1. Chapter 5 Switching

2. 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)

3. 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

4. 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

5. Bridges and Bridging 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

6. Bridges and Bridging 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

7. Bridges and Bridging 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

8. Bridges and Bridging 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

9. 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

10. Switches and Switching 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

11. Switches and 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

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

13. Switches and Switching 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

14. Switches and Switching 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

15. Switches and Switching 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

16. Switches and 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

17. Switches and Switching 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

18. Switches and Switching 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

19. Switches and Switching 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

20. Switches and Switching 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

21. Switches and Switching 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

22. Switches and Switching 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)

23. 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

24. Spanning Tree Algorithm 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

25. Spanning Tree Algorithm 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

26. Spanning Tree Algorithm 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

27. Spanning Tree Algorithm 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

28. Spanning Tree Algorithm 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

29. Spanning Tree Algorithm 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

30. Spanning Tree Algorithm 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

31. 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

32. Virtual Local Area Networks 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

33. Virtual Local Area Networks 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

34. Virtual Local Area Networks 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

35. Virtual Local Area Networks 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

36. Virtual Local Area Networks 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

37. 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

38. Integrating Switches 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

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

40. Integrating Switches 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