Understanding VLAN Trunking

Trunking, a term frequently used in IT and telecommunications, refers to a network configuration that efficiently conveys data between multiple entities without using one-to-one links. Similar to how a tree trunk carries water to every branch and leaf, a network trunk essentially carries multiple streams of signals to the correct locations. For managed services providers (MSPs), trunking in networking will typically relate either to link aggregation or virtual local area network (VLAN) trunking, a practice that is integral to VLAN configuration. IP trunking refers specifically to Voice over Internet Protocol (VoIP) services, which may be relevant to some MSP customers as well.

What does trunk mean in networking? 

A trunk is a single channel of communication that allows multiple entities at one end to correspond with the correct entity at the other end. It is a “link” that carries many signals at the same time, creating more efficient network access between two nodes. Trunking is perhaps best known in reference to telecommunications, where the method is used to connect switching centers and create multiple-signal links. But trunking is also important in computer networking, where it has two main advantages:

Link aggregation

Sometimes, trunking refers specifically to port trunking, also known as link aggregation or Ethernet bonding. This is a method of combining individual Ethernet links to act as one logical link, potentially making it possible to overcome certain bandwidth limitations. This allows you to connect several switches with Ethernet ports and allocate traffic among all the ports, relieving switch-switch or switch-to-server congestion. If failover occurs, link aggregration enables faster recovery, as the signals simply use another port while still routing to the correct endpoint. As a result, this simultaneous packet transmission may result in improved performance. The IEEE standard for link aggregation is referred to as 802.1AX.

 

VLAN trunking

What is VLAN trunking? To answer this question, it’s important to understand why networks have VLANs. VLANs came into use in part to mitigate some difficulties with switched networks, which replaced hubs. Switches offer enhanced control compared to hubs, including increased throughput, reduced collisions, and more. However, these switched networks have a flat topology that can create some congestion and redundancy issues. VLANs offer a solution.

A VLAN is a way to provide connectivity for subnets on a network. With a VLAN, it’s possible to configure a single switched network to better suit system requirements without making physical network changes. MSPs can assign switches to VLANs and create logical groups to partition communication. Network switches support VLANs and create a Layer 2 subnet implementation. On a practical level, this both prevents certain devices from interacting and allows others to connect more efficiently.

Why is trunking important to VLAN configuration? 

With VLAN trunking, it’s possible to extend a VLAN across the network. When you implement multiple VLANs across a network, trunk links are necessary to ensure that VLAN signals remain properly segregated for each to reach their intended destination. This is also more efficient, as multiple VLANs can be configured on a single port.

With these multiple VLANs on a single port, the system has to have a way of distinguishing and routing signals. A trunking VLAN port always uses identifying tags to mark frames as they pass between switches. The most common trunking protocol, IEEE 802.1Q, adds a tag to the Ethernet frame as it passes through, marking it as belonging to a specific VLAN. This tag, which includes the Media Access Control (MAC) address, ultimately helps route the Ethernet frame to the correct endpoint as it passes across the trunked link and the host port.

IEEE 802.1Q is an open standard protocol, and typically the best (and only) option if you’re using equipment from multiple vendors. The Cisco VLAN Trunking Protocol (VTP) is a proprietary protocol, and can potentially be useful—for instance, when you configure a new VLAN on a VTP server, it automatically distributes it across switches, reducing configuration time.

Some maintenance is necessary for VLAN trunking—for example, the switch-forward tables must stay up-to-date in case of network topology changes. This means continually refreshing the tables to allow new changes to emerge and old forwarding frames to be cleared out. It’s worth noting, too, that there are two types of possible trunking here: symmetrical and asymmetrical trunking. Symmetrical trunking allows any port within a group to transmit packets to any other port, allowing for higher data transmission and reception rates. Asymmetrical trunking allows only one port on a switch to receive packets, so transmission rates are high while data reception is slower.

What is the difference between an access port and a trunk port? 

Essentially, access ports handle traffic for one VLAN, while trunk ports are equipped to route traffic to and from multiple VLANs using a tagging protocol. Here is the basic difference between a trunk port vs. an access port:

Access ports

An access port is a connection on a switch that transmits data to and from a specific VLAN. Because an access port is only assigned to a single VLAN, it sends and receives frames that aren’t tagged and only have the access VLAN value. This doesn’t cause signal issues because the frames remain within the same VLAN. If it does happen to receive a tagged packet, it will simply avoid it. This is a simpler configuration, but not the most efficient choice if the network is even moderately complex.

Trunk ports

Unlike an access port, a trunk port can transmit data from multiple VLANs. If you have a dozen VLANs on a particular switch, you don’t need additional cables or switches for each VLAN—just that single link. A trunk port allows you to send all those signals for each switch or router across a single trunk link. In contrast to an access port, a trunk port must use tagging in order to allow signals to get to the correct endpoint. Trunk ports typically offer higher bandwidth and lower latency than access ports.

What is IP trunking?

IP trunking is a transmission method for the large-scale use of Voice over Internet Protocol (VoIP). VoIP involves sending voices over the internet as data, rather than as electrical signals (as with traditional telephone landlines). When an organization wants to use VoIP, the data must be handled properly—especially because the connection between internal and external networks typically consists of just one link.

While VoIP is used by millions of individuals for personal reasons, few of those individuals need IP trunking. However, businesses and larger organizations do require trunking, because they need a way to handle multiple call lines and the many services like call waiting and voicemail that are associated with VoIP. Trunking allows multiple calls to go through simultaneously, which is a must for many organizations. Although the same VoIP trunk connects all business users (both in-office and remote), the signals are sent over a single trunking link that allows each call to reach the correct destination.

IP trunking means the service provider utilizes trunking to both send and receive switches for each call. The typical trunking configuration is the Privacy Branch Exchange (PBX). As the voices are digitized into IP packets, the PBX system can address and route the packets to the receiving endpoint—that is, to the other caller. This transmission process occurs over the caller’s unique IP address.

IP trunking may happen on the backend, but VoIP is still relevant for many MSPs. If your customers might be interested in moving their phone service to the data network, it’s worth understanding how IP trunking can make that possible. In other cases, you may wish to utilize VLAN trunking in order to optimize a customer’s network. When managing these many moving parts, consider taking advantage of SolarWinds® RMM [https://www.solarwindsmsp.com/products/rmm] to utilize a comprehensive set of remote monitoring and management tools in a single dashboard.

For more networking tips and explanations check out our blog. Also read more about our IP Blacklist Prevention Solution.

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