I'm wondering what would happen in my network in the following configuration:

on switch1 -> interface1 in mode access, vlan4
on switch2 -> interfaceA in mode trunk, vlan4 and vlan8

switch1 is connected through interface1 to switch2's interfaceA.

So, when something from vlan4 goes through interface1 from switch1, it's tagged vlan4 right? My guess would be:

 it's be going to vlan4, just as if interfaceA was in access.

But what happens when something from vlan4 go through interfaceA from switch2? my guess:

it's not tagged so it goes into switch1's default VLAN?

Would all incoming traffic from switch2 to switch1 go into default VLAN?


So, when something from vlan4 goes through interface1 from switch1, it's tagged vlan4 right?

No, you seem to have it backwards. Frames exiting an access interface are not tagged because most access devices do not understand VLAN tags. VLAN tags are used on trunks to separate frames belonging to different VLANs.

Frames exiting switch1, interface1 will be untagged, and will be interpreted by switch2 as belonging to the native VLAN. Since you do not specifically call out which VLAN is the native VLAN on switch2, it should default to VLAN 1.

What happens when tagged frames from switch2 are received by an access interface on switch1 is undefined. Some switches will drop the frames as giants, and some switches will simply pass the frames through.

In Cisco switches, if the native VLAN on switch2, interfaceA is not VLAN 4, then you will get messages on switch2 about a native VLAN mismatch.

  • Indeed, I had it backward. does this mean (in this configuration) I can have multiple VLAN going out from interface1 in access , but they'll all end up in the default VLAN inswitch2?
    – pwnsauce
    May 10 '17 at 13:08
  • 1
    A switch will only send through an access interface frames in the VLAN for which the interface is configured. That means that if the interface is configured for VLAN 4, the switch will only send frames for VLAN 4 through that interface, and any (untagged) frames received on that interface will be placed in VLAN 4. You can get into trouble by leaking frames from one VLAN to another VLAN.
    – Ron Maupin
    May 10 '17 at 15:00

Trunk against Access --> Access the frame will not be tagged, it maybe will be dropped ,the messages from a vlan to the same vlan will arrive.

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