The term “trunking” is frequently used in the telecommunications and IT industries. Without depending on point-to-point connections, it explains a network configuration that effectively transfers data between several entities. A network trunk transmits several streams of signals to their intended locations, just like a tree trunk delivers water to its branches and leaves. VLAN trunking frequently refers to either link aggregation or Virtual Local Area Network (VLAN) trunking for managed service providers (MSPs), both of which are necessary for VLAN configurations. IP trunking specifically refers to Voice over Internet Protocol (VoIP) services, which some MSP clients may find very pertinent.
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What does trunk mean in networking?
Multiple entities on one end can connect to the relevant entity on the other end through a trunk, which acts as a single communication channel. It serves as a “link” that can transmit many signals at once, making network access between two nodes more effective. Trunking is essential to computer networking even though it is mainly related to telecommunications, where it is used to connect switching centers and provide various signal connections.
VLAN trunking
Understanding the function of VLANs in a network is crucial to comprehending VLAN trunking. Traditional hubs were replaced by VLANs to solve some of the issues with switched networks. Switches can produce a flat topology that can result in congestion and redundancy problems, even while they offer better control through increased throughput and fewer conflicts. A workable answer to these issues is provided by VLANs. Subnets within a network can link thanks to a VLAN (Virtual Local Area Network). Without physically altering the network infrastructure, a single switched network can be configured to better satisfy system needs by putting VLANs into place. Switches can be assigned to VLANs by Managed Service Providers (MSPs), forming logical groupings that divide communication. Network switches facilitate the establishment of Layer 2 subnets and support VLANs. In practice, this configuration not only stops some devices from communicating with one another, but it also makes it possible for other devices to communicate more effectively.
We can use VLAN trunking, another Ethernet technology, to get around the scale issues we talked about. With this method, switches are connected by a single link that can accommodate several VLANs if necessary. Furthermore, it guarantees that VLAN traffic stays distinct, so devices in VLAN 10 won’t receive frames from VLAN 20 and vice versa. Figure 3 shows an illustration of this configuration, with the connection between Switches 1 and 2 identified as a trunk link. Under these conditions, VLAN 10 and VLAN 20 can both move freely across the link.
What is IP trunking?
A transmission technique called IP trunking was created for the widespread use of Voice over Internet Protocol (VoIP). VoIP makes it possible for voice calls to be sent over the internet as data packets as opposed to electrical impulses, as is the case with conventional landlines. Effective data management is essential when a company chooses to use VoIP, particularly since there is usually just one link connecting the internal and external networks. Most people do not need IP trunking, even if many use VoIP for private purposes. To manage several call lines and the different VoIP services, such voicemail and call waiting, businesses and larger organizations do require trunking systems. For many businesses, the ability to handle several calls at once is crucial, and trunking makes this possible. All corporate users, both in-office and remote, are connected via the same VoIP trunk, which enables signals to be sent over a single trunk link and guarantees that every call reaches its intended recipient.
The technique used by service providers to send and receive switches for every call is known as IP trunking. The Private Branch Exchange (PBX) is the typical trunking arrangement. Voices are transformed into IP packets in this configuration, and the PBX system oversees addressing and forwarding these packets to the other caller, or the receiving endpoint. The caller’s unique IP address is used for this transmission process. VoIP is still important for many managed service providers (MSPs), even though IP trunking may take place on the backend. It’s crucial to comprehend how IP trunking can ease the transfer if your clients are thinking about moving their phone services to a data network. Additionally, to improve a customer’s network performance, you might wish to investigate VLAN trunking.
Multiswitch broadcast domains
We covered the process of forwarding a broadcast frame received on any switch port to every other port on that switch in the previous course. In light of this, if we connect two switches with their default configurations, as shown in Figure 1, any broadcast frame that any switch receives will be sent to the other switch, which will then send it out of all of its ports. This indicates that a broadcast domain is not limited to a single switch; rather, it includes any devices—regardless of whether they are linked to separate switches—that get a copy of any broadcast frame.A broadcast domain with hundreds of end devices might be created by applying this idea to a local area network (LAN) with many connected switches. This may eventually cause the network to become overloaded with broadcast, unknown, and multicast (BUM) traffic, which could make the LAN unusable. As a result, in big topologies with interconnected switches, it becomes more crucial to divide a single broadcast domain into several smaller ones.
VLAN on multiple switches
We may use Virtual LANs (VLANs) to partition the switch topology into several broadcast domains by putting the concepts covered in the previous lecture to use. There are several ways to accomplish this, but let’s start with the most straightforward one. This entails setting both switches’ ports 1 through 4 to VLAN 10 and ports 5 through 9 to VLAN 20. Despite being functional, this architecture is not scalable. Each VLAN requires a physical connection between the switches to use this strategy. If the topology calls for more than ten VLANs, then more than ten physical cables would need to be run between the switches, and each switch would need to have more than ten switch ports used for those connections. It is obvious that topologies with a small number of VLANs can use this architecture. On a broader scale, nevertheless, this method of spanning VLANs between switches is impractical in today’s contemporary enterprise networks, which frequently have several VLANs.
Trunking protocols
Cisco switches have used IEEE 802.1Q and Inter-Switch Link (ISL) as trunking protocols over the years. As a forerunner of 802.1Q, Cisco created the proprietary tagging protocol known as ISL, which is now outdated and no longer in use. IEEE 802.1Q is now the sole protocol supported by most contemporary switches and is acknowledged as the industry standard for trunking encapsulation. It’s crucial to remember that the tagging procedure expands the Ethernet header of the frames by 4 bytes. The VLAN ID, which is 12 bits long and identifies the VLAN to which the frame belongs, is the most important component of this tag. There are 4,094 different VLAN numbers that can be used because 0x000 and 0xFFF are reserved.
VLAN Tagging
Switches can forward frames from several VLANs via a single link called a trunk thanks to VLAN trunking. This is accomplished by adding extra header data—known as a tag—to the Ethernet frame. VLAN tagging is the process of inserting this little header. Take the situation where an end station is sending a broadcast frame, as shown in Figure 4. Switch 1 must send this frame out of all its ports since it interprets it as a broadcast. But Switch 1 needs to let Switch 2 know that this frame is part of VLAN 10.
Switch 1 adds a VLAN header identifying VLAN number 10 to the original Ethernet frame before forwarding it to Switch 2. Switch 2 notices that the frame is labelled with VLAN 10 when it gets it. After that, it forwards the original Ethernet frame to every interface set up for VLAN 10 after removing the VLAN header. In this case, the VLAN header is carried by Ethernet frames that are sent between switches via the trunk link. Untagged frames are sent when the receiving switch removes the VLAN tag after receiving the frames and distributes them to the clients inside the VLAN.
Switch interface modes
Any switch interface can serve as a trunk port or an access port. A technique called Dynamic trunking technique (DTP) helps network managers automatically configure the operational mode of interfaces in typical LAN deployments, where there may be hundreds or even thousands of switch ports. Every Cisco switch port is in a dynamic auto mode by default. This enables DTP to decide whether to establish the port as an access or trunk port by listening to and evaluating the configuration on the other end of the connection. For example, DTP will inform the other side if a link is present between Switch 1 (SW1) and Switch 2 (SW2) and the interface on SW1 is set up to function as a trunk port. As a result, a trunk link will be created between the two switches when SW2’s interface automatically switches to trunk mode. The switches’ ability to build a trunk link depends on the setups on both sides.
Configuring Trunk ports
As previously stated, dynamic auto is the default option for Cisco switch ports. As a result, only one end of the connection must be set up to actively negotiate the trunking to create a trunk link. For instance, we can configure Switch 1’s (SW1) interface Gi0/1 to actively try to build a trunk. Even if no configuration has been applied on the other end of the link on Switch 2 (SW2), we may verify that a trunk link has been established by using the command display interface trunk. This illustrates how the Dynamic Trunking Protocol (DTP) works. It’s crucial to observe that SW2’s interface is in operational mode auto, meaning it is awaiting SW1’s trunk negotiation.
Summary
- Locally important VLANs are kept in the local VLAN database of the switch.
- Tag frames with VLAN identification are connected to trunk links.
- Cisco switches now employ IEEE 802.1Q as their standard trunking technology; ISL, an earlier technique, is no longer in use due to deprecation.
- Trunk link negotiation is possible with Dynamic Trunking Protocol (DTP).
- Both switches must be set up to support trunking to create a trunk link between them.
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