Chapter 5 Switching. Introduction Look at: –Bridges and Bridging(5.1) –Switches and...

Chapter 5

Switching

Introduction

• Look at:– Bridges and Bridging(5.1)– Switches and Switching(5.2)– Spanning Tree Algorithm(5.3)– Virtual Local Area Networks (VLANs)(5.4)– Integrating Switches(5.5)

Introduction

• Moving electronic signals from one interface to another is the term used to describe switching

• Ethernet networks started as a single segment bus topology with all clients able to receive the electronic signals of all other clients on their network

Bridges and Bridging

• A bridge operates by simply plugging it into the wall and connecting network devices to it

• Bridging can isolate groups of network devices by segmenting the wire signals of one group from another group

• Each of the separate groups is referred to as a collision domain

• Fewer devices competing per network segment mean better communication


• There are several different implementations of bridging:– Transparent bridging– Source route bridging– Translational bridging

• Each has a specific purpose in controlling traffic flows and collisions


• In transparent bridging, the network devices are unaware of the presence of the bridge

• The bridge forwards traffic based on a first-in, first-out (FIFO) basis and combines the transmissions allowing multiple clients to communicate with the Server

• Network transmissions take a small amount of time, and the bridge must be fast enough to keep up


• Source route bridging (SRB) is designed for use with Token Ring networks

• In Token Ring networks, rings and bridges have an assigned numeric value

• The client determines the best path through the Layer 2 network

• The default behavior is to use the path identified in the first response received


• Translational bridging contains a mixture of Ethernet and Token Ring clients

• Very few standards exist for translational bridging, and implementations vary between vendors

• Some translational bridging implementations may not work between vendors

• It’s important to research compatibility if you plan on implementing translational bridging

Switches and Switching

• Switching offers the following improvements over bridging:– Higher port density– Faster packet processing capabilities– Quality of service (QoS) capabilities– Use of virtual LAN (VLAN) technology

• Each port is a single bridging device creating its own collision domain


• Switches achieve faster packet processing capabilities than bridges in a number of different ways:– Application-specific integrated circuits – Better CPUs – Cut-through switching – Fragment-free switching


• Circuit switching is the oldest form of switching

• It establishes calls over the most efficient route available at the time

• It can be wasteful compared to other types of communication

• It is wasteful because the circuit remains active even if the end stations are not currently transmitting


• Examples of circuit-switched networks include:– Asynchronous Transfer Mode (ATM)– Integrated Services Digital Network (ISDN)– Leased digital line– T-1– Analog dial-up line


• Types of switching include: – Time-division which is the switching of

time-division multiplexed (TDM) channels by shifting bits between time slots in a TDM frame

– Space-division in which single transmission-path routing is accomplished using a switch to physically separate a set of matrix contacts or cross-points


• Types of switching include: – Time-space division which precedes each

input trunk in a crossbar with a TSI, and delays samples so that they arrive at the right time for the space-division switch’s schedule

– Time-space-time division is where data passes through a space switch to create circuits for TDM outlets


• Packet switching is implemented by protocols that rearrange messages into packets before sending them

• Each packet is then transmitted individually • Because packets are send via different

routes, they may not arrive in the order in which they were originally sent

• Technologies such as Cellular Digital Packet Data and Voice over IP is currently implementing voice communication using packet switching


• In packet switching protocols, a port is represented by a value between 1 and 65535

• The port number indicates the type of packet• Port mapping, or port address translation is a

process where packets arriving for a particular socket can be translated and redirected to a different socket

• Port mapping is necessary only for incoming transmissions, not for returning traffic


• Blocking occurs when a network is unable to connect stations to complete a circuit

• In packet switching, blocking is caused by congestion

• Congestion management is used to help ensure that the blocked packet is not lost

• Congestion management is implemented using buffers or queues


• Asynchronous transfer mode (ATM) is a high-speed, connection orientated packet switching technique

• It uses short, fixed-length packets called cells • ATM is connection oriented• The switching elements have pre-defined

routing tables to minimize the complexity of single switch routing


• Switching fabric is the combination of hardware and software that transfers data coming into a node to the appropriate port on the next node

• The bandwidth of a switching fabric is defined by its data width in bits and the speed at which it can transmit these bits

• Switching fabric includes data buffers and shared memory


• A crossbar switch is a device that directly switches data between an input port and an output port without sharing a bus with any other data

• Crossbar topology is similar to bus topology• There is only one path that all devices share• A crossbar switch environment offers more

flexibility and greater scalability


• A broadcast connection is when a device simultaneously sends data to more than one receiving device

• A broadcast connection in a non-blocking multistage network that has an input port connected simultaneously to several unused output ports

• The network must be non-blocking for broadcast assignments


• A switching element is the basic building block of a switch

• Switching fabric consists of switching elements that facilitate a particular switching mechanism

• When a large number of switching elements are connected together in a network, this is a multistage interconnection network (MIN)

Spanning Tree Algorithm

• The Spanning Tree Algorithm (STA) was created to overcome the weakness inherent in bridges

• This weakness is the flooding behavior of broadcast, multicast and unknown traffic types

• Problems occur with broadcasts when you configure two or more bridges in a loop


• By interconnecting multiple switches with redundant paths, you overcome problems with faulty cables or port failures

• Another less well-known side effect of a loop is the corruption of the forwarding tables on all the switches

• It would take very little time before Layer 2 broadcast loops completely destroy the functionality of a network


• The Spanning Tree Protocol (STP) prevents bridging loops by identifying a preferred path through a series of looped bridges

• Administrators can provide redundancy and fault-tolerance by wiring a loop, and then using STP turn off ports that would cause loops to occur

• If a primary link fails, STP will reactivate the back-up port allowing normal operation of the network to continue


• The STP process is accomplished by:– Election of a root bridge– Each bridge, when first turned on, begins

the election process by sending a packet called a bridge protocol data unit (BPDU)

– Each bridge believes itself to be the root until it receives a BPDU with a lower root bridge ID


• The STP process is accomplished by:– Identification of a root port– Once the root bridge is determined, each

non-root bridge finds the best path to the root using path cost

– Path cost is a numeric value that bridges use to determine the preference of a given path. It is derived by taking 1000 MBps and dividing it by the speed of the link


• The STP process is accomplished by:– Identification of a designated port– After the switches determine the root

bridge and the best path to the root, the designated port is determined

– When this step finishes, the loop is broken because one of the switches is blocking traffic on one of it’s ports


• The switch continues to receive traffic on the port and evaluates each frame

• All user traffic is filtered including unicasts, broadcasts, and multicasts frames

• Anytime a new switch is plugged in, the process of determining the root bridge occurs


• Most bridges and switches go through the following phases when activating ports to determine which system is the root bridge and which ports are active or disabled– Blocking – Listening – Learning – Forwarding

Virtual Local Area Networks

• Prior to the concept of VLANs the only way for administrators to block broadcast traffic was to implement a router

• VLANs represent a solution allowing administrators to group ports on their switches so that broadcast traffic is only passed among ports within the group

• Once a VLAN is established the ports will only flood broadcast traffic to their own members


• Once the VLAN is created, it is necessary to have a Layer 3 router

• The clients on separate VLANs will no longer communicate with each other

• The network devices can no longer discover each other by using ARP broadcasts


• The basic process of creating a VLAN involves tagging the inbound packet with a VLAN ID

• Tagging is accomplished in one of two ways:– Frame insertion– Frame encapsulation


• Frame insertion involves inserting a small identifier into the frame as it is received at the switch’s port

• In Frame encapsulation, the entire frame is simply encapsulated inside a VLAN ID header and checksum

• Encapsulation is still a proprietary method of VLAN tagging but is gaining in popularity

• It is fast at frame insertion and less prone to damaging the underlying frame


• VLAN trunking uses the VLAN concept and applies it to more than one switch

• The solution for routing frames is trunking or making one port forward traffic for all VLANs

• Doing so cuts port usage down considerably • While VLANs appear complex at first, they

offer a great deal of flexibility for administrators


• If a particular device needs to be moved from one broadcast domain to another, only the port configuration on the switch needs to be changed, not the physical location of the device

• Fewer ports are used on the routers, saving money and configuration time

Integrating Switches

• Integrating hubs and switches provide a migration path as networks are upgraded from hub environments to switching topologies

• This can be done by connecting a crossover network cable from one of the hub ports into a switch port

• If the hub or switch includes a crossover port then you use a regular networking cable


• All ports on a hub are in the same broadcast and collision domain

• All ports on a switch are in the same broadcast domain but each port is its own collision domain unless VLANs are used

• Connect your servers and other high traffic workstations directly to a switch so that they are free from collisions


• Some switches can allow you to add functions such as: – Packet filtering– Encryption– Auditing/Accounting– Tunneling– Routing


• By adding these directly to the switch, frames are copied into the memory of the switch once and then forwarded on to the next destination without further copying

• This allows a switch to route frames at wire speed

• An additional side benefit of integrating switches with routing technology is the simplification of network design

Chapter 5 Switching. Introduction Look at: –Bridges and Bridging(5.1) –Switches and...

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