What IS the purpose of the command witchport mode Trunks allowed vlan

[Flavio Miranda]

Hi @Bukasa  

 This command add the vlan 13 to the trunk port.

 When you use switchport trunk allow vlan X you are creating a filter and defining which vlan can be used on the trunk

If you need one more vlan to be allowed on the trunk, you need to add this new vla n with the commad

Switchport trunk allowed add vlan x

[Flavio Miranda]

Hi @Bukasa  

The command is

Switchport trunk allowed vlan add 13.

This coomand permit the vlan 13 to pass traffic on the trunk port along with the other vlans.

 When you specify the vlan on the commamd

Switchport trunk allowed vlan X you are crearing a filter and allowing only vlan X. If you later on need vlan 13 to also use the trunk interface,  you need to add this new vlan to the trunk.

[MHM Cisco World]

• When you connect two Cisco switches through 802.1Q trunks, the switches exchange spanning-tree bridge protocol data units (BPDUs) on each VLAN allowed on the trunks. The BPDUs on the native VLAN of the trunk are sent untagged to the reserved IEEE 802.1d spanning-tree multicast MAC address (01-80-C2-00-00-00). The BPDUs on all other VLANs on the trunk are sent tagged to the reserved Shared Spanning Tree Protocol (SSTP) multicast MAC address (01-00-0c-cc-cc-cd).

even if the SW not use this VLAN the STP exchange BPDU for each vlan allow in trunk. 
so we allow only VLAN in SW

[Joseph W. Doherty]

As @Flavio Miranda already described, it allows you to add VLANs to an list of allowed VLANs.  Specifically:

Switch(config-if)#switchport trunk allowed vlan ?
  WORD    VLAN IDs of the allowed VLANs when this port is in trunking mode
  add     add VLANs to the current list
  all     all VLANs
  except  all VLANs except the following
  none    no VLANs
  remove  remove VLANs from the current list

The alternative, I believe, would be to reenter the whole list, with the changes embedded.

For many years now, such an "editing" feature seems needless when you might cut, edit, paste, but once upon a time, you had to type everything out.  Basically, the above "editing" features, save you typing.