Why does show interfaces trunk list the VLANs four times, and how is the allowed list different from pruning?
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The sections narrow down: VLANs allowed by config; of those, the ones that exist and are active on this switch; of those, the ones in spanning-tree forwarding state on this port and not pruned. A VLAN has to survive all of them to cross. The allowed list is static config per trunk. VTP pruning is dynamic: a switch tells its neighbor which VLANs it needs, and the neighbor stops flooding the rest over that trunk.
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SW1# show interfaces trunk
Port Mode Encapsulation Status Native vlan
Gi1/0/48 on 802.1q trunking 999
Port Vlans allowed on trunk
Gi1/0/48 10,20,30,40
Port Vlans allowed and active in management domain
Gi1/0/48 10,20,30
Port Vlans in spanning tree forwarding state and not pruned
Gi1/0/48 10,20- VLAN 40 is allowed but does not exist in SW1's VLAN database, so it drops out of the second list.
- VLAN 30 exists but is either blocked by spanning tree on this port or pruned.
show spanning-tree vlan 30tells you which: anAltn BLKrole on Gi1/0/48 means STP.
Editing the allowed list safely
interface GigabitEthernet1/0/48
switchport trunk allowed vlan add 50
switchport trunk allowed vlan remove 40Without add or remove, the command replaces the whole list.
VTP pruning
Enabled domain-wide with vtp pruning on a server. VLAN 1 and 1002 to 1005 are never pruned.
switchport trunk pruning vlan limits which VLANs are eligible on a trunk.
They will ask next
Allowed lists or VTP pruning, which do you prefer?
Allowed lists. They are explicit, visible in the config, work with any vendor, and work in VTP transparent or off mode, where pruning does not apply.
Traps
- Typing
switchport trunk allowed vlan 50on a live trunk. It removes every other VLAN.