Spanning tree¶

Spanning tree finds loops between switches and blocks ports so that frames cannot circle forever. It is off by factory default. Turn it on if the switch is connected to other switches over more than one path, or if loops could happen by mistake.

Three protocols are available. Only one can be on at a time:

Protocol	Value for `enabled-protocol`	Configured under
RSTP, IEEE 802.1w (recommended)	`oc-stp-types:RSTP`	`stp rstp`
MSTP, IEEE 802.1s	`oc-stp-types:MSTP`	`stp mstp`
STP, IEEE 802.1D (legacy)	`sw:STP`	`stp rstp` (same settings as RSTP)

RSTP and MSTP fall back to classic STP on their own on ports that face an old 802.1D switch. The settings of a protocol that is not on can stay; they have no effect.

Turning spanning tree on or changing the protocol blocks the ports until they are known to be loop-free: a few seconds with RSTP, about 30 seconds with STP.

All switch ports take part in spanning tree. Per-port settings are only needed to change the defaults.

Bridge settings¶

Setting	Default	Allowed
`bridge-priority`	32768	0 … 61440 in steps of 4096. Lower wins the root election: give the switch that should be the root a lower priority than the others.
`hello-time`	2	2 only
`max-age`	20	Must satisfy `max-age <= 2 × (forwarding-delay − 1)`
`forwarding-delay`	15	See `max-age`
`hold-count`	6

Values that break the rules are rejected with an explanation:

validate: stp: rstp: bridge-priority 5000 is not a multiple of 4096 in 0..61440
validate: stp: rstp: max-age 20 is greater than 2 * (forwarding-delay 10 - 1)

Port settings¶

Path cost and port priority change which port is blocked when there is a loop. They are set per protocol (and per MSTI with MSTP):

Cost: path cost. Without it, the cost follows the link speed.
Port priority: a multiple of 16, default 128. Lower is preferred.

These port features apply to all protocols:

Setting	Values	Meaning
`edge-port`	`oc-stp-types:EDGE_AUTO` (default), `oc-stp-types:EDGE_ENABLE`, `oc-stp-types:EDGE_DISABLE`	An edge port has an end device, not a switch, behind it and forwards at once. `EDGE_AUTO` detects this. Set `EDGE_ENABLE` on ports to PCs to save the start-up delay.
`link-type`	`P2P`, `SHARED`	Whether the port is a point-to-point link. Detected automatically when unset.
`guard`	`ROOT`, `NONE`	Root guard: the port may never become the root port. Blocks a foreign switch from taking over the root role.
`bpdu-guard`	`true`, `false`	Shut the port down when a spanning tree frame (BPDU) arrives. For ports where no switch may be connected.
`bpdu-filter`	`true`, `false`	Neither send nor process BPDUs on the port. Use with care: this can create loops.

bpdu-guard and bpdu-filter can also be turned on for all ports at once. A port setting overrides the global one.

MSTP¶

MSTP runs several spanning tree instances (MSTIs), each for a group of VLANs, so different VLANs can use different paths. All switches of an MSTP region must have the same region name, revision and VLAN-to-MSTI mapping.

The region has a name, a revision, a max hop count (6 … 40, default 20), the timers (as for RSTP), and the bridge priority of the common spanning tree (CIST). VLANs that are in no MSTI belong to the CIST.
Each MSTI (id 1 … 4094) has its VLAN list (ids and x..y ranges) and its own bridge priority. A VLAN belongs to at most one MSTI. The list counts as written, whether the VLANs are declared or not, because it has to match the other switches of the region.
Port cost and priority can be set per MSTI and for the CIST.

Status¶

For the bridge (for RSTP and STP, or for the CIST and each MSTI with MSTP):

Field	Meaning
`bridge-address`, `bridge-priority`	This switch.
`designated-root-address`, `designated-root-priority`	The root bridge. If it equals the bridge, this switch is the root.
`root-port`, `root-cost`	The port towards the root and the path cost to it.
`topology-changes`	How often the topology changed.

Per port:

Field	Meaning
`role`	`ROOT`, `DESIGNATED`, `ALTERNATE` or `BACKUP`
`port-state`	`FORWARDING`, `LEARNING`, `BLOCKING` (RSTP's "discarding"), `DISABLED`
`cost`, `port-priority`, `port-num`	Values in use
`designated-*`	The bridge and port this port hears from
`forward-transisitions`	How often the port went to forwarding (spelled this way in OpenConfig)
`counters`: `bpdu-sent`, `bpdu-received`	Spanning tree frames counted

Web UI¶

The Spanning tree card of the status page shows the protocol (off, STP, RSTP or MSTP), the switch's bridge priority and address, the root bridge and the root port, and the number of topology changes. Below, one row per port with its role, state, whether it is an edge port, and its path cost and priority.

Edit on the card sets the protocol or turns spanning tree off, and changes the bridge priority and the timers.
Edit on a port sets edge port, link type, path cost and port priority.

While the ports are blocked after turning spanning tree on or changing the protocol, the dialog may say that the switch did not answer in time. The page catches up once the ports forward again.

Not in the web UI

MSTP regions and instances, root guard, BPDU guard and BPDU filter are configured with the CLI or RESTCONF.

CLI¶

Known problem

In the current firmware the CLI on the switch rejects the timers, hold-count, max-hop, cost, port-priority and MSTI ids. Turning a protocol on, bridge-priority and the port features work. See Known problems.

Turn on RSTP¶

switch> set stp global config enabled-protocol oc-stp-types:RSTP
switch> commit
switch> save

That is enough for most networks. To make this switch the root bridge:

switch> set stp rstp config bridge-priority 4096
switch> commit

The other bridge settings are under stp rstp config too, for example set stp rstp config forwarding-delay 10.

Port cost and priority¶

switch> set stp rstp interfaces interface lan3 config name lan3
switch> set stp rstp interfaces interface lan3 config cost 20000
switch> set stp rstp interfaces interface lan3 config port-priority 64
switch> commit

Port features¶

Set under stp interfaces:

switch> set stp interfaces interface lan8 config name lan8
switch> set stp interfaces interface lan8 config edge-port oc-stp-types:EDGE_ENABLE
switch> commit

For all ports at once: stp global config bpdu-guard and stp global config bpdu-filter.

MSTP¶

switch> set stp global config enabled-protocol oc-stp-types:MSTP
switch> set stp mstp config name region1
switch> set stp mstp config revision 1
switch> set stp mstp config bridge-priority 8192
switch> set stp mstp mst-instances mst-instance 1 config mst-id 1
switch> set stp mstp mst-instances mst-instance 1 config vlan 20
switch> set stp mstp mst-instances mst-instance 1 config vlan 30..40
switch> set stp mstp mst-instances mst-instance 1 config bridge-priority 4096
switch> commit

stp mstp config holds the region (name, revision), max-hop, the timers, and bridge-priority of the CIST.
Port cost and priority per MSTI: stp mstp mst-instances mst-instance 1 interfaces interface lan8 config .... For the CIST: stp mstp interfaces interface lan8 config ....

Change protocol or turn it off¶

enabled-protocol is a list. To switch protocols, delete the old entry and add the new one before you commit. Two entries are rejected (only one protocol may be enabled):

switch> delete stp global config enabled-protocol oc-stp-types:RSTP
switch> set stp global config enabled-protocol oc-stp-types:MSTP
switch> commit

To turn spanning tree off, delete the entry and commit.

Status¶

switch> show state text stp
...
   rstp {
      state {
         hello-time 2;
         max-age 20;
         forwarding-delay 15;
         hold-count 6;
         bridge-priority 4096;
         bridge-address 92:b6:82:b9:ac:79;
         designated-root-priority 4096;
         designated-root-address 92:b6:82:b9:ac:79;
         root-cost 0;
         topology-changes 0;
      }
      interfaces {
         interface lan1 {
            state {
               name lan1;
               port-priority 128;
               port-num 1;
               role oc-stp-types:DESIGNATED;
               port-state oc-stp-types:FORWARDING;
               ...
               forward-transisitions 1;
               counters {
                  bpdu-sent 10;
                  bpdu-received 0;
               }
            }
         }
...

Here the switch is the root bridge itself: the root address is its own, and there is no root-port. Ports without a link show port-state oc-stp-types:DISABLED and no role. The bridge status is under rstp state (mstp state for the CIST, and per mst-instance), the port status under ... interfaces interface lanN state.

RESTCONF¶

Coming later

Examples are not written yet. See Using RESTCONF for how the CLI paths above map to RESTCONF resources.

Spanning tree is configured under:

/restconf/data/openconfig-spanning-tree:stp

In RESTCONF the protocol identities are spelled out: openconfig-spanning-tree-types:RSTP, openconfig-spanning-tree-types:MSTP and clixon-switch:STP. The clixon-switch-rs README has an example that turns on RSTP.

Not supported¶

Rapid PVST, loop guard, bridge assurance, EtherChannel guard and automatic recovery after BPDU guard are rejected, for example stp/global/config/loop-guard true is not supported, only false.