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Tobias Waldekranz says:
====================
net: bridge: Multiple Spanning Trees
The bridge has had per-VLAN STP support for a while now, since:
https://lore.kernel.org/netdev/20200124114022.10883-1-nikolay@cumulusnetworks.com/
The current implementation has some problems:
- The mapping from VLAN to STP state is fixed as 1:1, i.e. each VLAN
is managed independently. This is awkward from an MSTP (802.1Q-2018,
Clause 13.5) point of view, where the model is that multiple VLANs
are grouped into MST instances.
Because of the way that the standard is written, presumably, this is
also reflected in hardware implementations. It is not uncommon for a
switch to support the full 4k range of VIDs, but that the pool of
MST instances is much smaller. Some examples:
Marvell LinkStreet (mv88e6xxx): 4k VLANs, but only 64 MSTIs
Marvell Prestera: 4k VLANs, but only 128 MSTIs
Microchip SparX-5i: 4k VLANs, but only 128 MSTIs
- By default, the feature is enabled, and there is no way to disable
it. This makes it hard to add offloading in a backwards compatible
way, since any underlying switchdevs have no way to refuse the
function if the hardware does not support it
- The port-global STP state has precedence over per-VLAN states. In
MSTP, as far as I understand it, all VLANs will use the common
spanning tree (CST) by default - through traffic engineering you can
then optimize your network to group subsets of VLANs to use
different trees (MSTI). To my understanding, the way this is
typically managed in silicon is roughly:
Incoming packet:
.----.----.--------------.----.-------------
| DA | SA | 802.1Q VID=X | ET | Payload ...
'----'----'--------------'----'-------------
|
'->|\ .----------------------------.
| +--> | VID | Members | ... | MSTI |
PVID -->|/ |-----|---------|-----|------|
| 1 | 0001001 | ... | 0 |
| 2 | 0001010 | ... | 10 |
| 3 |
|
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Linux kernel
============
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In order to build the documentation, use ``make htmldocs`` or
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Please read the Documentation/process/changes.rst file, as it contains the
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the problems which may result by upgrading your kernel.