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Enterprise LAN Routing and Switching

Definition

In networking, switching and routing refers basically to packet management over a LAN or a WAN. Through different switching technologies that are crucial to network design, switches allow traffic to be sent only where it is needed in most cases, using fast, hardware-based methods. On the other side, routing is the process of selecting paths in a network along which to send network traffic (the packets).

User Benefits

The classical role of routers and switches evolved once with the more and more dynamic and complexity required today. Adding on top of it the business and performance requirements, this mix has rewritten the roles of the traditional switch and router respectively.

Business Impact

Thus, independent of the purpose it serves, an infrastructure should be designed with the following principles:

·         Open architecture – allow integration with other technologies – e.g.: security, application delivery from different vendors

·         Resilience – the infrastructure should be robust enough to withstand to a change of any kind to regular or forecasted behavior

·         Service-oriented – the architecture should follow the application delivery principles – e.g.: Service Oriented Architecture - rather than just packet forwarding.

·         Easy to manage – the less operating systems and versions in the network, the less the effort to manage it and less risk of service outage induced by different versions of operating systems in the network.

To all the above requirements, there are also specific requirements to each environment the infrastructure is deployed.

In Data Centers, the infrastructure should be regarded as a unified data center fabric between the applications within the data center and the users. Other principles that would apply in the Data Center could also be:

·         Reduced power and cooling requirements

·         Application consolidation through virtualization leads automatically to application-to-application communication within the Data Center – this translates into distributed chassis and aggregation

·         Service SLA leads to application QoS mechanisms to be enforced, rather than packet based QoS.

In the Government arena, there are various compliance requirements which lead to supplemental security integration.

For all these reasons above, the infrastructure has to be built with applications in mind.


Products supporting this technology

Ruckus Wireless

Introduction:

The Internet is the largest data network on earth. The Internet consists of many large and small networks that are interconnected. Individual computers are the sources and destinations of information through the Internet. Connection to the Internet can be broken down into the physical connection, the logical connection, and applications.

A physical connection is made by connecting an adapter card, such as a modem or a NIC, from a computer to a network. The physical connection is used to transfer signals between computers within the local-area network (LAN) and to remote devices on the Internet.

The logical connection uses standards called protocols. A protocol is a formal description of a set of rules and conventions that govern how devices on a network communicate. Connections to the Internet may use multiple protocols. The Transmission Control Protocol/Internet Protocol (TCP/IP) suite is the primary set of protocols used on the Internet. The TCP/IP suite works together to transmit and receive data, or information.

The last part of the connection consists in applications, or software programs, that interpret and display data in an understandable form. Applications work with protocols to send and receive data across the Internet. A Web browser displays HTML as a Web page. Examples of Web browsers include Internet Explorer and Mozilla. File Transfer Protocol (FTP) is used to download files and programs from the Internet. Web browsers also use proprietary plug-in applications to display special data types such as movies or flash animations.

In networking, which is the foundation of Internet and Intranet communications, switches and routers are the main actors.

In the following pages, readers can found valuable information covering these fundamental network resources, associated principles and protocols. Some of the covered areas:

·         Network devices

·         Switches and switching modes

·         Routers and connections (serial, ISDN, DSL, cable)

·         Routable and routed protocols

·         Routing tables

·         Routing aalgorithms and metrics

·         Routing versus switching

Before any start, readers must accommodate with language and acronyms composing network terminology and network models.

 

1.  Network terminology

1.1.              Network devices

A repeater is a network device used to regenerate a signal. Repeaters regenerate analog or digital signals that are distorted by transmission loss due to attenuation. A repeater does not make intelligent decision concerning forwarding packets like a router.

Hubs concentrate connections. In other words, they take a group of hosts and allow the network to see them as a single unit. This is done passively, without any other effect on the data transmission. Active hubs concentrate hosts and also regenerate signals.

Bridges convert network data formats and perform basic data transmission management.
Bridges provide connections between LANs. They also check data to determine if it should cross the bridge. This makes each part of the network more efficient.

Workgroup switches add more intelligence to data transfer management. They can determine if data should remain on a LAN and transfer data only to the connection that needs it. Another difference between a bridge and switch is that a switch does not convert data transmission formats.

Routers have all the capabilities listed above. Routers can regenerate signals, concentrate multiple connections, convert data transmission formats and manage data transfers. They can also connect to a WAN, which allows them to connect LANs that are separated by great distances. None of the other devices can provide this type of connection.

1.2.              Network topology

Network topology defines the structure of the network. One part of the topology definition is the physical topology, which is the actual layout of the wire or media. The other part is the logical topology, which defines how the hosts access the media to send data. The physical topologies that are commonly used are as follows:  

  • A bus topology uses a single backbone cable that is terminated at both ends. All the hosts connect directly to this backbone.
  • A ring topology connects one host to the next and the last host to the first. This creates a physical ring of cable.
  • A star topology connects all cables to a central point.
  • An extended star topology links individual stars together by connecting the hubs or switches.
  • A hierarchical topology is similar to an extended star. However, instead of linking the hubs or switches together, the system is linked to a computer that controls the traffic on the topology.
  • A mesh topology is implemented to provide as much protection as possible from interruption of service. For example, a nuclear power plant might use a mesh topology in the networked control systems. Although the Internet has multiple paths to any one location, it does not adopt the full mesh topology.

The logical topology of a network determines how the hosts communicate across the medium. The two most common types of logical topologies are broadcast and token passing.

1.3.              Network protocols

Protocol suites are collections of protocols that enable network communication between hosts. A protocol is a formal description of a set of rules and conventions that govern a particular aspect of how devices on a network communicate. Protocols determine the format, timing, sequencing, and error control in data communication. Without protocols, the computer cannot make or rebuild the stream of incoming bits from another computer into the original format.

Protocols control all aspects of data communication, which include the following:

  • How the physical network is built
  • How computers connect to the network
  • How the data is formatted for transmission
  • How that data is sent
  • How to deal with errors

These network rules are created and maintained by many different organizations and committees. Included in these groups are the Institute of Electrical and Electronic Engineers (IEEE), American National Standards Institute (ANSI), Telecommunications Industry Association (TIA), Electronic Industries Alliance (EIA) and the International Telecommunications Union (ITU), formerly known as the Comité Consultatif International Téléphonique et Télégraphique (CCITT).

1.4.              Local-area networks (LANs)

LANs consist of the following components:

  • Computers
  • Network interface cards (NICs)
  • Peripheral devices
  • Networking media
  • Network devices

LANs allow businesses to locally share computer files and printers efficiently and make internal communications possible. A good example of this technology is e-mail. LANs manage data, local communications, and computing equipment.

Some common LAN technologies include the following:

  • Ethernet
  • Token Ring
  • FDDI

1.5.              Wide-area networks (WANs)

WANs interconnect LANs, which then provide access to computers or file servers in other locations. Because WANs connect user networks over a large geographical area, they make it possible for businesses to communicate across great distances. WANs allow computers, printers, and other devices on a LAN to be shared with distant locations. WANs provide instant communications across large geographic areas.

Collaboration software provides access to real-time information and resources and allows meetings to be held remotely. WANs have created a new class of workers called telecommuters. These people never have to leave their homes to go to work.

WANs are designed to do the following:

  • Operate over a large and geographically separated area
  • Allow users to have real-time communication capabilities with other users
  • Provide full-time remote resources connected to local services
  • Provide e-mail, Internet, file transfer, and e-commerce services

Some common WAN technologies include the following:

  • Modems
  • Integrated Services Digital Network (ISDN)
  • Digital subscriber line (DSL)
  • Frame Relay
  • T1, E1, T3, and E3
  • Synchronous Optical Network (SONET)

 

2.  Network models

2.1.              OSI Model

Knowledge of the structure of the Open Systems Interconnect (OSI) Model is essential to understanding network technology because it is the most commonly cited protocol model when describing any network protocol.

OSI Model Layers

The OSI model consists of seven layers that perform specific functions. Each layer passes its results to another layer. A sending station formats a network request to send data. The request is submitted to the protocol at the Application Layer.

Application Layer

The Application layer is the top layer. The protocol that runs at the Application layer performs an operation on the request and then passes it to the presentation layer. The protocols at layers underneath the Application Layer perform their own calculations. They also append their information to the data sent from the layer above. At the receiving station, information flows from the bottom layer, back to the Application layer.

The Application layer of the OSI model is responsible for defining interactions between network services (applications) and the network.

Application layer services include, but are not limited to: file, print, and messaging services. The Application layer may also support error recovery.

Presentation Layer

The Presentation layer formats data exchange, converts character sets, and encrypts data.The Presentation layer is also responsible for data compression. Sometimes it is responsible for data stream redirection.

Session Layer

The Session layer defines how computers establish, synchronize, maintain, and end their sessions together. Security authentication, connection ID establishment, data transfer, acknowledgements, and connection release are some of the practical functions that occur at this layer.

Transport Layer

Data is checked for errors at the Transport layer, which is responsible also for segmentation. It can divide a long message into segments, or combine a series of short messages into one segment. These broken up or combined segments must later be correctly reassembled. This is accomplished through segment sequencing, where a number is appended to each of the segments.

Logical address/name resolution is performed at the Transport layer. Also, the Transport layer sends acknowledgment that it received a data packet.

The same, error and flow control in network communications is the responsibility of the Transport layer.

Network Layer

The Network layer handles logical addressing. It is also responsible for translating logical names into physical addresses. The Network layer prioritizes data because not all data is of equal importance. This prioritization is known as Quality of Service, or QoS. Additional functions: the Network layer controls congestion, routes data from source to destination, and builds and tears down packets. Most routing protocols function at this layer.

Data Link Layer

The Data Link layer takes raw data from the Physical layer and gives it a logical structure.

PhysicalLayer

The Physical layer is responsible for controlling the functional interface. For example, this layer controls the transmission technique, pin layout, and connector type.

2.2.              TCP/IP Model

The U.S. Department of Defense (DoD) created the TCP/IP reference model, because it wanted to design a network that could survive any conditions, including a nuclear war. In a world connected by different types of communication media such as copper wires, microwaves, optical fibers and satellite links, the DoD wanted transmission of packets every time and under any conditions. This very difficult design problem brought about the creation of the TCP/IP model.

Unlike other proprietary networking technologies, TCP/IP was developed as an open standard. This meant that anyone was free to use TCP/IP. This helped speed up the development of TCP/IP as a standard.

The TCP/IP model has the following four layers:

  • Application layer
  • Transport layer
  • Internet layer
  • Network access layer

Although some of the layers in the TCP/IP model have the same name as layers in the OSI model, the layers of the two models do not correspond exactly. Most notably, the application layer has different functions in each model.

The designers of TCP/IP felt that the application layer should include the OSI session and presentation layer details. They created an application layer that handles issues of representation, encoding, and dialog control.

The transport layer deals with the quality of service issues of reliability, flow control, and error correction. One of its protocols, the transmission control protocol (TCP), provides excellent and flexible ways to create reliable, well-flowing, low-error network communications.

TCP is a connection-oriented protocol. It maintains a dialogue between source and destination while packaging application layer information into units called segments. Connection-oriented does not mean that a circuit exists between the communicating computers. It does mean that Layer 4 segments travel back and forth between two hosts to acknowledge the connection exists logically for some period.

The purpose of the Internet layer is to divide TCP segments into packets and send them from any network. The packets arrive at the destination network independent of the path they took to get there. The specific protocol that governs this layer is called the Internet Protocol (IP). Best path determination and packet switching occur at this layer.

The relationship between IP and TCP is an important one. IP can be thought to point the way for the packets, while TCP provides a reliable transport.

The name of the network access layer is very broad and somewhat confusing. It is also known as the host-to-network layer. This layer is concerned with all of the components, both physical and logical, that are required to make a physical link. It includes the networking technology details, including all the details in the OSI physical and data link layers.

Some of the most commonly used application layer protocols include the following:

  • File Transfer Protocol (FTP)
  • Hypertext Transfer Protocol (HTTP)
  • Simple Mail Transfer Protocol (SMTP)
  • Domain Name System (DNS)
  • Trivial File Transfer Protocol (TFTP)

The common transport layer protocols include:

  • Transport Control Protocol (TCP)
  • User Datagram Protocol (UDP)

The primary protocol of the Internet layer is:

  • Internet Protocol (IP)

Regardless of which network application services are provided and which transport protocol is used, there is only one Internet protocol, IP. This is a deliberate design decision. IP serves as a universal protocol that allows any computer anywhere to communicate at any time.

The network access layer refers to any particular technology used on a specific network.

Some well-known TCP protocols:

Layer

Protocol(s)

Application

HTTP, FTP, NFS, Telnet, SMTP, SNMP

Transport

TCP, UDP

Internet

IP, ICMP, ARP, RARP, DHCP

Network

SLIP, PPP

 

 

2.2.1.  Comparing TCP/IP with OSI

 

 

3.  Data flow

Data flow in the context of collision and broadcast domains focuses on how data frames propagate through a network. It refers to the movement of data through OSI Layers 1, 2 and 3 devices and how data must be encapsulated to effectively make that journey. Remember that data is encapsulated at the network layer with an IP source and destination address, and at the data-link layer with a MAC source and destination address.

A good rule to follow is that a Layer 1 device always forwards the frame, while a Layer 2 device wants to forward the frame. In other words, a Layer 2 device will forward the frame unless something prevents it from doing so. A Layer 3 device will not forward the frame unless it has to. Using this rule will help identify how data flows through a network.

·         Layer 1 devicesdo no filtering, so everything that is received is passed on to the next segment. The frame is simply regenerated and retimed and thus returned to its original transmission quality. Any segments connected by Layer 1 devices are part of the same domain, both collision and broadcast.

·         Layer 2 devicesfilter data frames based on the destination MAC address. A frame is forwarded if it is going to an unknown destination outside the collision domain. The frame will also be forwarded if it is a broadcast, multicast, or a unicast going outside of the local collision domain. The only time that a frame is not forwarded is when the Layer 2 device finds that the sending host and the receiving host are in the same collision domain. A Layer 2 device, such as a bridge, creates multiple collision domains but maintains only one broadcast domain.

·         Layer 3 devicesfilter data packets based on IP destination address. The only way that a packet will be forwarded is if its destination IP address is outside of the broadcast domain and the router has an identified location to send the packet. A Layer 3 device creates multiple collision and broadcast domains.

Data flow through a routed IP based network, involves data moving across traffic management devices at Layers 1, 2, and 3 of the OSI model. Layer 1 is used for transmission across the physical media, Layer 2 for collision domain management, and Layer 3 for broadcast domain management.

 

4.  Switches

A switch is sometimes described as a multiport bridge.
A typical bridge may have only two ports that link two network segments. A switch can have multiple ports based on the number of network segments that need to be linked. Like bridges, switches learn information about the data frames that are received from computers on the network. Switches use this information to build tables to determine the destination of data that is sent between computers on the network.

Although there are some similarities between the two, a switch is a more sophisticated device than a bridge. A bridge determines whether the frame should be forwarded to the other network segment based on the destination MAC address. A switch has many ports with many network segments connected to them. A switch chooses the port to which the destination device or workstation is connected. Ethernet switches are popular connectivity solutions because they improve network speed, bandwidth, and performance.

Switching is a technology that alleviates congestion in Ethernet LANs. Switches reduce traffic and increase bandwidth. Switches can easily replace hubs because switches work with the cable infrastructures that are already in place. This improves performance with minimal changes to a network.

All switching equipment performs two basic operations.

·         The first operation is called switching data frames. This is the process by which a frame is received on an input medium and then transmitted to an output medium.

·         The second is the maintenance of switching operations where switches build and maintain switching tables and search for loops.

Switches operate at much higher speeds than bridges and can support new functionality, such as virtual LANs.

An Ethernet switch has many benefits. One benefit is that it allows many users to communicate at the same time through the use of virtual circuits and dedicated network segments in a virtually collision-free environment.
This maximizes the bandwidth available on the shared medium. Another benefit is that a switched LAN environment is very cost effective since the hardware and cables in place can be reused.

4.1.              Layer 2 switching

Generally, a bridge has only two ports and divides a collision domain into two parts. All decisions made by a bridge are based on MAC or Layer 2 addresses and do not affect the logical or Layer 3 addresses. A bridge will divide a collision domain but has no effect on a logical or broadcast domain. If a network does not have a device that works with Layer 3 addresses, such as a router, the entire network will share the same logical broadcast address space. A bridge will create more collision domains but will not add broadcast domains.

A switch is essentially a fast, multi-port bridge that can contain dozens of ports. Each port creates its own collision domain. In a network of 20 nodes, 20 collision domains exist if each node is plugged into its own switch port. If an uplink port is included, one switch creates 21 single-node collision domains. A switch dynamically builds and maintains a content-addressable memory (CAM) table, which holds all of the necessary MAC information for each port.

4.2.              Switch modes

How a frame is switched to the destination port is a trade off between latency and reliability.

1.       In this respect, a switch can start to transfer the frame as soon as the destination MAC address is received. This is called cut-through packet switchingand results in the lowest latency through the switch. However, no error checking is available.

2.       The switch can also receive the entire frame before it is sent to the destination port. This gives the switch software an opportunity to verify the Frame Check Sequence (FCS). If the frame is invalid, it is discarded at the switch. Since the entire frame is stored before it is forwarded, this is called store-and-forward packet switching.

3.       A compromise between cut-through and store-and-forward packet switching is the fragment-free mode. Fragment-free packet switchingreads the first 64 bytes, which includes the frame header, and starts to send out the packet before the entire data field and checksum are read. This mode verifies the reliability of the addresses and LLC protocol information to ensure the data will be handled properly and arrive at the correct destination (the Logical Link Control (LLC) data communication protocol layer is the upper sub-layer of the Data Link Layer in the seven-layer OSI reference model)

When cut-through packet switching is used, the source and destination ports must have the same bit rate to keep the frame intact. This is called symmetric switching. If the bit rates are not the same, the frame must be stored at one bit rate before it is sent out at the other bit rate. This is known as asymmetric switching. Store-and-forward mode must be used for asymmetric switching. 

Asymmetric switching provides switched connections between ports with different bandwidths. Asymmetric switching is optimized for client/server traffic flows in which multiple clients communicate with a server at once. More bandwidth must be dedicated to the server port to prevent a bottleneck.

4.2.1.  Spanning-Tree Protocol

When multiple switches are arranged in a simple hierarchical tree, switching loops are unlikely to occur. However, switched networks are often designed with redundant paths to provide for reliability and fault tolerance.
Redundant paths are desirable but they can have undesirable side effects such as switching loops. Switching loops are one such side effect. Switching loops can occur by design or by accident, and they can lead to broadcast storms that will rapidly overwhelm a network. STP is a standards-based protocol that is used to avoid switching loops. Each switch in a LAN that uses STP sends messages called Bridge Protocol Data Units (BPDUs) out all its ports to let other switches know of its existence. This information is used to elect a root bridge for the network. The switches use the spanning-tree algorithm (STA) to resolve and shut down the redundant paths.

Each port on a switch that uses STP exists in one of the following five states:

  • Blocking
  • Listening
  • Learning
  • Forwarding
  • Disabled

A port moves through these five states as follows:

  • From initialization to blocking
  • From blocking to listening or to disabled
  • From listening to learning or to disabled
  • From learning to forwarding or to disabled
  • From forwarding to disabled

STP is used to create a logical hierarchical tree with no loops. However, thealternate paths are still available if necessary.

 

5.  Routers

For long distance communication, WANs use serial transmission. This is a process by which bits of data are sent over a single channel. This process provides reliable long distance communication and the use of a specific electromagnetic or optical frequency range.

Frequencies are measured in terms of cycles per second and expressed in Hz. Signals transmitted over voice grade telephone lines use 4 kHz. The size of the frequency range is referred to as bandwidth. In networking, bandwidth is a measure of the bits per second that are transmitted.

5.1.              Routers connections

5.1.1.  Serial connections

Routers are responsible for routing data packets from source to destination within the LAN, and for providing connectivity to the WAN. Within a LAN environment the router contains broadcasts, provides local address resolution services, such as ARP and RARP, and may segment the network using a subnetwork structure. In order to provide these services the router must be connected to the LAN and WAN.

In addition to determining the cable type, it is necessary to determine whether DTE (data terminal equipment) or DCE (data communications equipment) connectors are required. The DTE is the endpoint of the user’s device on the WAN link. The DCE is typically the point where responsibility for delivering data passes into the hands of the service provider.

When connecting directly to a service provider, or to a device that will perform signal clocking, the router is a DTE and needs a DTE serial cable.
This is typically the case for routers. However, there are cases when the router will need to be the DCE.

When cabling routers for serial connectivity, the routers will either have fixed or modular ports.

·         Interfaces on routers with fixed serial ports are labelled for port type and port number.

·         Interfaces on routers with modular serial ports are labelled for port type, slot, and port number.
The slot is the location of themodule. To configure a port on a modular card, it is necessary to specify the interface using the syntax “port type slot number/port number”. Use the label “serial 1/0”, when the interface is serial, the slot number where the module is installed is slot 1, and the port that is being referenced is port 0.

5.1.2.  ISDN BRI connections

With ISDN BRI, two types of interfaces may be used, BRI S/T and BRI U. In order to decide which interface type is needed, it has to be determined who is providing the Network Termination 1 (NT1) device .

An NT1 is an intermediate device located between the router and the service provider ISDN switch. The NT1 is used to connect four-wire subscriber wiring to the conventional two-wire local loop. In North America, the customer typically provides the NT1, while in the rest of the world the service provider provides the NT1 device.

It may be necessary to provide an external NT1 if the device is not already integrated into the router. Reviewing the labelling on the router interfaces is usually the easiest way to determine if the router has an integrated NT1. A BRI interface with an integrated NT1 is labelled BRI U. A BRI interface without an integrated NT1 is labelled BRI S/T.

5.1.3.  DSL connections

The ADSL router provides business-class functionality for small businesses, small remote offices and corporate teleworkers. It enables service providers and resellers to increase service revenue by supporting features for business-class security, integrated toll-quality voice/data, differentiated classes of service, and managed network access.

5.1.4.  Cable connections

The cable access router provides high-speed network access on the cable television system to residential and small office, home office (SOHO) subscribers.

5.2.              Broadcast domains

To better understand how routers behave in their critical role on the network infrastructure, there are some concepts that have to be detailed first, like: broadcasting, packet propagation, connectionless and connection-oriented delivery.

A broadcast domain is a group of collision domains that are connected by Layer 2 devices.
When a LAN is broken up into multiple collision domains, each host in the network has more opportunities to gain access to the media. This reduces the chance of collisions and increases available bandwidth for every host. Broadcasts are forwarded by Layer 2 devices. Excessive broadcasts can reduce the efficiency of the entire LAN. Broadcasts have to be controlled at Layer 3 since Layers 1 and 2 devices cannot control them. A broadcast domain includes all of the collision domains that process the same broadcast frame. This includes all the nodes that are part of the network segment bounded by a Layer 3 device. Broadcast domains are controlled at Layer 3 because routers do not forward broadcasts. Routers actually work at Layers 1, 2, and 3. Like all Layer 1 devices, routers have a physical connection and transmit data onto the media. Routers also have a Layer 2 encapsulation on all interfaces and perform the same functions as other Layer 2 devices. Layer 3 allows routers to segment broadcast domains.

In order for a packet to be forwarded through a router it must have already been processed by a Layer 2 device and the frame information stripped off. Layer 3 forwarding is based on the destination IP address and not the MAC address. For a packet to be forwarded it must contain an IP address that is outside of the range of addresses assigned to the LAN and the router must have a destination to send the specific packet to in its routing table.

5.3.              Packet propagation and switching within a router

As a packet travels through an internetwork to its final destination, the Layer 2 frame headers and trailers are removed and replaced at every Layer 3 device.
This is because Layer 2 data units, or frames, are for local addressing. Layer 3 data units, or packets, are for end-to-end addressing.

Layer 2 Ethernet frames are designed to operate within a broadcast domain with the MAC address that is burned into the physical device. Other Layer 2 frame types include PPP serial links and Frame Relay connections, which use different Layer 2 addressing schemes. Regardless of the type of Layer 2 addressing used, frames are designed to operate within a Layer 2 broadcast domain. When the data is sent to a Layer 3 device, the Layer 2 information changes.

As a frame is received at a router interface, the destination MAC address is extracted. The address is checked to see if the frame is directly addressed to the router interface, or if it is a broadcast. In either situation, the frame is accepted. Otherwise, the frame is discarded since it is destined for another device on the collision domain.

The CRC information is extracted from the frame trailer of an accepted frame. The CRC is calculated to verify that the frame data is without error.

If the check fails, the frame is discarded. If the check is valid, the frame header and trailer are removed and the packet is passed up to Layer 3. The packet is then checked to see if it is actually destined for the router, or if it is to be routed to another device in the internetwork. If the destination IP address matches one of the router ports, the Layer 3 header is removed and the data is passed up to the Layer 4. If the packet is to be routed, the destination IP address will be compared to the routing table. If a match is found or there is a default route, the packet will be sent to the interface specified in the matched routing table statement. When the packet is switched to the outgoing interface, a new CRC value is added as a frame trailer, and the proper frame header is added to the packet. The frame is then transmitted to the next broadcast domain on its trip to the final destination.

5.4.              Connectionless and connection-oriented delivery

These two services provide the actual end-to-end delivery of data in an internetwork.

Most network services use a connectionless delivery system. 
Different packets may take different paths to get through the network. The packets are reassembled after they arrive at the destination. In a connectionless system, the destination is not contacted before a packet is sent. A good comparison for a connectionless system is a postal system. The recipient is not contacted to see if they will accept the letter before it is sent. Also, the sender does not know if the letter arrived at the destination.

In connection-oriented systems, a connection is established between the sender and the recipient before any data is transferred.
An example of a connection-oriented network is the telephone system. The caller places the call, a connection is established, and then communication occurs.

Connectionless network processes are often referred to as packet-switched processes.As the packets pass from source to destination, packets can switch to different paths, and possibly arrive out of order. Each packet contains the instructions, such as destination address and order in a message, that coordinate its arrival with other associated packets. Packets are reassembled into the proper sequence at the destination. Devices make the path determination for each packet based on a variety of criteria. Some of the criteria, such as available bandwidth, may differ from packet to packet.

Connection-oriented network processes are often referred to as circuit-switched processes. A dedicated connection between the originator and the recipient is first established, and then data transfer begins. All packets travel sequentially across the same physical or virtual circuit in one continuous stream.

The Internet is a gigantic, connectionless network in which the majority of packet deliveries are handled by IP. TCP adds Layer 4 connection-oriented reliability services to connectionless IP communications.

5.5.              Routing Protocols

5.5.1.  Routing overview

Routing is an OSI Layer 3 function.
Routing is a hierarchical organizational scheme that allows individual addresses to be grouped together. These individual addresses are treated as a single unit until the destination address is needed for final delivery of the data.
Routing finds the most efficient path from one device to another. The primary device that performs the routing process is the router.

The following are the two key functions of a router:

  • Routers must maintain routing tables and make sure other routers know of changes in the network topology. They use routing protocols to communicate network information with other routers – function performed by the CONTROL PLANE
  • When packets arrive at an interface, the router must use the routing table to determine where to send them. The router switches the packets to the appropriate interface, adds the frame information for the interface, and then transmits the frame – function performed by the FORWARDING PLANE

A router is a network layer device that uses one or more routing metrics to determine the optimal path along which network traffic should be forwarded. Routing metrics are values that are used to determine the advantage of one route over another.
Routing protocols use various combinations of metrics to determine the best path for data.

Routers interconnect network segments or entire networks. Routers pass data frames between networks based on Layer-3 information. Routers make logical decisions about the best path for the delivery of data. Routers then direct packets to the appropriate output port to be encapsulated for transmission.
Stages of the encapsulation and de-encapsulation process occur each time a packet transfers through a router. The router must de-encapsulate the Layer 2 data frame to access and examine the Layer 3 address. The complete process of sending data from one device to another involves encapsulation and de-encapsulation on all seven OSI layers. The encapsulation process breaks up the data stream into segments, adds the appropriate headers and trailers, and then transmits the data. The de-encapsulation process removes the headers and trailers and then recombines the data into a seamless stream.

5.5.2.  Routing versus switching

Routers and switches may seem to perform the same function. The primary difference is that switches operate at Layer 2 of the OSI model and routers operate at Layer 3. This distinction indicates that routers and switches use different information to send data from a source to a destination.

The relationship between switching and routing can be compared to local and long-distance telephone calls. When a telephone call is made to a number within the same area code, a local switch handles the call. The local switch can only keep track of its local numbers. The local switch cannot handle all the telephone numbers in the world. When the switch receives a request for a call outside of its area code, it switches the call to a higher-level switch that recognizes area codes. The higher-level switch then switches the call so that it eventually gets to the local switch for the area code dialled.

The router performs a function similar to that of the higher-level switch in the telephone example. Each computer and router interface maintains an ARP table for Layer 2 communication. The ARP table is only effective for the broadcast domain to which it is connected. The router also maintains a routing table that allows it to route data outside of the broadcast domain. Each ARP table entry contains an IP-MAC address pair.

The Layer 2 switch builds its forwarding table using MAC addresses. When a host has data for a non-local IP address, it sends the frame to the closest router. This router is also known as its default gateway. The host uses the MAC address of the router as the destination MAC address.

A switch interconnects segments that belong to the same logical network or subnetwork.
For non-local hosts, the switch forwards the frame to the router based on the destination MAC address. The router examines the Layer 3 destination address of the packet to make the forwarding decision. Host X knows the IP address of the router because the IP configuration of the host contains the IP address of the default gateway.

Just as a switch keeps a table of known MAC addresses, the router keeps a table of IP addresses known as a routing table.
MAC addresses are not logically organized. IP addresses are organized in a hierarchy. A switch can handle a limited number of unorganized MAC addresses since it only has to search its table for addresses within its segment. Routers require an organized address system that can group similar addresses together and treat them as a single network unit until the data reaches the destination segment.

If IP addresses were not organized, the Internet would not work. This could be compared to a library that contained millions of individual pages of printed material in a large pile. This material is useless because it is impossible to locate an individual document. If the pages are identified and organized into books and each book is listed in a book index, it will be a lot easier to locate and use the data.

Another difference between switched and routed networks is switched networks do not block broadcasts.
As a result, switches can be overwhelmed by broadcast storms. Routers block LAN broadcasts, so a broadcast storm only affects the broadcast domain from which it originated. Since routers block broadcasts, they also provide a higher level of security and bandwidth control than switches.

5.5.3.  Routable and routed protocols

A protocol is a set of rules that determines how computers communicate with each other across networks. Computers exchange data messages to communicate with each other. To accept and act on these messages, computers must have sets of rules that determine how a message is interpreted. Examples include messages used to establish a connection to a remote machine, e-mail messages, and files transferred over a network.

A protocol describes the following:

  • The required format of a message
  • The way that computers must exchange messages for specific activities

A routed protocolallows the router to forward data between nodes on different networks.
A routable protocol must provide the ability to assign a network number and a host number to each device. Some protocols, such as IPX, require only a network number. These protocols use the MAC address of the host for the host number. Other protocols, such as IP, require an address with a network portion and a host portion. These protocols also require a network mask to differentiate the two numbers. The network address is obtained by ANDing the address with the network mask.

The reason that a network mask is used is to allow groups of sequential IP addresses to be treated as a single unit.
If this grouping were not allowed, each host would have to be mapped individually for routing. This would be impossible, because according to the Internet Software Consortium there are approximately 250,000,000 hosts on the Internet.

IP as a routed protocol: IP is the most widely used implementation of a hierarchical network-addressing scheme. IP is a connectionless, unreliable, best-effort delivery protocol. The term connectionless means that no dedicated circuit connection is established prior to transmission. IP determines the most efficient route for data based on the routing protocol. The terms unreliable and best-effort do not imply that the system is unreliable and does not work well. They indicate that IP does not verify that data sent on the network reaches its destination. If required, verification is handled by upper layer protocols. 

As information flows down the layers of the OSI model, the data is processed at each layer.
At the network layer, the data is encapsulated into packets. These packets are also known as datagrams.  IP determines the contents of the IP packet header, which includes address information. However, it is not concerned with the actual data. IP accepts whatever data is passed down to it from the upper layers.

Routers use routing protocols to exchange routing tables and share routing information. In other words, routing protocols enable routers to route routed protocols.

Some functions of a routing protocol are as follows:

  • Provides processes used to share route information
  • Allows routers to communicate with other routers to update and maintain the routing tables

Examples of routing protocols that support the IP routed protocol include RIP, IGRP, OSPF, BGP, and are detailed in subchapter “Protocols – detailed”.

5.5.4.  Path determination

Path determination occurs at the network layer.
A router uses path determination to compare a destination address to the available routes in its routing table and select the best path. The routers learn of these available routes through static routing or dynamic routing. Routes configured manually by the network administrator are static routes. Routes learned by others routers using a routing protocol are dynamic routes.

The router uses path determination to decide which port to send a packet out of to reach its destination.
This process is also referred to as routing the packet. Each router that the packet encounters along the way is called a hop. The hop count is the distanced travelled. Path determination can be compared to a person who drives from one location in a city to another. The driver has a map that shows which streets lead to the destination, just as a router has a routing table. The driver travels from one intersection to another just as a packet travels from one router to another in each hop. At any intersection, the driver can choose to turn left, turn right, or go straight ahead. This is similar to how a router chooses the outbound port through which a packet is sent.

The decisions of a driver are influenced by factors such as traffic, the speed limit, the number of lanes, tolls, and whether or not a road is frequently closed. Sometimes it is faster to take a longer route on a smaller, less crowded back street instead of a highway with a lot of traffic. Similarly, routers can make decisions based on the load, bandwidth, delay, cost, and reliability of a network link.

The following process is used to determine the path for every packet that is routed:

  • The router compares the IP address of the packet that it received to the IP tables that it has.
  • The destination address is obtained from the packet.
  • The mask of the first entry in the routing table is applied to the destination address.
  • The masked destination and the routing table entry are compared.
  • If there is a match, the packet is forwarded to the port that is associated with that table entry.
  • If there is not a match, the next entry in the table is checked.
  • If the packet does not match any entries in the table, the router checks to see if a default route has been set.
  • If a default route has been set, the packet is forwarded to the associated port. A default route is a route that is configured by the network administrator as the route to use if there are no matches in the routing table.
  • If there is no default route, the packet is discarded. A message is often sent back to the device that sent the data to indicate that the destination was unreachable.

5.5.5.  Routing tables

Routers use routing protocols to build and maintain routing tables that contain route information. This aids in the process of path determination. Routing protocols fill routing tables with a variety of route information. This information varies based on the routing protocol used. Routing tables contain the information necessary to forward data packets across connected networks. Layer 3 devices interconnect broadcast domains or LANs. A hierarchical address scheme is required for data transfers. 

Routers keep track of the following information in their routing tables:

  • Protocol type– Identifies the type of routing protocol that created each entry.
  • Next-hop associations– Tell a router that a destination is either directly connected to the router or that it can be reached through another router called the next-hop on the way to the destination. When a router receives a packet, it checks the destination address and attempts to match this address with a routing table entry.
  • Routing metric– Different routing protocols use different routing metrics. Routing metrics are used to determine the desirability of a route. For example, RIP uses hop count as its only routing metric. Others use bandwidth, load, delay, or reliability metrics to create a composite metric value.
  • Outbound interfaces– The interface that the data must be sent out of to reach the final destination.

Routers communicate with one another to maintain their routing tables through the transmission of routing update messages. Some routing protocols transmit update messages periodically. Other protocols send them only when there are changes in the network topology. Some protocols transmit the entire routing table in each update message and some transmit only routes that have changed. Routers analyze the routing updates from directly-connected routers to build and maintain their routing tables.

5.5.6.  Routing algorithms and metrics

An algorithm is a detailed solution to a problem. Different routing protocols use different algorithms to choose the port to which a packet should be sent. Routing algorithms depend on metrics to make these decisions.

Routing protocols often have one or more of the following design goals:

  • Optimization– This is the capability of a routing algorithm to select the best route. The route will depend on the metrics and metric weights used in the calculation. For example, one algorithm may use both hop count and delay metrics, but may consider delay metrics as more important in the calculation.
  • Simplicity and low overhead– The simpler the algorithm, the more efficiently it will be processed by the CPU and memory in the router. This is important so that the network can scale to large proportions, such as the Internet.
  • Robustness and stability– A routing algorithm should perform correctly when confronted by unusual or unforeseen circumstances, such as hardware failures, high load conditions, and implementation errors.
  • Flexibility– A routing algorithm should quickly adapt to a variety of network changes. These changes include router availability, router memory, changes in bandwidth, and network delay.
  • Rapid convergence– Convergence is the process of agreement by all routers on available routes. When a network event causes changes in router availability, updates are needed to reestablish network connectivity. Routing algorithms that converge slowly can cause data to be undeliverable.

Routing algorithms use different metrics to determine the best route.
Each routing algorithm interprets what is best in its own way. A routing algorithm generates a number called a metric value for each path through a network. Sophisticated routing algorithms base route selection on multiple metrics that are combined in a composite metric value. Typically, smaller metric values indicate preferred paths.

Metrics can be based on a single characteristic of a path, or can be calculated based on several characteristics. The following metrics are most commonly used by routing protocols:

  • Bandwidth– Bandwidth is the data capacity of a link.
  • Delay– Delay is the length of time required to move a packet along each link from a source to a destination. Delay depends on the bandwidth of intermediate links, the amount of data that can be temporarily stored at each router, network congestion, and physical distance.
  • Load– Load is the amount of activity on a network resource such as a router or a link.
  • Reliability– Reliability is usually a reference to the error rate of each network link.
  • Hop count– Hop count is the number of routers that a packet must travel through before reaching its destination. Each router is equal to one hop. A hop count of four indicates that data would have to pass through four routers to reach its destination. If multiple paths are available to a destination, the path with the least number of hops is preferred.
  • Ticks– The delay on a data link using IBM PC clock ticks. One tick is approximately 1/18 second.
  • Cost– Cost is an arbitrary value, usually based on bandwidth, monetary expense, or other measurement, that is assigned by a network administrator.

5.5.7.  Protocols – detailed

An autonomous system is a network or set of networks under common administrative control, such as the juniper.net domain. An autonomous system consists of routers that present a consistent view of routing to the external world. Routing protocols can be classified in two families of routing protocols: Interior Gateway Protocols (IGPs) – route data within an autonomous system – and Exterior Gateway Protocols (EGPs) – route data between autonomous systems.

IGPs can be further categorized as either distance-vector or link-state protocols.

The distance-vector routing approach determines the distance and direction, vector, to any link in the internetwork. The distance may be the hop count to the link. Routers using distance-vector algorithms send all or part of their routing table entries to adjacent routers on a periodic basis. This happens even if there are no changes in the network. By receiving a routing update, a router can verify all the known routes and make changes to its routing table. This process is also known as “routing by rumour”. The understanding that a router has of the network is based upon the perspective of the adjacent router of the network topology.

Examples of distance-vector protocols include the following:

  • Routing Information Protocol (RIP) – The most common IGP in the Internet, RIP uses hop count as its only routing metric.
    RIP is a distance vector routing protocol that uses hop count as its’ metric to determine the direction and distance to any link in the internetwork. If there are multiple paths to a destination, RIP selects the path with the least number of hops. However, because hop count is the only routing metric used by RIP, it does not always select the fastest path to a destination. Also, RIP cannot route a packet beyond 15 hops. RIP Version 1 (RIPv1) requires that all devices in the network use the same subnet mask, because it does not include subnet mask information in routing updates. This is also known as classful routing.
    RIP Version 2 (RIPv2) provides prefix routing, and does send subnet mask information in routing updates. This is also known as classless routing. With classless routing protocols, different subnets within the same network can have different subnet masks. The use of different subnet masks within the same network is referred to as variable-length subnet masking (VLSM).
  • Interior Gateway Routing Protocol (IGRP) – IGRP is a distance-vector routing protocol developed by Cisco. IGRP was developed specifically to address problems associated with routing in large networks that were beyond the range of protocols such as RIP. IGRP can select the fastest available path based on delay, bandwidth, load, and reliability. IGRP also has a much higher maximum hop count limit than RIP (255). IGRP uses only classful routing.
  • Enhanced IGRP (EIGRP) – This IGP includes many of the features of a link-state routing protocol. Because of this, it has been called a balanced-hybrid protocol, but it is really an advanced distance-vector routing protocol. Specifically, EIGRP provides superior operating efficiency such as fast convergence and low overhead bandwidth.

Link-state routing protocols were designed to overcome limitations of distance vector routing protocols. Link-state routing protocols respond quickly to network changes sending trigger updates only when a network change has occurred. Link-state routing protocols send periodic updates, known as link-state refreshes, at longer time intervals, such as every 30 minutes.

When a route or link changes, the device that detected the change creates a link-state advertisement (LSA) concerning that link. The LSA is then transmitted to all neighbouring devices. Each routing device takes a copy of the LSA, updates its link-state database, and forwards the LSA to all neighbouring devices. This flooding of LSAs is required to ensure that all routing devices create databases that accurately reflect the network topology before updating their routing tables.

Link-state algorithms typically use their databases to create routing table entries that prefer the shortest path. Examples of link-state protocols include Open Shortest Path First (OSPF) and Intermediate System-to-Intermediate System (IS-IS).

·         Open Shortest Path First (OSPF) – is a link-state routing protocol developed by the Internet Engineering Task Force (IETF) in 1988. OSPF was written to address the needs of large, scalable internetworks that RIP could not.

·         Intermediate System-to-Intermediate System (IS-IS) is a link-state routing protocol used for routed protocols other than IP. Integrated IS-IS is an expanded implementation of IS-IS that supports multiple routed protocols including IP.

An example of External Gateway Protocol (EGP) is BGP.

·         Border Gateway Protocol (BGP) exchanges routing information between autonomous systems while guaranteeing loop-free path selection. BGP is the principal route advertising protocol used by major companies and ISPs on the Internet. BGP4 is the first version of BGP that supports classless interdomain routing (CIDR) and route aggregation. Unlike common Internal Gateway Protocols (IGPs), such as RIP, OSP

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