CCNA Trunking Concepts Part I
We would like to welcome you again to a new CCNA 640-802 article related to VLANs. We will talk about VLAN trunking and we would like to note this is an important requirement in the CCNA exam. When you will take your CCNA certification exam, you will find that Cisco is considering this an important topic because you have a great chance to find a lot of questions related to VLAN trunking.
Trunking is what makes VLANs so great. Cisco describes a trunk as a point-to-point link between two network devices that carries more than one VLAN. Trunks are giving you the possibility to extend your VLAN across the whole physical network and sometimes are used even to extend your network across multiple service providers to reach the geographical destination of your other offices.
There are two types of trunks used by Cisco and you will see on the CCNA exam. The most common these days is the IEEE 802.1Q, but older deployments and some networks also use Cisco’s proprietary Inter-Switch Link (ISL). However, ISL is not covered anymore in the CCNA course so I will cover only the basic informations about it. Regardless of the type of trunk type used, trunks can be used only with Fast and Gigabit Ethernet interfaces.
Inter-Switch Link (ISL)
ISL is Cisco’s proprietary protocol used to create VLANs in a network. ISL is functioning at the Data-Link layer of the OSI model (layer 2). The size of the encapsulated ISL frames is starting from 94 bytes and can increase up to 1548 due to the additional fields the protocol creates during the encapsulation. The VLAN ID is a 15-bit value found in a 26-byte header of the frame. ISL also adds a 4-byte CRC in the frame for error correction and control.
IEEE 802.1Q
Today’s standard in trunking is 802.1Q. To better understand how other switches in the network are able to identify a frame belonging to a VLAN, you must know what does the first switch with that frame.
When a switch receives a frame on a port configured in access mode with a static VLAN, it takes that frame and inserts a VLAN tag, recalculates the FCS and sends the frame out through the trunking port. The VLAN tag field contains an Ethertype filed, a Tag control information field and the FCS field. By convention, Ethertype is set to the value of 0×8100. Tag control information field contains 3 bits of user priority, 1 bit of canonical Format Identifier (used to transport Token Ring frames across Ethernet links) and 12 bits of VLAN ID (VID).
Trunking ports support the transmission of tagged frames as well as untagged frames through the interface.
Some devices are sending by default are tagging the VLAN traffic. If such traffic arrives in a native VLAN interface, the switch will drop the frame. The traffic sent to a native VLAN interface should always be untagged. The only solution for this problem if you need that switch port to be configured as native is to configure the other devices to send the traffic untagged.
Untagged Frames arriving to the switch are automatically forwarded to the native VLAN. The default native VLAN is 1, but when you configure a 802.1Q trunk port, you can specify another VLAN as native. When you configure this, a default Port VLAN ID (PVID) is assigned to the interface. For example, if you create the native VLAN 20, the PVID will also 20. To enable a port in trunking mode, you can use the switchport mode trunk interface configuration command. To specify a VLAN as a native VLAN use switchport trunk native vlan vlan-id. You can also check the status of the interface with show interfaces interface switchport.
Switch(config-if)#switchport mode trunk
Switch(config-if)#switchport trunk native vlan 20
Switch(config-if)#ctrl-Z
Switch#show interfaces Fa0/1 switchport
Name: Fa0/1
Switchport: Enabled
Administrative Mode: dynamic auto
Operational Mode: down
Administrative Trunking Encapsulation: dot1q
Negotiation of Trunking: On
Access Mode VLAN: 50
Trunking Native Mode VLAN: 20 (VLAN0020)
Administrative Native VLAN tagging: enabled
……….
Administrative private-vlan trunk Native VLAN tagging: enabled
Administrative private-vlan trunk encapsulation: dot1q
………..
Trunking VLANs Enabled: ALL
We hope you found this Cisco CCNA certification article covering VLAN concepts helpful. We pride ourselves with industry leading CCNA training courses and we hope you will make great use of the information found here while taking your CCNA exam.
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