
Port states live in `RTL837X_MSTP_STATES (0x5310)`, two bits per port:
`00` disabled, `01` blocking, `10` learning, `11` forwarding. A port in
blocking forwards nothing between ports, but it still sends what the CPU
@@ -109,19 +85,80 @@ encodes. All configured values are in seconds:
| `stp fwd <n>` | 15 | 4–30 |
| `stp txhold <n>` | 6 | 1–10 |
A port entering the tree spends `fwd` seconds in blocking before it forwards
(an edge port skips the wait). Root information is discarded after `maxage`
seconds without a BPDU, and the switch then reclaims the root role.
802.1D also requires `2 * (fwd - 1) >= maxage >= 2 * (hello + 1)`. A
command typed on the console or sent by the web page that would break it is
refused and the value stays as it was, so change the values in an order that
keeps the rule at every step (for a larger max age raise the forward delay
first). The web page shows the error, and a refused command is not recorded
in the command history, so saving the configuration does not pick it up. Lines of the saved configuration are taken one by one whatever their
order, and `stp on` checks the result: if the rule is broken it warns and
starts with 2/20/15 instead.
The values above are what the switch uses while it is the root. Otherwise it
takes max age and forward delay from the BPDUs on its root port, as 802.1D
requires, and passes them on in its own BPDUs with the message age one second
higher; the hello time stays its own.
A port entering the tree discards for one forward delay, then learns
addresses without forwarding for a second one, and only then forwards (an
edge port skips the wait). In RSTP mode on a point-to-point link
(`p2p auto` or `on`) it can go faster: a designated port that is still
discarding sets the proposal flag, and forwards as soon as the bridge on the
other side answers with an agreement. When the root port receives a proposal,
the switch first puts its other non-edge designated ports back to discarding
(they propose in turn), then forwards the root port and answers with an
agreement; this sync happens once per root port, later proposals are only
answered. An alternate or backup port that receives a proposal does the same
sync once for the information it holds and answers with an agreement in a
BPDU with the alternate role, so the bridge on the other side does not have
to wait out its timers; it stays discarding itself. An agreement is accepted
from a root or an alternate port on the other side. A new root port forwards at once, after every port that was root within
the last forward delay has been put back to discarding; such a recent root
port stays closed, even to an agreement, until that forward delay is over.
A new root port that was a backup port within the last two hello times does
not take the fast path and waits for its timers.
A designated port that hears worse information from a bridge that still
claims to be designated and learning or forwarding on the same link has a
dispute: usually one direction of the link is lost and the other bridge does
not hear this switch's BPDUs. The port goes back to discarding, and every
further such BPDU restarts its listen period, so it does not forward while
the dispute lasts.
With `stp version rstp` each port still talks 802.1D to a neighbour that
does: once a Config BPDU or TCN arrives and the migrate time (3 s since the
port came up or last switched) is over, the port sends Config BPDUs, uses
the 802.1D TC window and reports changes with TCN if it is the root port. An
RST BPDU after the migrate time switches it back, and so does a link that
comes up again or `stp port <n> mcheck`. Any worse information heard on a designated port, disputed or
not, is answered with a BPDU at once instead of at the next hello, so the
other bridge learns sooner that it is not the designated one. Information heard on a port stays
valid for three of the sender's hello times; a BPDU whose message age has
reached its max age is not used at all. When the root port's information
expires the switch moves to the next best port, or reclaims the root role if
there is none.
Among ports offering the same root, the lowest root path cost wins, then the
designated bridge, the designated port ID, and finally this switch's own port
ID. A worse BPDU from a different bridge does not replace what a port holds;
a worse one from the same designated bridge and port does.
## Topology changes
A change on a local non-edge port (the link coming or going, a port promoted
to forwarding) flushes the addresses learned on it and sets the TC flag in
our BPDUs for `maxage + fwd` seconds. A TC flag received in a BPDU is passed
on: the switch flushes the other non-edge ports once and keeps the flag in
its own BPDUs until one hello after the last flagged frame, so the
notification crosses the switch instead of dying at it. A legacy TCN is
acknowledged with TCA and then treated like a local change.
A topology change is detected when a non-edge port enters forwarding. That
port, and every other forwarding non-edge port, then carries the TC flag in
its BPDUs for hello time plus one second (max age plus forward delay with
`stp version stp`), and each of the other ports flushes the addresses it
learned. The root port takes part too: while its TC window runs it sends
BPDUs toward the root, so the change reaches the rest of the network. With
`stp version stp` the root port sends TCN instead, until a Config BPDU with
TCA comes back.
A TC flag received on a root or designated port is passed on the same way to
the other ports, not back to the port it came in on. A legacy TCN is
acknowledged with TCA and then treated like a received TC. A port that turns
alternate, loses its link, or is blocked for a loop flushes its own addresses
without raising a topology change, and an edge port never raises one.
## Bridge settings
@@ -129,6 +166,8 @@ acknowledged with TCA and then treated like a local change.
stp prio <0-15> # bridge priority = n * 4096, default 8 (32768)
stp version rstp|stp # RST BPDUs (default) or legacy Config BPDUs
stp hello|maxage|fwd|txhold <seconds>
stp pathcost long|short # automatic port costs, 802.1D-2004 (default) or 802.1D-1998
stp bpdu flood|filter # BPDUs while STP is off: pass them on (default) or drop them
```
The bridge with the lowest priority wins the root election; ties are broken by
@@ -136,6 +175,17 @@ the MAC address. If you do not want this switch to become the root of an
existing network, give it a worse priority than the current root — `stp prio 15`
(61440) is the usual "never me" value.
A port without a configured cost takes it from its link speed. The long
method uses the 802.1D-2004 values (20000 for 1G, 2000 for 10G); the short
one the 802.1D-1998 table for bridges that still use 16-bit costs (100 for
10M, 19 for 100M, 4 for 1G, 2 for 10G, and 3 for 2.5G and 5G, which that
table does not list). All bridges in a network should use the same method.
`stp bpdu filter` keeps BPDUs from being passed between ports while STP is
off, instead of the default flooding that lets a surrounding spanning tree
run through the switch. It takes effect at once when STP is off, otherwise
when STP is turned off.
## Per-port settings
```
@@ -146,6 +196,7 @@ stp port <1-9> prio <0-240> # port priority, steps of 16
stp port <1-9> guard none|bpdu|root
stp port <1-9> filter on|off # neither send nor accept BPDUs
stp port <1-9> p2p auto|on|off
stp port <1-9> mcheck # try RSTP again on a port that fell back to STP
```
**edge** — an edge port forwards immediately and does not trigger a
@@ -162,26 +213,67 @@ claims a better priority.
on the far side reacts badly to them (some unmanaged switches with loop
prevention cut the link) but you still want STP on the rest of the ports.
## Link aggregation
A LAG (Link Aggregation Group) is one port of the tree, with its own roles,
timers and states. The switch supports up to 4 LAGs; each one takes the place
of its member ports in the STP status and gets its own row on the Spanning
Tree page. A group carries its own path cost, priority, edge, guard and
point-to-point settings, and takes the same commands as a port:
```
stp lag <1-4> on|off|mcheck|edge|cost|prio|guard|filter|p2p ...
stp lag 1 cost 10000 # the group decides, not its members
```
LAGs are configured with the `lag` command. Once a port is a member of a LAG
it can no longer be configured on its own for STP, and `stp port <n>` on a
member names the group to configure instead. Membership is read from the
aggregation registers once a second, so a group changing under LACP is picked
up without any coordination between the two; a port that joins or leaves a
group, and the group itself, start over from discarding. The automatic path
cost of a group is the one of its lowest member that has a link.
The hardware keeps no STP state for a LAG as a whole, so every state change is
written to all member ports, in a single register write. BPDUs go out through
the lowest member that has a link and carry the group's own port id, and a
BPDU received on any member belongs to the group. Losing one member of a live
LAG is not a topology change; the group only goes down with its last link.
## Status
The Spanning Tree page shows the elected root (priority and MAC), the path cost
to it, the root port, the topology-change counter and, per port, the live state
read from the ASIC together with the configured options. The same data is
available as JSON:
to it, the root port, the topology-change counter and the time since it last
moved, the max age and forward
delay in use (the root's, when this switch is not the root) and, per port, the
live state read from the ASIC, or "No link", together with the configured
options. For a port taking part in the tree it also shows the designated
bridge, port and cost, and whether it talks RSTP or 802.1D to its neighbour;
the Check button runs `stp port <n> mcheck`. The same data is available as
JSON:
```
GET /stp.json
```
The `stp status` command prints the same view on the serial console.
The Counters section of the same page (or `/stpcnt.json`) shows per port the
BPDUs received and sent, those of them that carried the topology change flag
(TCNs included), and the time since the last BPDU arrived. The counters start
from zero with `stp on`, and `stp clear` resets them; the topology change
counter is not reset.
The `stp status` command prints the same view on the serial console. A port
is reported with one of the roles Root, Designated, Alternate, Backup or
Disabled; Disabled covers a port with STP turned off on it, one shut down by
BPDU guard and one without link.
BPDUs shorter than their type allows, judged by the 802.3 length field, are
dropped: 7 bytes for a TCN, 38 for a Config BPDU and 39 for an RST BPDU,
LLC header included.
## Limitations
* One spanning-tree instance; no MSTP, no per-VLAN trees.
* No proposal/agreement handshake — an RST-capable neighbour will still
converge, but through the timers rather than the fast transition.
* Port roles are approximated: the root port and designated ports are
distinguished, alternate/backup are not.
* Topology changes propagate away from the root only: nothing is announced
on the root port (no TCN and no BPDUs at all), so bridges upstream rely on
their own detection.
* A backup port, one that hears the BPDUs of another port of this switch,
is blocked and reported as Backup, but keeps sending BPDUs: loop detection
on that segment relies on them.