About forward or drop of 1Q-tagged Ethernet frame by default

[Makoto. Y]

Could you tell me about question below,

By default, when a C2960 switch receive an "IEEE802.1Q-tagged Ethernet frame" , forward it to other port or drop it ?

[MHM Cisco World · Accepted solution]

Hmm,

if trunk switch port then forward 

if access as I know depend on vendor some forward some drop

[Makoto. Y]

I appreciate your answer.
On access, Brand X, other than Cisco, forward.
Brand Y drop.
So it is right in thinking that IEEE standards don't define the behavior (forward or drop) !?

[TJ-20933766 · Accepted solution]

Correct. The IEEE standard only dictates how one switch tells another switch the VLAN membership of a frame. Once the switch receives the frame, it is up to the manufacturer of the switch to determine the logic behind how it processes the received frame.

[Makoto. Y]

It opened up my eyes.

Thank you very much for your lucid explanations.

[TJ-20933766 · Accepted solution]

1) Existence of VLAN on Switch - If a Cisco switch does not have the VLAN for which the frame is tagged, the frame will be dropped. For example: If a frame with a VLAN 80 tag enters an interface on the switch that is configured as a trunk and VLAN 80 does not exist in the VLAN database, the frame is dropped. Use the following command to view the VLAN database and ensure the VLAN has been configured on the switch:

show vlan brief

2) Switch Interface Configuration - If the frame is tagged and enters the switch on an interface not configured as a trunk, the frame will be discarded. By default, switch interfaces are configured as "dynamic auto" and will become trucks should they be connected to another device that is capable of sending Dynamic Trunking Protocol (DTP) frames such as another Cisco switch. The best practice is to statically configure the mode of the interface to be either an access port or a trunk port but not dynamic auto. To configure a switch interface to be a trunk, using the following command:

interface GigabitEthernet0/1
 switchport encapsulation dot1q
 switchport mode trunk

Note: the command "switchport encapsulation dot1q" may not be necessary depending on the switch you are using. If you type this command and get an error then you do not need it.

[Makoto. Y · Accepted solution]

I appreciate your answer.

Thank you very much for your kind instruction.

I now have a deeper understanding of Cisco switch.

[RedNectar]

@Makoto. Y ,

If @TJ-20933766  has answered your question, please mark the question as answered to help others.

[Makoto. Y]

Thank you for your comment.

Does that mean pushing the button " Acceot as Solution" ?

[RedNectar]

Correct