Spanning Tree and Link Aggregation
What you must understand
Spanning tree prevents Layer 2 loops. Aruba supports STP (802.1D), RSTP (802.1w), MSTP (802.1s), and on AOS-S the proprietary PVST-style per-VLAN mode. Key roles: root bridge, designated port, root port, alternate/blocking. Root selection uses bridge priority (default 32768) then MAC. MSTP maps VLANs to instances; the CIST is instance 0.
Link aggregation: LACP (802.3ad/802.1AX) forms dynamic bundles; static/manual trunks also exist. On CX, use interface lag <n> and add members; on AOS-S, use trunk <n> lacp. Members must match in speed, duplex, and VLAN config.
Traps
- Lower priority wins the root election; the default is 32768, and increments are in multiples of 4096.
- A port can be in a LAG and running STP - the LAG is treated as one logical link, so it is not blocked against itself.
- LACP has an active and passive side; two passive sides never form a bundle.
- Mismatched member settings silently prevent the LAG from coming up.
Rule of thumb
Set your intended root bridge to priority 4096 or 8192 so it always wins, and verify with show spanning-tree. For LAGs, always configure LACP active on at least one side and confirm with show lacp interfaces.