difference between 2 commands in CBS 350 model switches

[13jobsp90]

I have a doubt regarding Cisco CBS 350 model switch. For a WIFI AP to configure in a port we give 2 commands switchport mode trunk and switchport trunk allowed vlan ____ & switchport trunk native vlan ___. 

The above 3 commands are given for wifi ap to get it configured.

Why do you need to have/ what is the difference between switchport mode trunk and switchport trunk allowed vlan ____ ? If I pass switchport mode trunk it means it passes all vlans to that interface port. then why we need switchport trunk allowed vlan ____ ?

[M02@rt37]

Hello @13jobsp90 

The switchport mode trunk command configures a port to act as a trunk, allowing it to carry traffic for multiple VLANs. By default, a trunk port will pass traffic from all VLANs, making it essential for devices like wireless APs that need to handle multiple VLANs, such as one for management and another for user traffic.

The switchport trunk allowed vlan ____ command, on the other hand, is used to restrict which VLANs are allowed on the trunk. Even though a trunk port can handle traffic for all VLANs by default, this command ensures that only the specified VLANs can pass through the trunk, improving network security and reducing unnecessary traffic.

[13jobsp90]

then why need switchport mode trunk command even though we have the other command which passes only specified vlan which we need?

[M02@rt37 · Accepted solution]

@13jobsp90 

Without the switchport mode trunk command, the port would not be a trunk and would not be able to carry traffic for more than one VLAN. The switchport trunk allowed vlan ____ command only makes sense once the port is in trunk mode because it defines which VLANs can be carried on the trunk.

These 2 commands serve distinct roles in configuring a trunk port:

• switchport mode trunk:  This command enables trunking on the port, allowing it to carry traffic for multiple VLANs. Without this command, the port would operate in access mode, where it only carries traffic for a single VLAN. In other words, this command is necessary to tell the switch that the port should handle traffic for multiple VLANs, not just one.

• switchport trunk allowed vlan ____ : Once the port is in trunk mode (thanks to the switchport mode trunk command), this command is used to specify which VLANs are allowed to pass through the trunk. By default, a trunk port carries traffic for all VLANs, but this command limits it to the VLANs you specify, improving control and security.

[13jobsp90]

Thank you for the explanation.

[M02@rt37]

You're so welcome @13jobsp90