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.

Switching Concept in Networking and Telecommunications

The switch frame forwarding is common in networking and telecommunication. Different types of switches are used in networking(LAN, WAN, and PSTN). The basic concept of switching decision is based on the following two criteria.

• Ingress port


• Destination address

The traffic forwarding decision is completed in relation to the traffic flow. The term ingress is used to explain where a frame enters to the device on a particular port. The term egress is used to describe the frames leaving the device from the particular port. When a switch makes a frame forwarding decision, it is based on the ingress port and the address of the destination host.  The networking switch maintains a MAC address table. The table is used to decide how to forward traffic through the switch. The figure below illustrates the switch MAC address table.

• If a data enters to switch port 1 and the message has a destination address of DB-CD-AC-3D-26-25, then the switch forwards the data out to port 7.


• If a data enters to switch port 6 and the message has a destination address of C3-A3-A2-35-A6-66, then the switch forwards the data out port 8.


• If a data enters switch port 2 and has a destination address of EE-01-A1-AF-00-01, then the switch forwards the data out port 1.

The network switch has only one intelligence, the ability to maintain MAC address and table make a decision to forward traffic based on the ingress port and the destination address of the message. Every switch has only one master table that describes an association between address and port. The Cisco switches forward Ethernet frames based on the destination MAC address of the frames.

The concept of Dynamically Populating a MAC Address Table

Network Switches use MAC address table to forward network traffic towards destination through the proper port. It is necessary for a switch to know which port to use to transmit data. The switch must first learn which devices exist on each port. When the switch learns the devices connected to each port, its prepare MAC address. The table also called content addressable memory (CAM) table. The CAM is a memory which is used in high-speed searching applications. The switches decide how to handle incoming data frames by maintaining the table of MAC addresses. The switch builds MAC address table by reading the MAC address of each device connected to the port of a switch and then uses MAC address table to send frames destined for a particular out the specific port which has been assigned to that device. The switch populates the table of the MAC address based on the source MAC addresses.

When a switch receives an incoming frame with a destination MAC address, if the destination MAC address not available in the table, the switch forwards the frame out of all ports except for the ingress port of the frame, this technique called flooding of data.  When destination device responds, the switch adds the source MAC address of the frame and the port where the just received to the MAC address table. Network, where multiple switches are interconnected, The MAC address table contains multiple addresses for a single port. The switch kept MAC address for a specific time, the aging time is five minutes for a single host entry. The steps below describe the process of the building MAC address table.   

The following steps describe the process of building the MAC address table:

• The switch receives a frame from Host 1 on Port 1, the MAC address table is empty (Figure 1).

• The switch examines the MAC address of the source (Host 1) and compares it to the MAC address table.


• If the address is not found in the MAC address table, it associates the MAC address of host 1 with port 1 (ingress port) in the MAC address table. (Figure 2)

• If the MAC address already exists in the table, it resets the aging timer for an entry.


• when the switch has recorded the source address information in the MAC table then switch examines the destination MAC address.


• If the destination address is not already recorded in the MAC table or if it’s a broadcast MAC address containing all Fs, the switch floods the data frame to all ports, except the ingress port (Figure 3).

• The destination Host (Host 4) will reply to the frame with a unicast frame addressed to Host 1 (Figure 4). The switch enters the source MAC address of Host 4 and the port number of the ingress port into the address table. The destination address (Host 1) of the frame and its associated egress ports found in the MAC address table.

• The switch now complete entries for host 1 and host 4 and they can now forward frames between these source and destination devices without flooding. 

Role of Switched Networks

A few years ago the flat Layer 2 switched networks were the standard. Flat Layer 2 data networks work on the basic principles of Ethernet and the common use of hub and repeaters to broadcast LAN traffic throughout a campus. As shown in Figure 1, networks have primarily changed to switched LANs in a hierarchical network. A switched LAN allows traffic management, additional flexibility, and extra features, such as:

• QoS


• Wireless network connectivity and support


• IP telephony and mobility services


• Additional security

Form Factors

Different types of switches are used in a business network. Considering common business requirement the appropriate switched equipment selection is a very important factor.  Selecting the type of switch the network administrator and designer must select a modular or fixed configuration and stackable or non-stackable. The physical installation is also an important consideration of the switch. The thickness of the switch decides where to install the switch. The switch can be installed in a rack or in a cabinet. These options are sometimes referred to as switch form factors, which I am going to explain below:-

Fixed Configuration Switches

Fixed Configuration switches are switches with a fixed number of ports and are normally not expandable. These switches do not support features or options further than those that originally came with the switch. The switch model determines the features and options available. Such as, if you have a 16-port gigabit fixed switch, you cannot add additional ports when you need them. This category is discussed in further detail below. Cisco Catalyst 2K, 3K, and the Cisco 300/500 series are good examples of Fixed Configuration switches. The Fixed configuration switch category is further broken down into:

• Unmanaged Switches


• Smart Switches


• Managed L2 and L3 Switches

Modular Configuration Switches

Modular switches are expendable switches comes with different sized chassis which allow a different number of modular line cards inside the chassis. The modular switches offer more flexibility in their configuration. If you have a 24 port modular switch you can easily expand the switch to 48 port.  

Stackable Configuration Switches

A stackable switch is a switch that is completely functional operating standalone but which can also be set up to operate jointly with one or more other switches in a group. This group of switches will show the characteristics of a single switch but have the port capacity of the sum of all switches. Stackable switches can be interconnected using a special cable that provides high-bandwidth throughput between the switches. The Cisco Stack Wise Technology allows the interconnection of nine switches in the group. These stacked switches effectively operate as a single larger switch.

Cisco Borderless Network

With the rising demands of the converged network, the recent developments in network design are the Cisco Borderless Network. The Cisco Borderless Network is a network design that allows organizations to support a borderless network that can connect any person, everywhere on any device very securely and consistently. This design address IT and business challenges.

The Cisco Borderless Network structure merge wired and wireless access, as well as access control, policy; and performance management of different device types. Using the Cisco Borderless Network architecture is built on a hierarchical infrastructure of hardware that is scalable and flexible, the Figure below illustrates the Cisco Borderless Network which provides two primary sets of services: network services and endpoint services; all services managed by an integrated management solution. This design enables different network elements to work mutually and also allow to access resources from any place at any time; as providing optimization, scalability as well as security.

Hierarchy in the Cisco Borderless Network

Availability, flexibility, security, and manageability is the primary requirement of the borderless switched network. The borderless switched network should deliver current and future requirement. The basic principles of the Borderless switched network are the following:

• hierarchical network Design


• Modularity


• Resiliency


• Flexibility

Understanding the principle to fits in the different situation is very important. The hierarchical borderless 

switched network is providing a base for the network designer to cover security, mobility, and unified communication features. Cisco designs three-tier and two-tier hierarchical network for a campus. The figure below illustrates the Cisco hierarchical network design. The three-tier of this design are the access, distribution, and core layers.  Every layer of this design can be seen as a clear structured module with particular roles and functions in the campus network.

The campus hierarchical network design which containing modularity provide critical network services with resilient and flexible.  The modularity also provide assistance for increase and change that occurs over time

Access, Distribution, and Core Layers

Access Layer

The access layer is the place where traffic enters or exits the campus network. It is the edge of the campus network. Usually, the main function of an access layer is to provide network connectivity and access to end users.  The switches of the access layer connect to switches of the distribution layer. The switches in the distribution layer apply network foundation technologies such as routing, QoS, and security.

To meet user requirement and network application, the next-generation switching platforms currently present extra converged integrated; and quick services to different types of endpoints at the edge of the network. Creating intelligence into the switches of an access layer allows applications to function on the network more capable and securely.

 Distribution Layer

The distribution layer provides connectivity between access layer and core layer with many important functions. This layer provides intelligent switching and routing. The distribution layer provides network access policy functions to access the whole network as long as differentiated services to different classes of service applications at the edge of the network. This layer providing high ease of use throughout redundant distribution layer switches to the end-user and equal cost paths to the core layer. The distribution layer also aggregate large-scale wiring closet networks and aggregate Layer 2 broadcast domains and Layer 3 routing boundaries

Core Layer

This layer is the backbone of the Cisco borderless network. The core layer connects a number of layers of the campus network and work as the aggregator for all of the other campus blocks and interconnects the campus with each other inside the network. The core layer main function is to provide speedy fault isolation and high-speed backbone connectivity.

Collapsed Core Network

Three-tier campus network usually planed for organizations where the access, distribution, and core, each are required as separate layers. This is required because of a basic, cost-effective, scalable, and efficient physical layout design for the large-scale network where many campuses exist. The best practice is to make an extended-star 

physical topology from the main campus to all other campuses.

The campus where fewer users accessing the network or single building campus usually not required a separate core and distribution layers. Network structure required in this situation is called a collapsed network or two-tier campus network.  In a collapsed network the role of the core switches moves to the distribution switches, merging the core- and distribution layer.

The figure below illustrates the collapsed campus network example for where the distribution and core layers are collapsed into a single layer.