See [Spanning Tree Protocol]

802.1Q is an IEEE trunking mechanism that is an open standard. Both ISL and 802.1Q add VLAN information to the Ethernet frames explicitly. However, the way in which they perform this process is different. With ISL, a 26-byte header and a 4-byte trailer are added to the frame: The original frame is not modified. This process is referred to as encapsulation. With 802.1Q, the actual frame is modified, or tagged. To denote VLAN information, a 4-byte Tag Protocol Identifier (TPID) and a 2-byte Tag Control Information (TCI) are inserted between existing fields in the Ethernet frame.

802.1Q Tunneling

Q-in-Q tunneling, proprietary to Cisco, is commonly referred to as tag stacking. When you send tagged ...

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