Frame Tagging: Frame tagging is used to identify the VLAN that the frame belongs to in a network with multiple VLANs. The VLAN ID is placed on the frame when it reaches a switch from an access port, which is a member of a VLAN. That frame can then be forwarded out of the trunk link port. Each switch can see what VLAN the frame belongs to and can forward the frame to corresponding VLAN access ports or to another VLAN trunk port.
Before forwarding a tagged frame to an end host, the switch will remove the VLAN ID and the VLAN membership information, since end-host devices don’t understand tagging.
Two trunking protocols are usually used today for frame tagging:
Inter-Switch Link (ISL) – Cisco’s proprietary VLAN tagging protocol.
IEEE 802.1q – IEEE VLAN tagging protocol. Since it is an open standard, it can be used for tagging between switches from different vendors.
Example: There are two VLANs in the topology. VLAN 5 and VLAN 10. Host C sends a broadcast packet to switch SW1. Switch SW1 receives the packet, tags the packet with the VLAN ID of 5, and sends it to SW2. SW2 receives the packet, looks up at the VLAN ID, and forwards the packet only out the port in VLAN 5. Host A and Host B will not receive the packet because they are in different VLAN (VLAN 10).
Inter-Switch Link (ISL): Inter-Switch Link (ISL) is a Cisco proprietary protocol for frame tagging. Since it is a proprietary protocol, it can be used only between Cisco switches. It supports up to 1000 VLANs and can be used over Fast Ethernet and Gigabit Ethernet links only.
ISL works by encapsulating an Ethernet frame is an ISL header and trailer. The encapsulated frame remains unchanged. The VLAN ID is included in the ISL header.
Original frame:
| Destination MAC | Source MAC | Length/type | Data | FCS |
ISL encapsulates the frame:
| ISL Header | Destination MAC | Source MAC | Length/ type | Data | FCS | ISL FCS |
ISL Is considered to be deprecated, and some newer Cisco switches don’t even support it. 802.1q is commonly used instead.
802.1q: 802.1q us a VLAN tagging protocols developed by IEEE (Institute of Electrical and Electronics Engineers). Since it is an open standard, it can be used between switches from different vendors, so if you are trunking between a Cisco switch and a different brand of a switch, you’ve can use 802.1q for the trunk to work.
Unlike ISL, which encapsulates the whole frame in an ISL header and trailer, 802.1q insert an extra 4-byte 802.1q VLAN field into the original frames Ethernet header. The 802.1q field includes the 12-but VLAN ID field, which specifies the VLAN to which the frame belongs. 802.1q tagged frame can carry information for 4,094 VLANs.
Original frame
| Destination MAC | Source MAC | Length/type | Data | FCS |
802.1Q frame:
| Destination MAC | 802.1Q Tag | Source MAC | Length/ type | Data | FCS |
802.1q defines one special VLAN ID on each trunk as the native VLAN (by default VLAN 1). 802.1q does not add an 802.1Q header to frame in the native VLAN when the switch on the other side of the trunk receives a frame that does not have an 802.1q header, the receiving switch knows that the frame is part of the native VLAN. Because of this behavior, both switches must agree on switch VLAN is the native VLAN.
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