Types of VLANs

There are different types of VLANs used in networking. Some VLAN is defined by classes of traffic and some other are defined by the specific function that they serve. Each switch has a default VLAN.

Default VLAN

VLAN-1 is the default VLAN in Cisco switches. After initial boot up process, the switch loads the default configuration and all switch ports became a part of the default VLAN (VLAN-1). The switch port that is the part of the default VLAN work in the same broadcast domain.  The figure below illustrates the default VLAN of a Cisco switch, the show VLAN brief command was executed on a switch running the default configuration. You can see that all ports are assigned to VLAN 1 by default. There is no difference between the features and function of VLAN1 and other VLAN; excluding that it cannot be renamed or deleted. By default, all Layer 2 control traffic is associated with VLAN 1.

Data VLAN

A data VLAN is also referred to as a user VLAN. It is used to separate the network into different groups of users or devices. Data VLAN is used to forward user-generated traffic. It is also separate voice and management traffic from data traffic.

Native VLAN

A native VLAN is assigned to an 802.1Q trunk port that was created for backward compatibility with old devices that don’t support VLANs just like a hub. Frames belonging to the native VLAN are not tagged when sent out on the trunk links so older devices can simply understand these frames. Frames received untagged on the trunk links are set to the native VLAN. The trunk is the links between switches that maintain the transmission of traffic connected with more than one VLAN. An 802.1Q trunk port supports traffic coming from many VLANs (tagged traffic); with traffic that does not come from a VLAN. Tagged traffic is traffic that has a 4-byte tag inserted in the original Ethernet frame header; specifying the VLAN to which the frame belongs. The 802.1Q trunk port places untagged traffic on the native VLAN, which by default is VLAN 1.

Management VLAN

The separate VLAN for management like monitoring, system logging, SNMP, and other sensitive management jobs is best practice in networking. It also ensures that bandwidth for management will also be available even when user traffic is high. VLAN 1 is the management VLAN by default. To create the management VLAN, the switch virtual interface of that VLAN is assigned an IP address and subnet mask, which management remotely via HTTP, Telnet, SSH, or SNMP. Because the out-of-the-box configuration of a Cisco switch has VLAN 1 as the default VLAN, VLAN 1 would be a bad choice for the management VLAN.

If your organization uses voice over IP (VoIP), a separate VLAN is needed. This will save bandwidth for other applications and ensure VoIP quality. The Voice Over  Internet Protocol (VoIP) traffic requires, assured bandwidth to ensure quality, transmission priority, ability to be routed around congested areas on the network and delay of less than 150ms across the network. To meet these requirements, the entire network has to be designed to support VoIP.

Router File System

Benefits of VLANs

VLANs make it simple to plan a network to maintain the goals of an organization. The most important benefits of  VLANs are as follows:

Security

Security in one of the primary benefits of VLANs. it makes possible that the hosts that have sensitive data are separated from the rest of the network. The separation decreases the possibility of confidential information breaches. The figure below illustrates that management, sells and IT section are totally separate from each other, so they can’t access each other’s files. 

Cost

VLANs reduce the cost of the network and make possible of more efficient use of existing hardware and bandwidth because segmenting a network into smaller VLAN is cheaper than creating a routed network with routers.

Better Network Performance 

Dividing flat Layer 2 networks into multiple broadcast domains reduces unnecessary traffic on the network and help increase network performance. VLANs manage traffic very efficiently so that your end users experience better performance. The administrator will have a smaller amount of latency problems on the network and more reliability for important applications.

Shrink Broadcast Domains 

Division of large network into smaller VLANs reduce devices in the broadcast domain. As shown in the figure above, total nine hosts working in this network but only three hosts are in broadcast domains.

Improved IT staff efficiency 

VLANs make network management very easy for IT staff because users with related network requirements share the same VLAN.

Simpler Project and Application Management 

VLANs combined users and network devices to support both business and geographic requirements.

Simplified Administration for the Network Manager

VLAN simplifies network management. Grouping of users into the virtual networks, make it easy to set up and control network policies at a group level.

Easy Troubleshooting

Network troubleshooting can be simpler and faster because of different user groups are segmented and isolated from one another. If the network administrator knows that complaints are only coming from a different subset of users, the network administrator will be able to quickly narrow down where to look to find the issue.

Virtual Local Area Network (VLANs)

One of the technologies that improve network performance is the separation of large broadcast domains into smaller ones. By design, routers block broadcast traffic. But, routers usually have a limited number of LAN interfaces. A router’s main role is to move data between different networks. The router does not provide network access to end devices. The access layer switches are responsible for connectivity to end devices. The VLAN on Layer 2 switches reduce the size of broadcast domains. VLANs are incorporated into network design making it easier for a network to support the goals of an organization. Mostly VLAN is used within switched LANs.

The VLAN create a logical broadcast domain that consists of different physical LAN segments. it improves network performance by separating large broadcast domains into smaller broadcast domains. If a device in one VLAN sends a broadcast data, all devices in that VLAN receive the data, but devices in other VLANs do not.

What is VLANs

Virtual Local Area Network provides segmentation within a switched network. It is a technique that group devices within a LAN. A group of devices within a VLAN communicate like they are connected to the same wire. VLANs are based on logical connections, in place of physical connections. it permits a network administrator to divide larger networks based on factors like function, project team, or application, without hold for the physical location of the user or device.

Devices inside a VLAN work like they are in their own independent network; though they share a common infrastructure with other VLANs. Any switch port can belong to any VLAN, and unicast; broadcast and multicast packets are forwarded and flooded only to end station within the VLANs. Every VLAN is work like a separate logical network; and data destined for stations, not in the VLAN must be forwarded through a router or device that supports routing. VLANs make possible the implementation of access and security policies to particular groupings of users. Each switch port can be assigned to only one VLAN except some cases. The figure below illustrates the layout of VLANs.

 

Cisco Borderless Networks

Collision Domains and Broadcast Domains

Collision Domains

In hub-based Ethernet segments, network devices contribute to getting the medium, because devices must take turns when transmitting. The segment of the network that shares the same bandwidth between devices is known as collision domains, in other words, a collision domain is the element of a network where collisions of packets can occur. A collision can occur when two or more devices send a packet at the same time on the shared segment.  When the packets collide, both devices must send the packets again, which reduces network efficiency. Usually, collisions occur in a hub environment, as each port on a hub is in the same collision domain. But each port on a bridge, switch and a router is in separate collision domains.

However, the switch can divide a network into segments and which reduce the number of devices that contribute to bandwidth.  Each port of the switch represents a new segment and each new segment is a new collision domain. Which provide more bandwidth to the devices on the segment, and one collision domain does not interfere with other collision domain. The figure below illustrates the collision and broadcast domain.

Broadcast Domains

A broadcast domain is a domain where the sending device sends a single copy of data and that copy of data will be delivered to every device in the network segment. A broadcast domain consists of devices that can reach each other at the data link layer by using broadcast. Each port on a hub and switch are by default in the same broadcast domain. broadcast domains. Although, switches filter frames based on MAC addresses, they do not filter broadcast frames. Switch forward must flood broadcast to all ports for other switches. If a collection of two or more switches forms as a single broadcast domain.

Network layer devices, such as a router, can feature to divide a Layer 2 broadcast domain, so each port of the router are used to segment both collision and broadcast domains. When the device sends layer 2 broadcast, the destination MAC address in the frame should be all F,s. A frame containing all F in the destination field is received by all devices in the broadcast domain.

When a switch in the network segment receives a broadcast frame, it floods the frame out to each of its ports, except the ingress port. All devices connected to the switch receive a copy of the broadcast frame and processes it. Broadcasts are an important part of the network, therefore we can’t avoid it. Broadcasts are used to initially locate other devices and network services. Many protocols like ARP and DHCP are dependent on Broadcast to function.

The broadcasts reduce the network efficiency because network bandwidth is used to propagate the broadcast traffic. A lot of broadcasts and a heavy traffic load on a network can cause in congestion: a slow-down in the network performance. When two or more switches are connected together, the broadcast domain is increased. The figure below illustrates the broadcast domains.

Data Frames Forwarding and Switching Method

With network growing, its facing slower network performance, Ethernet bridges were added to networks to limit the size of the collision domains. The advancement in integrated circuits permitted for LAN switches to replace the early bridges. The modern switches were able to move the layer 2 forwarding decision from software to application-specific-integrated-circuits (ASICs). The ASICs decrease the packet-handling time within the device and permit the device to handle an increased number of ports without degrading performance.  There are two methods of data switching and forwarding:-

• Store-and-forward method


• Cut-through Method

Store-and-Forward Switching

The store-and-forward method makes a forwarding decision when the complete frame received and then checked the frame for errors using a mathematical error-checking mechanism known as a cyclic redundancy check (CRC). if the CRC is valid, the switch looks up the destination address, which determines the outgoing interface. The frame is then forwarded out the correct port.

The Store-and-forward method has two primary characteristics that differentiate it from cut-through:

• Error checking


• Automatic buffering.

Error Checking

A switch using store-and-forward switching technique performs an error check on each incoming frame. When received the entire frame on the ingress port, as shown in the figure, the switch compares the frame-check-sequence (FCS) value in the last field of the datagram against its own FCS calculations. The FCS is the process that helps to make sure that the frame is free of physical and data-link layer errors. If the frame has no error, the switch forwards the frame to the destination; otherwise, the frame is dropped.

 

Automatic Buffering

With any difference in data speeds between the ingress and egress ports, the switch stores the whole frame in a buffer computes the FCS check; forwards it to the egress port buffer and then sends it. For example, when an incoming frame traveling into a Fast Ethernet port that must be sent out a Giga Ethernet interface would require using the store-and-forward method. The store-and-forward switching is the primary method for Cisco switches.

Cut-Through Switching

The Cut-Through Switching is another method of switching, as shown in Figure 2; this method starts the forwarding process when the destination MAC address of an incoming frame and the egress port has been determined. The advantage to this method is the ability to switch data earlier than store-and-forward method. The primary characteristics of cut-through switching are the following:

• Rapid Frame Switching


• Fragment Free

 

Rapid Frame Forwarding

A switch using the cut-through method make forwarding immediately when it has found the destination MAC address of the frame in its MAC address table. The switch doesn’t need to wait for the complete frame to receive like store-and-forward method.

A switch using the cut-through method can quickly decide because of ASICs and MAC controller. The cut-through method needs to check a larger portion of a frame’s headers for additional filtering purposes. For example, the switch can examine source MAC address; destination MAC, and the Ether Type fields which are total 14 bytes and also examine an additional 40 bytes in order to carry out more difficult functions Layers 3 and 4.

This method does not drop invalid frames. The frames with errors are forwarded next segments of the network. If there are too much invalid frames in the network, which produce a negative impact on bandwidth.

Fragment Free Switching

It is a modified form of a cut-through switching. In this form of switching the switch waits for the collision window (64 bytes) to pass before forwarding the frame. Each frame will be checked into the data field to make sure there is no fragmentation has occurred. This is provides enhanced error checking than cut-through; without any further latency and delay. The lower latency speed of cut-through switching makes it more suitable for high-performance computing (HPC) applications that need process-to-process latencies of 10 microseconds or less.