Understanding VLAN Trunking

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.

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