Layer 2 use case
The access port of the PE1 device is an untagged member of the stack VLAN, which is connected to CE1. The Ingress TPID check on this access port is disabled, so the port treats all the ingress traffic as untagged.
When traffic egresses from the stack VLAN trunk port, an additional tag (outer Tag or S-Tag) with the stack VLAN ID is added to the frame. The TPID value used in the S-Tag is the TPID value configured on that trunk port.
- The default TPID value on all ports is 0x8100.
- If CE1 sends untagged traffic, it is single-tagged when it egresses out of the stack VLAN trunk port.
- If CE1 sends tagged traffic (C-Tag), it is double-tagged (C-Tag + S-Tag) when it egresses out of the stack VLAN trunk port.
When the traffic reaches the PE2 and leaves the egress access port, the access port removes the added S-TAG. This action changes the single-tagged packets to untagged and double-tagged packets to single-tagged, which is similar to the packets sent by CE1, and then reaches CE2.
![](https://dl.dell.com/content/guides/public/Html/smartfabric-os-user-guide-10-5-4/images/GUID-6FD6F98F-1A6A-419F-BC21-BDA7E607AE12-low.png)