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README.VLANs.md

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Using mstpd with Trunk Ports / VLANs

There are currently two different ways to implement spanning tree on trunk ports using Linux bridging and mstpd:

  1. Single Spanning Tree (RSTP/STP)
  2. Per-VLAN Spanning Tree (PVST+)

Single Spanning Tree (RSTP/STP)

  • Create a Linux bridge and attach the trunk interfaces to it.
  • Enable STP on the bridge.
  • Force mstpd to use RSTP (mstpctl setforcevers <bridge> rstp).
  • Create Linux VLAN interfaces on top of the bridge interface.
  • If necessary, create a Linux bridge for each VLAN and attach the relevant VLAN interface to it. Do not enable STP on these VLAN bridges.

This should be compatible with most switches. If a connected switch speaks MSTP, it will fall back to RSTP on our port. If a connected switch speak STP, mstpd will fall back to STP on that port.

However, note that Cisco switches in 'pvst' or 'rapid-pvst' mode will not fall back to RSTP in this case. To use a single Spanning Tree with Cisco switches, you must put the Cisco switches in 'mst' mode (spanning-tree mode mst). Cisco switches in 'mst' mode will fall back to PVST+ on ports connected to other switches that speak PVST+.

Per-VLAN Spanning Tree (PVST+)

  • Create Linux VLAN interfaces on top of each trunk interface.
  • For each VLAN, create a Linux bridge and attach the relevant VLAN interfaces to it.
  • If the native VLAN is used (if the trunk may carry untagged packets), then create another Linux bridge, attach the trunk interfaces to it, and run ebtables -t broute -A BROUTING -i <trunk iface> -p 802_1Q -j DROP for each trunk interface to prevent this bridge from processing tagged packets.
  • Enable STP on each bridge.
  • Force mstpd to use RSTP on each bridge (mstpctl setforcevers <bridge> rstp).

This is only compatible with other switches that speak PVST+.