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WEEK-VIII

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Ethernet-Data link Protocols

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Ethernet Data Link Protocols

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• One of the most significant strengths of the Ethernet family of protocols is that these protocols use the same data link standard.
• In fact, the core parts of the data link standard date back to the original Ethernet standards.
• The Ethernet data link protocol defines the Ethernet frame: an Ethernet header at the front, the encapsulated data in the middle, and an Ethernet trailer at the end.
• Ethernet actually defines a few alternate formats for the header, with the frame format shown in Figure below being commonly used today.

Commonly Used Ethernet Frame Format

Table below lists the fields in the header and trailer, and a brief description for reference.

IEEE 802.3 Ethernet Header and Trailer Fields

The Ethernet MAC address and Ethernet Addressing

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• Ethernet addresses, also called Media Access Control (MAC) addresses, are 6-byte-long (48-bitlong) binary numbers.
• For convenience, most computers list MAC addresses as 12-digit hexadecimal numbers.
• Most MAC addresses represent a single NIC or other Ethernet port, so these addresses are often called a unicast Ethernet address.
• The term unicast is simply a formal way to refer to the fact that the address represents one interface to the Ethernet LAN.
• The entire idea of sending data to a destination unicast MAC address works well, but it only works if all the unicast MAC addresses are unique.

• If two NICs tried to use the same MAC address, there could be confusion.
• If two PCs on the same Ethernet tried to use the same MAC address, to which PC should frames sent to that MAC address be delivered?
• Ethernet solves this problem using an administrative process so that, at the time of manufacture, all Ethernet devices are assigned a universally unique MAC address.
• Before a manufacturer can build Ethernet products, it must ask the IEEE to assign the manufacturer a universally unique 3-byte code, called the Organizationally Unique Identifier (OUI).
• The manufacturer agrees to give all NICs (and other Ethernet products) a MAC address that begins with its assigned 3-byte OUI.

• The manufacturer also assigns a unique value for the last 3 bytes, a number that manufacturer has never used with that OUI.
• As a result, the MAC address of every device in the universe is unique.

Structure of Unicast Ethernet Addresses

• In addition to unicast addresses, Ethernet also uses group addresses.
• Group addresses identify more than one LAN interface card.
• A frame sent to a group address might be delivered to a small set of devices on the LAN, or even to all devices on the LAN.
• In fact, the IEEE defines two general categories of group addresses for Ethernet:

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Broadcast address: Frames sent to this address should be delivered to

all devices on the Ethernet LAN. It has a value of FFFF.FFFF.FFFF.

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Multicast addresses: Frames sent to a multicast Ethernet address will

be copied and forwarded to a subset of the devices on the LAN that

volunteers to receive frames sent to a specific multicast address.

Identifying Network Layer Protocols with the Ethernet Type Field

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• The Ethernet Type field, or EtherType, sits in the Ethernet data link layer header, but its purpose is to directly help the network processing on routers and hosts.
• Basically, the Type field identifies the type of network layer (Layer 3) packet that sits inside the Ethernet frame.
• First, think about what sits inside the data part of the Ethernet frame shown earlier.
• Typically, it holds the network layer packet created by the network layer protocol on some device in the network.
• Today, the most common network layer protocols are both from TCP/IP: IP version 4 (IPv4) and IP version 6 (IPv6).

• For example, a host can send one Ethernet frame with an IPv4 packet and the next Ethernet frame with an IPv6 packet.
• Each frame would have a different Ethernet Type field value, using the values reserved by the IEEE, as shown in Figure below.

Use of Ethernet Type Field

Error Detection with FCS

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• Ethernet also defines a way for nodes to find out whether a frame’s bits changed while crossing over an Ethernet link.
• The Ethernet Frame Check Sequence (FCS) field in the Ethernet trailer—the only field in the Ethernet trailer—gives the receiving node a way to compare results with the sender, to discover whether errors occurred in the frame.
• The sender applies a complex math formula to the frame before sending it, storing the result of the formula in the FCS field.
• The receiver applies the same math formula to the received frame.
• The receiver then compares its own results with the sender’s results.
• If the results are the same, the frame did not change; otherwise, an error occurred, and the receiver discards the frame.

• Note that error detection does not also mean error recovery.
• Ethernet defines that the errored frame should be discarded, but Ethernet does not attempt to recover the lost frame.

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ENCAPSULATION

• Protocol Data Unit (PDU) is the term which describes data as it moves through the layers of the OSI model.
• It consists of protocol control information and user data.

Fig.: PDUs across the layers

ETHERNET FRAME

• The encapsulated data defined by the Network Access layer is called an Ethernet frame.
• An Ethernet frame starts with a header, which contains the source and destination MAC addresses, among other data.
• The middle part of the frame is the actual data. The frame ends with a field called Frame Check Sequence (FCS).
• The Ethernet frame structure is defined in the IEEE 802.3 standard.
• Here is a graphical representation of an Ethernet frame and a description of each field in the frame:

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• Preamble – informs the receiving system that a frame is starting and enables synchronisation.
• SFD (Start Frame Delimiter) – signifies that the Destination MAC Address field begins with the next byte.
• Destination MAC – identifies the receiving system.
• Source MAC – identifies the sending system.
• Type – defines the type of protocol inside the frame, for example IPv4 or IPv6.

• Data and Pad – contains the payload data. Padding data is added to meet the minimum length requirement for this field (46 bytes).
• FCS (Frame Check Sequence) – contains a 32-bit Cyclic Redundancy Check (CRC) which allows detection of corrupted data.
• The FCS field is the only field present in the Ethernet trailer. It allows the receiver to discover whether errors occurred in the frame. Note that Ethernet only detects in-transit corruption of data – it does not attempt to recover a lost frame. Other higher level protocols (e.g. TCP) perform error recovery.

HIERARCHICAL NETWORK DESIGN

• The hierarchical network design model splits the network into several modular layers.
• The modular layer in the design model has its own area to implement their own specific functions.
• The hierarchical network design model provides an easy to way to scale the network and keeps a consistent deployment method.
• Several modular layers keep the LAN design from needing a flat and fully measured network where all nodes are interconnected.
• The Hierarchical internetworking model is a three-layer model for network design first proposed by Cisco.
• The hierarchical network design model has three modular blocks known as layers, from the top layer to bottom layer:

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• the core layer
• the distribution layer
• the access layer.

 Fig, illustrates the Hierarchical Network Design Model

• The core layer provides connections between distribution layers in larger environments.
• The core network provides high-speed, highly redundant forwarding services to move packets between distribution-layer devices in different regions of the network.
• Core switches and routers are usually the most powerful, in terms of raw forwarding power, in the enterprise; core network devices manage the highest-speed connections, such as 10 Gigabit Ethernet or 100 Gigabit Ethernet.
• The distribution layer provides an aggregation point for the access layer and acts as a boundary between the access and core layers.
• The distribution layer is the smart layer in the three-layer model.
• Routing, filtering, and QoS policies are managed at the distribution layer.

• Distribution layer devices also often manage individual branch-office WAN connections.This layer is also called the Workgroup layer.
• the access layer provides direct access to the network for the users.
• End-stations and servers connect to the enterprise at the access layer.
• Access layer devices are usually commodity switching platforms, and may or may not provide layer 3 switching services.
• The traditional focus at the access layer is minimizing "cost-per-port": the amount of investment the enterprise must make for each provisioned Ethernet port.
• This layer is also called the desktop layer because it focuses on connecting client nodes, such as workstations to the network.

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PHYSICAL AND LOGICAL ADDRESSES

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PHYSICAL ADDRESS

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• A physical address is a local address.
• It is called a physical address because it is implemented in hardware.
• Example of a physical address is the 48-bit MAC address in the Ethernet protocol, which is imprinted on the NIC installed in the host or router.

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Fig.: Relationship of layers and addresses in TCP

LOGICAL ADDRESS (IP)

• Logical addresses are necessary for universal communications that are independent of underlying physical networks.
• Physical addresses are not adequate in an internetwork environment where different networks can have different address formats.

• A universal addressing system is needed in which each host can be identified uniquely, regardless of the underlying physical network.
• The logical addresses are designed for this purpose.
• A logical address in the Internet is a 32- bit address that can uniquely define a host connected to the Internet.
• No two publicly addressed and visible hosts on the Internet can have the same IP address.
• An IP address of the system is called logical address.
• This address is used by network layer to identify a particular network (source to destination) among the networks.
• This address can be changed by changing the host position on the network. So it is called logical address.

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BENEFITS OF A HIERARCHICAL DESIGN

There are many benefits associated with hierarchical network designs. viz:

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• Scalability
• Redundancy
• Performance
• Security
• Manageability
• Maintainability

ACCESS, DISTRIBUTION, AND CORE LAYERS

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• The three layered model is the basic foundation for creating small and larger Networks.
• Using this it is possible to design a hierarchical network with dividing the network into three different layers which also help us in reducing the network complexity
• Today’s networks are complex and large, wide variety of technology, running multiple services and also having challenges with functionality, increasing demand of bandwidth and compatibility with other businesses and venders.
• So for designing large networks there is a need to have such hierarchical model for designing our network.

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Advantages of Cisco 3-Layered model

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• Provide the flexibility in our network with three layers distribution, each layer is mapped with physical implementation and each of layers has its own features and functionality.
• 3-layer model is easier to understand and easy to grow your network.
• 3-layer model is easy to troubleshoot because of its logical distribution into layer, as each layer has its own functionality.
• Allows the lower cost in implementation.

Access Layer

• The access layer represents the network edge, where traffic enters or exits the campus network.
• Traditionally, the primary function of an access layer switch is to provide network access to the user.
• Access layer switches connect to distribution layer switches, which implement network foundation technologies such as routing, quality of service, and security.

Some of key characteristics of access-layer are as following:

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• High availability
• Layer 2 switching
• Port security
• Broadcast suppression
• QoS classification and marking and trust boundaries
• Rate limiting/policing
• Address Resolution Protocol (ARP) inspection
• Virtual access control lists (VACL)
• Spanning tree protocol (STP)
• Trust classification
• Power over Ethernet (PoE) and auxiliary VLANs for VoIP

Distribution Layer

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The distribution layer interfaces between the access layer and the core layer to provide many important functions, including:

• Aggregating large-scale wiring closet networks
• Aggregating Layer 2 broadcast domains and Layer 3 routing boundaries
• Providing intelligent switching, routing, and network access policy functions to access the rest of the network
• Providing high availability through redundant distribution layer switches to the end-user and equal cost paths to the core
• Providing differentiated services to various classes of service applications at the edge of the network

Some of key characteristics of distribution-layer are as following:

• Route filtering by source or destination address and filtering on input or output ports
• Hiding internal network numbers by route filtering
• Policy-based connectivity
• Static routing
• QoS mechanisms, such as priority-based queuing
• Redundancy and load balancing
• Aggregation of LAN wiring closets and WAN connections
• Security filtering
• Route summarization
• Departmental or workgroup access

• Broadcast or multicast domain definition Routing between virtual LANs (VLAN)
• Media translations (for example, between Ethernet and Token Ring)
• Redistribution between routing domains (for example, RIP redistribution into OSPF)

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Core Layer - The core layer is the network backbone. It connects several layers of the campus network. The core layer serves as the aggregator for all of the other campus blocks and ties the campus together with the rest of the network. The primary purpose of the core layer is to provide fault isolation and high-speed backbone connectivity.

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Some of key characteristics of core-layer are as following:

• Fast transport and large amount of data
• Redundancy
• High reliability and availability
• Low latency and good manageability
• Quality of service (QoS) classification, or other processes
• Fault tolerance
• Limited and consistent diameter

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Core layer Devices:

• High end routers and switches
• Layer-3 switches
• Gateways and media converters
• Soft Switches for IP telephone

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Fig. illustrates a three-tier campus network design for organizations where the access, distribution, and core are each separate layers. To build a simplified, scalable, cost-effective, and efficient physical cable layout design, the recommendation is to build an extended-star physical network topology from a centralized building location to all other buildings on the same campus.

SENDING ETHERNET FRAMES WITH SWITCHES AND HUBS

• Ethernet LANs behave slightly differently depending on whether the LAN has mostly modern devices, in particular, LAN switches rather than some older LAN devices called LAN hubs.
• Basically, the use of more modern switches allows the use of full-duplex logic, which is much faster and simpler than half-duplex logic, which is required when using hubs.

SENDING IN MODERN ETHERNET LANS USING FULL DUPLEX

• Modern Ethernet LANs use a variety of Ethernet physical standards, but with standard Ethernet frames which will flow over any of those sorts of physical links.
• Each individual link can run at a special speed, but each link allows the attached nodes to send the bits within the frame to the next node.
• they need to work together to deliver the data from the sending Ethernet node to the destination node. The process is relatively simple, on purpose; the simplicity lets each device send a large number of frames per second. Figure 8.17 shows an example during which PC1 sends an Ethernet frame to PC2.

Fig.: Sending Data in a Modern Ethernet LAN

USING HALF DUPLEX WITH LAN HUBs

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• LAN hubs forward data using physical layer standards, and are therefore considered to be Layer 1 devices.
• When an electrical signal comes in one hub port, the hub repeats that electrical signal out all other ports (except the incoming port).
• The data reaches all the rest of the nodes connected to the hub, therefore the data hopefully reaches the correct destination.
• The hub has no concept of Ethernet frames, of addresses, and so on.
• The downside of using LAN hubs is that if two or more devices transmitted a signal at the same instant, the electrical signal collides and becomes garbled.
• The hub repeats all received electrical signals, albeit it receives multiple signals at the same time.

• Figure shows the idea, with PCs Archie and Bob sending an electrical signal at the same instant of your time (at Steps 1A and 1B) and therefore the hub repeating both electrical signals out toward Larry on the left (Step 2).

Fig.:Collision Occurring Because of LAN Hub Behavior

• In Figure., If the hub is replaced with a LAN switch, the switch prevents the collision on the left.
• The switch operates as a Layer 2 device, meaning that it is at the data-link header and trailer.
• A switch would check out the MAC addresses, and even if the switch needed to forward both frames to Larry on the left, the switch would send one frame and queue the opposite frame until the first frame was finished.
• To stop the hub’s logic collisions, the Ethernet nodes must use half-duplex logic rather than full-duplex logic a problem occurs only two or more devices send at the same time; half-duplex logic tells the nodes that if somebody else is sending, wait before sending.

Fig.: Full and Half Duplex in an Ethernet LAN

ETHERNET ACCESS LAYER DEVICES

• Ethernet is a network access layer protocol.
• The network access layer, which accepts IP datagrams and transmits them over a specific network such as an Ethernet or Token Ring, etc.
• The network access layer which is an interface to a specific network technology corresponds to the physical and data link layers in OSI.

Fig.: Classification of Protocols in the Protocol Suite

• In the Cisco hierarchical model, also known as the hierarchical internetworking model, the access layer is responsible for providing end user devices with a connection to network resources. 
• The access layer, which is the lowest level of the Cisco three tier network model, ensures that packets are delivered to end user devices.
• This layer is sometimes referred to as the desktop layer, because it focuses on connecting client nodes to the network.
• Access layer devices include hubs, multi-station access units and switches.
• The access layer represents the network edge, where traffic enters or exits the campus network.
• Traditionally, the primary function of an access layer switch is to provide network access to the user.

• Access layer switches connect to distribution layer switches, which implement network foundation technologies such as routing, quality of service, and security.
• To meet network application and end-user demand, the next-generation switching platforms now provide more converged, integrated, and intelligent services to various types of endpoints at the network edge.
• Building intelligence into access layer switches allows applications to operate on the network more efficiently and securely.

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HUB

• Hubs/repeaters are used to connect together two or more Ethernet segments of any type of medium.
• In larger designs, signal quality begins to deteriorate as segments exceed their maximum length.
• Hubs provide the signal amplification required to allow a segment to be extended a greater distance.
• A hub repeats any incoming signal to all ports.

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SWITCH

• The access switches are the network switches that connect the access layer with the subnets.
• The subnets are integrated with access devices like routers, IP devices, control, and monitoring panels, etc.
• An access layer of a hierarchy network features multiple subnets to which the access switches are directly connected.

THE MAC ADDRESS TABLE

• The MAC address table contains address information that the Switch uses to forward traffic between the inbound and outbound ports.
• All MAC addresses in the address table are associated with one or more ports.
• The switch maintains an address table called the MAC address table in order to efficiently switch frames between interfaces.
• So basically a switch stores information about the other (Ethernet interfaces) to which it is connected on a network.
• when a switch receives a frame, it associates the MAC address of the sending device with the switch port on which it was received.

ETHERNET BROADCAST AND BROADCAST DOMAIN

• Ethernet Broadcast is possible on the underlying data link layer in Ethernet networks.
• Frames are addressed to reach every computer on a given LAN segment if they are addressed to MAC address FF:FF:FF:FF:FF:FF.
• Ethernet frames that contain IP broadcast packages are usually sent to this address.
• In simple Ethernet (without switches or bridges), data frames are transmitted to all other nodes on a network.
• Each receiving node checks the destination address of each frame, and simply ignores any frame not addressed to its own MAC address or the broadcast address.

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• The broadcast domain is used to describe a group of devices on a specific network segment that can reach each other with Ethernet broadcasts.
• Broadcasts sent by a device in one broadcast domain are not forwarded to devices in another broadcast domain.
• This improves the performance of the network because not all devices on a network will receive and process broadcasts.
• Routers separate a LAN into multiple broadcast domains (every port on a router is in a different broadcast domain).
• Switches (by default) flood Ethernet broadcast frames out all ports, just like bridges and hubs.
• All ports on these devices are in the same broadcast domain.

Fig.: Illustration of Broadcast Domain

• In the picture above there is a network of six computers, two hubs, a bridge, a switch, and a router.
• The broadcast domains are marked with little thick lines.
• All the devices connected to a hub, a bridge, and a switch are in the same broadcast domain.
• Only routers separate the LAN into multiple broadcast domains. Ethernet broadcasts are usually used by Address Resolution Protocol (ARP) to translate IP addresses to MAC addresses.

ARP

• Address Resolution Protocol (ARP) is a communication protocol used to find the MAC (Media Access Control) address of a device from its IP address.
• This protocol is used when a device wants to communicate with another device on a Local Area Network or Ethernet.