Catalyst 2960 dot1q

[nandigamamanikanta]

• What will happen if don't configure dot1q on cisco 2960 switches? 

[balaji.bandi]

all the ports act as access port default to vlan 1

[MHM Cisco World]

I think when you need to config trunk you should first config the encapsulation and then config 
switchport mode trunk .

[Spooster IT Services · Accepted solution]

- Dot1Q is an Open standard Trunking Protocol 

- If it is an access port then it is fine if you don't configure Dot1Q or any other tagging protocol (ISL ) , because access ports don't have the concept of  tagging as they carry only single vlan traffic .

- But While configuring Trunking it is must to configure tagging . As on trunk Links Multiple Vlan traffic is Forwarded , So for proper delivery of the Vlan traffic , Tagging is performed , so that there can be some differentiating factor between the Vlan traffic .

[Joseph W. Doherty]

(Wink) pretty much the same as what happens on other Cisco switches (and routers).  I.e. L2 frames would usually then not get a .Q tag.

As @Spooster IT Services describes, .Q tags are generally used so you can mix various VLANs on the same link, yet logically keep then distinct.

@Spooster IT Services also writes "because access ports don't have the concept of  tagging as they carry only single vlan traffic .", well that's an "it depends" situation.

Later Cisco switches often support a data and voice VLAN on the same (access) port.  The voice VLANs uses tagged frames, which, I believe, only using .Q tagging.  I.e. basically, such "access" ports are a special/restricted trunk port.

BTW, Cisco switches, for their trunk ports transmits one of the VLAN's frames, the "native" VLAN, without tagging its frames.  However, I believe such a trunk port will accept either untagged or tagged frames for the native VLAN.  (The latter, because, non-Cisco equipment generally tags all "trunk" type link frames.)

Also, BTW, I understand it's acceptable (by the .Q standard) to have .Q tags with a zero VLAN ID, this to use the tag only for L2 CoS.  Don't know if Cisco can generate such frames and/or whether Cisco switches would accept such frames.