What is VLAN trunking?
Virtual Local Area Networks (VLANs) can span several switches and other network devices, such routers or servers, using a single physical link to a critical networking feature called VLAN tripping. Traffic from several VLANs is routed through this one link, also referred to as a trunk link or trunk port.
VLAN trunking = combining multiple VLANs over a single physical link using tagging.
Why VLAN Trunking is Needed
The main objective of VLAN trunking is to extend VLANs beyond physical boundaries throughout a network. Without trunking, you would require a different physical connection for each VLAN between switches if you wanted to extend VLANs across several of them (for example, VLAN 10 for HR, Finance VLAN 20, IT VLAN 30). Particularly in contemporary enterprise networks with tens or even hundreds of VLANs, this design is extremely inefficient, expensive, and scalable. By enabling a single cable to transport all VLAN traffic, trunking addresses this issue. Additionally, communication between devices on various switches that are part of the same VLAN is crucial.
How VLAN Trunking Works
It is necessary to include information about the VLAN a frame belongs to when sending it over a trunk link. This procedure is known as frame tagging or VLAN tagging. The following actions are taken by the transmitting switch:
Single Link, Multiple VLANs: Trunking enables a single physical link between two switches to transport traffic for numerous VLANs, eliminating the requirement for distinct physical links for each VLAN.
Frame Tagging: The switch appends a VLAN tag to the Ethernet frame whenever traffic from a VLAN leaves a switch port that is a part of a trunk link.
VLAN ID: The 12-bit VLAN ID contained in this tag designates the particular VLAN to which the frame belongs.
Adding and removing tags: The tag is appended prior to the frame being transmitted via the trunk link and eliminated upon arrival at the destination switch.
VLAN Segregation: After identifying the frame’s VLAN from the tag, the destination switch forwards it to the relevant VLAN segment.
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Important Ideas and Protocols
VLAN trunking relies on a number of fundamental ideas and protocols:
Trunk Ports
Switch ports that are specifically set up to transport traffic for several VLANs are known as trunk ports. They send and receive tagged Ethernet frames and are commonly used for connections between switches or between a switch and a router.
Access Ports
Unlike trunk ports, access ports are set up to transport data for a single VLAN. When this data is transmitted to an endpoint, it is not tagged.
VLAN Tagging Protocols
The most widely used and industry-standard protocol for VLAN tagging is IEEE 802.1Q (Dot1Q). After the Source Address field, it appends a 4-byte (32-bit) tag to the original Ethernet frame. There are up to 4,094 functional VLANs (out of 4,096 available IDs, since 0 and 4095 are reserved) to the 802.1Q tag’s 12-bit VLAN ID (VID). It also has a 1-bit Canonical Format Identifier (CFI) and a 3-bit User Priority field for Quality of Service (QoS). The majority of modern switches support and default to 802.1Q. After the 802.1Q header is introduced, the frame size changes, requiring a recalculation of the Frame Check Sequence (FCS).
Inter-Switch Link (ISL)
The Cisco-proprietary Inter-Switch Link (ISL) trunking protocol is outdated and typically no longer utilised in contemporary switches. In contrast to 802.1Q, ISL supports up to 1,000 VLANs and wraps the entire Ethernet frame with a new header and trailer.
Native VLAN
The native VLAN in 802.1Q trunking is a unique VLAN in which frames are usually transmitted via the trunk link without tags. A switch will presume that an untagged frame on an 802.1Q trunk port is part of the native VLAN. On many switches, VLAN 1 is the native VLAN by default. Changing the native VLAN from VLAN 1 to an unused or “dummy” VLAN is strongly advised for security reasons, and tagging native VLAN traffic is regarded as best practice. Frames may “jump” across VLANs on a trunk if the native VLANs are mismatched.
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VLAN Trunking Protocol (VTP)
Cisco switches inside a VTP domain exchange VLAN configuration data (VLAN ID and name) via the VLAN Trunking system (VTP), a messaging system that is proprietary to Cisco. VTP makes administration easier and helps avoid configuration discrepancies by using 802.1Q trunks to spread VLAN definitions throughout a network of switches. There are three modes of operation for VTP: Server, Client, and Transparent. However, many businesses decide to disable it because of the security dangers and the potential for configuration problems if not handled appropriately. It’s crucial to remember that VLANs are kept in the local VLAN database of every switch and have local significance.
Dynamic Trunking Protocol (DTP)
Another Cisco-proprietary protocol that has the ability to dynamically negotiate the trunking status and encapsulation type (802.1Q) of trunk ports is called Dynamic Trunking Protocol (DTP). Through message exchange, DTP assists switches in automatically determining whether they are in access or trunk mode. Cisco switch ports are in dynamic auto mode by default, passively awaiting trunk negotiation from the other party. At least one side of the link needs to be actively set up to negotiate, such as switchport mode dynamic desirable or switchport mode trunk desired, in order to form a trunk. DTP only functions on switch-to-switch links and between Cisco devices. While negotiating a non-trunk link, a port can still send DTP messages out even if it is set up for switchport mode access.
Advantages of VLAN Trunking
VLAN trunking provides a number of noteworthy benefits for network administration and architecture, including:
Scalability: VLANs can spread throughout the network without a separate physical link for each VLAN, allowing logical network segmentation without the need for additional physical cable as the network expands.
Flexibility: Regardless of a user’s physical location, network managers can assign them to various VLANs and logically divide a network into departments or workgroups.
Efficiency: It makes better use of physical ports and cabling by drastically lowering the number of physical connections required, which makes network management and growth easier.
Inter-Switch Communication: It gives devices connected to various switches but in the same VLAN the necessary means of communication.
Troubleshooting Trunking
Typical trunking problems include:
- Mismatched Trunking Operational States: Tagged frames may be deleted if one switch is set up for trunking while the other is not.
- Mismatched Native VLAN: Frames sent in the native VLAN may “jump” to another VLAN if the native VLAN IDs on each end of a trunk do not match.
- Inaccurate Allowed VLANs: Unlisted VLAN traffic will not flow if the list of VLANs allowed on a trunk link is not identical on both ends.
- Trunk not formed: Verify that the encapsulation techniques (such as 802.1Q) are compatible on both sides and that the interface is operational (up/up).
- Disabled VLANs: In order for traffic to flow via a switch, a VLAN needs to be defined and active.
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