Trunk Ports

In a small network with few VLANs, it is possible to use a separate interface for each VLAN when connecting switches to switches, and switches to routers.

However, when the number of VLANs increases, this is not viable. It will result in wasted interfaces, and often routers won't have enough interfaces for each VLAN.

You can use trunk ports to carry traffic from multiple VLANs over a single interface.

Tagging

Switches will 'tab' all frames that they send over a trunk link. This allows the receiving switch to know which VLAN the frame belongs to.

• Trunk ports = tagged ports.

• Access ports = untagged ports.

Protocols

There ate two main trunking protocols: ISL (Inter-Switch Link) and IEEE 802.1Q.

ISL is an old Cisco proprietary protocol created before the industry standard IEEE 802.1Q.

IEEE 802.1Q is an industry standard protocol created by the IEEE (Institute of Electrical and Electronics Engineers).

Note

You will probably NEVER use ISL in the real world. Even modern Cisco equipment doesn't support it. For the CCNA you only need to learn 802.1Q.

802.1Q Tag

The 802.1Q tag is inserted between the Source and Type/Length fields of the Ethernet frame.

The tag is 4 bytes (32 bits) in length.

The tag consists of two main fields:

• Tag Protocol Identifier (TPID);

• Tag Control Information (TCI).

The TCI consists of three sub-fields.

TPID

16 bits (2 bytes) in length

Always set to a value of 0x8100. This indicates that the frame is 802.1Q-tagged.

TCI

PCP - Priority Code Point

3 bits in length

Used for Class of Service (CoS), which prioritizes important traffic in congested networks.

DEI - Drop Eligible Indicator

1 bit in length

Used to indicate frames that can be dropped if the network is congested.

VID - VLAN ID

12 bits in length

Identifies the VLAN the frame belongs to.

12 bits = 4096 total VLANs, range of 0 - 4095.

VLANs 0 and 4095 are reserved and can't be used.

Therefore, the actual range of VLANs is 1 - 4094.