Transcript Switches
Larger Site Networks Chapter 8 Copyright 2001 Prentice Hall Revision 2: July 2001 Multi-hub LANs Multiple hubs Multiple hubs in 10Base-T Multiple hubs in 100Base-TX Multiple hubs in Gigabit Ethernet 3 Hubs Chapter 7 Single-hub or single-switch LAN 200 meter maximum distance span between farthest two stations with UTP Y 100 m X 100 m 200 m 4 Hubs Chapter 8 Multiple-hub LANs To increase maximum distance span 100 m 100 m 100 m 5 Multiple Hubs in 10Base-T Farthest stations in 10Base-T can be five segments (500 meters apart) 100 meters per segment 100m Separated by four hubs 100m 10Base-T hubs 100m 100m 100m 500m, 4 hubs 6 Multiple Hubs in 10Base-T No loops allowed Only one possible path between any two stations AB=1,2,3,4,5 AC=1,2,3,4,6 BC=5,4,6 First two have too many hubs 4 3 6 2 5 1 No! A C No Loops B Multiple Hubs in 10Base-T Practical Limit in 10Base-T is Number of Stations Degradation of service beyond 100 stations Unacceptable service beyond 200 stations Maximum possible span normally embraces more than 200 stations In 10Base-T, the number of stations is the real limit to distance spans Still, it is possible to have a LAN with more than a 200 meter maximum span 7 Multiple Hubs in 100Base-TX Limit of Two Adjacent Hubs in 100Base-TX Must be within a few meters of each other Maximum span is 200 meters Shorter maximum span than 10Base-T 2 Collocated Hubs 100Base-TX Hubs 100m 100m ~200 m 8 Multiple Hubs with 1000Base-T Limit of One Hub in Gigabit Ethernet Maximum span is 200 meters Same limit as 100Base-TX Shorter maximum span than 10Base-T 100m 100m 9 Switched Ethernet Site Networks No Maximum Distance Spans Hierarchies and Single Possible Paths High Speeds and Low Prices Ethernet Switched Networks There is No Limit on the Number of Switches Between the Farthest Stations So there is no maximum distance span Ethernet Switch No Limit On Number of Switches or Span 11 12 Hierarchies Ethernet Switches Must be in a Hierarchy Usually, Fastest Switches are at the Top (Root) New Gigabit Ethernet Campus Switch Root 100Base-X Building Switch 10Base-T Workgroup Switch 13 Hierarchies Only a Single Possible Path (2,1,3,4) Between Any Two Stations 1 Single 3 Possible Path Ethernet Switch B 5 2 A 4 14 Hierarchies Vulnerable to Single Points of Failure Switch or Link (trunk line between switches) Divide the network into pieces Ethernet Switch X X 15 Hierarchies 802.1D Spanning Tree Allows Redundant Links Automatically deactivated to prevent loops Reactivated if there is a failure Ethernet Switch Deactivated Redundant Link 16 Hierarchies Link Aggregation Protocol Allows Multiple Links Between Stations If one link fails, others continue Switch failures or cuts of all links still fatal Ethernet Switch Multiple Links 17 Hierarchies Single Possible Path Simplifies Switch Forwarding Decisions When frame arrives, only one possible output port (no multiple alternative routes to select among) Switch sends frame out that port Simple Forwarding Decision Ethernet Switch 18 Hierarchies Switches allow only a single path for each MAC destination address Associated with a single port on each switch So switch forwarding table has one and only one row for each MAC address Ethernet Switch Address Port A3.. 3 B2.. 5 19 Hierarchies Ethernet switch only has to find the single row that matches the destination MAC address Only has to examine half the rows on average; less if the table is alphabetized Comparison at each row is a simple match of the frame and row MAC addresses; much less work that row comparison in routers Overall, this is much less work than routers must do Address Port A3.. 3 B2.. 5 Box More on Switched Ethernet Switch Learning Purchase Considerations VLANs Intelligent Switched Network Design 21 Switch Learning Box Situation: Switch with NIC A1-33-B6-47-DD-65 (A1) on Port 1 NIC BF-78-C1-34-17-F4 (BF) on Port 2 NIC C9-34-78-AB-DF-96 (C9) on Port 5 Switch Forwarding Table is Initially Empty Ethernet Switch A1 BF Address Port C9 22 Switch Learning Box A1 on Port 1 Sends to C9 on Port 5 Switch does not know port for C9 Broadcasts the frame, acting as a hub Notes from source address that A1 is on Port 1 Adds this information to switch forwarding table Ethernet Switch A1 BF Address Port A1 1 C9 23 Switch Learning C9 on Port 5 Sends to A1 on Port 1 Box Table shows that A1 is on Port 1 Switch only sends out Port 1: Acts like a switch! Source address shows that C9 is on Port 5 Switch adds this information to forwarding table Ethernet Switch A1 BF Address Port A1 1 C9 5 C9 24 Switch Learning Box Every Few Minutes, Switch Erases Switch Forwarding Table To eliminate obsolete information Relearning is very fast Ethernet Switch A1 BF Address Port C9 25 Switch Learning Box Switches Can be in Hierarchy Switches only learn that stations are out certain ports Do not Learn of switch in Between Switch A Port 1 Address A1 BF C9 Switch B A1 BF C9 Port 1 1 1 Switch Purchasing Decisions Box Maximum Number of MAC address-port entries Small switches may not be able to store many MAC addresses For addresses that cannot be stored, switch must act like a hub, broadcasting and so creating latency Address Port A1 1 C9 5 26 Switch Purchasing Decisions Box 27 Queue Size Incoming frames are placed in queues if they cannot be processed immediately May have several queues If queues are too small, frames will be lost during brief peak loads Frames Input Ports Queues Switch Matrix Output Ports Switch Purchasing Decisions Box Switching Matrix Receives input from multiple input ports, via queues Switches each frame to the correct output port Switch Matrix Frames Input Ports Queues Output Ports 28 Switch Purchasing Decisions Box Switching Matrix Aggregate Throughput The number of bits it can switch per second Nonblocking if aggregate throughput equals the number of ports times the speed of the ports. Can handle the load even if all ports are receiving input simultaneously Frames Input Ports Queues Switch Matrix Output Ports 29 Switch Purchasing Decisions Box Nonblocking Calculation 12 input ports 100 Mbps each Maximum possible input: 1,200 Mbps (1.2 Gbps) Nonblocking switch needs 1.2 Gbps of aggregate switching capacity Frames Input Ports Queues Switch Matrix Output Ports 30 Switch Purchasing Decisions Reliability through Redundancy Box Redundant power supplies and cooling fans May even have redundant switch matrix for backup Frames Input Ports Queues Switch Matrix Output Ports 31 Switch Purchasing Decisions 32 Box Manageability Can be managed remotely from the network administrator’s desk Network administrator can check on status of switch Network administrator can modify how the switch functions We will see remote management in Chapter 12 Remote management greatly reduces labor Frames Input Ports Queues Switch Matrix Output Ports Ethernet Virtual LANs Box 33 Hubs versus Switches Hubs broadcast bits out all ports Switches usually send a frame out a one port More fundamentally In unicasting, a message is only intended to go to one machine, as when a client sends a message to a server Switches assume unicasting; it is the basis for sending a frame out a single port 34 Ethernet Virtual LANs Box Broadcasting Sometimes, station needs to send a frame to all other stations; this is broadcasting For example, servers send a frame to advertise their presence with a broadcast message every minute or so 35 Ethernet Virtual LANs Box Broadcasting with Ethernet Switches Broadcaster sets the destination MAC address to all ones (48 ones) When switch broadcast such frames Can create congestion Broadcast Frame Ethernet Switch Ethernet Virtual LANs 36 Box In multicasting, messages are only intended to go to some stations For instance, from a server only to the client PCs it serves If Ethernet switches can implement multicasting, traffic overload would be avoided Multicast Frame Ethernet Virtual LANs 37 Box Ethernet switches do implement multicasting A server and the clients it serves are treated as a single virtual LAN (VLAN) Can only communicate among themselves, as if they were on their own LAN Marketing VLAN Server Frame Marketing VLAN Client Ethernet Virtual LANs 38 Box VLAN Benefits VLANs reduce traffic on the switched network Other benefits New They provide weak security because clients cannot reach all servers (easily defeated but good first line of defense) VLANs give ease of management because if a user changes organizational membership, VLAN membership is easily changed centrally Ethernet Virtual LANs 39 Box VLAN Problems VLANs have not been standardized A network of switches from different vendors cannot implement VLANs Standardization is beginning Using tagging (Chapter 7) Tag Control Information field has a 12-bit VLAN ID (VID) number, allowing 212 VLANs to be identified Ethernet Virtual LANs VLAN Interconnection For cross-VLAN communication, routers actually connect multiple switches Ethernet Switch 40 Box When are Frames Forwarded? Box Cut-Through Ethernet Switches Forward after seeing only part of a frame Minimum is destination address to determine output port May need to see tag fields for priority, VLAN May wait until 46 octets of data plus PAD Fast operation Forward the Frame FCS PAD Data Len SA DA SFD Pre 41 When are Frames Forwarded? Box Store-and-Forward Ethernet Switches Forwarded only after receiving full frame Allows error checking (CRC field) Hybrid Ethernet Switches Start in cut-through mode but check errors If many errors, go to store-and-forward mode Forward the Frame FCS PAD Data Len SA DA SFD Pre 42 43 Bad Switch Organization Box One Server for All Clients All traffic goes to and from server Bottlenecks: no simultaneous conversations No major benefits compared to hub Ethernet Switch Bottleneck 44 Bad Switch Organization Box Multiple Servers for Clients Allows simultaneous conversations Brings switching’s main benefit Ethernet Switch Congestion, Latency, and Remedies Peak Loads Congestion and Latency Overprovisioning Capacity Priority Quality of Service Traffic Shaping The Peak Load Problem Capacity Sufficient Most of the Time Otherwise, get bigger switches and trunk lines! Brief Traffic Peaks can Exceed Capacity Frames will be delayed in queues or even lost if queue gets full Capacity Traffic Peak 46 47 Overprovisioning Overprovisioning: Install More Capacity than Will be Needed Nearly All of the Time Wasteful of capacity Still, usually the cheapest solution today because of its simplicity Overprovisioned Capacity Traffic Peak 48 Priority Assign Priorities to Frames High priority for time-sensitive applications (voice) Low priority for time-insensitive applications (e-mail) In traffic peaks, high-priority frames still get through Low-priority applications do not care about a brief delay for their frames Low-Priority Frame Waits Briefly High-Priority Frame Goes 49 Priority Standardizing Priority 802 Tag Fields are standardizing priority for Ethernet and other 802 LAN technologies Priority is also being standardized by the IETF for IPv4 and IPv6 (Diffserv for differentiated services) 802 and IETF are harmonizing efforts for end-to-end priority Low-Priority Frame Waits Briefly High-Priority Frame Goes Full Quality of Service (QoS) Priority Makes no Quantitative Promises of Maximum Latency, etc. Quality of Service (QoS) Makes Quantitative Promises for such things Reserves capacity; if not used, this capacity is wasted Low or No Guarantee High Guarantee 50 Full QoS is Not a Cure-All 51 Traffic with no guarantees will not benefit It may not get through at all Often, voice traffic is given strong guarantees while data traffic is given low or no guarantees Low or No Guarantee High Guarantee Reserved Capacity 52 Traffic Shaping Overprovisioning, Priority, and QoS are Ways to Cope with Brief Congestion Traffic Shaping Prevents recognizes that congestion is beginning, acts to stop it Switch Tells Some Sources to Slow or Stop if Congestion is Beginning, based on Policies Source A Source B Slow or Stop Continue Network ATM Switches Cells Scalable QoS Perspective Virtual Circuits ATM Switches Asynchronous Transfer Mode Basic Standards Set by ITU-T Partner with ISO in OSI standards ATM standards developed within OSI architecture ATM Forum Sets Detailed Standards Group of mostly ATM vendors Moves quickly Also tests for interoperability 54 55 ATM Switches Has fixed-length frames are called cells Always 5 octet header, 48 octet payload, So always 53 octets total Small cell reduces latency (delay) at each switch Switch may only be able to send frame out after whole frame is read With short frames, this is not a problem ATM Cell Payload (48 octets) Header (5 octets) ATM Switches Highly Scalable Comparable to Ethernet Very sophisticated Offers quality of service guarantees Very expensive to purchase and manage ATM has high overhead (extra characters) 5 overhead octets for 48 data octets (10% overhead) Actually even worse (see Module E) 56 ATM Switches 57 Unfortunately, very expensive Has lost the desktop It is usually cheaper to use high-capacity Ethernet switches with overprovisioning, so that latency does not grow to the point where QoS is critical In LANs, usually used only where service quality is critical, typically when voice is being carried. Even losing there. ATM QoS Categories ATM Offers Varying Levels of QoS Parameters Peak cell rate (maximum burst speed) Maximum burst size (bits per burst) Sustainable cell rate (always allowed) Cell Delay Variation Tolerance (CDVT): how exact cell-to-cell timing is; Critical for voice and video Cell Loss Ratio: Losses during transmission 58 ATM QoS Categories ATM Offers Varying Levels of QoS For Voice and Video ITU-T Class A ATM Forum Service Category: Constant Bit Rate (CBR) Low latency Low Cell Delay Variation Tolerance Strong guarantees for voice and video! 59 ATM QoS Categories For IP and LAN Data ITU-T Class D Several ATM Forum Service Categories Developed several categories over Time Available bit rate (ABR) weak: send if capacity is available Unspecified bit rate (UBR) weak: simpler than ABR, but can get almost no share of capacity Guaranteed frame rate (GFR) gets roughly fair share of capacity during congestion 60 ATM QoS Categories 61 For IP and LAN Data Several ATM Forum Service Categories ABR, UBR, and even GFR give very low status to data transmission Not even as good as Ethernet priority of service Yet costs far more So ATM QoS makes little sense if used entirely for data • Has other data transmission benefits, however ATM QoS Categories Other Categories For Videoconferencing May need momentary bandwidth increase if there is a burst of motion on the screen Needs Low Cell Delay Variation Tolerance ATM: Class B ATM Forum Service Category: Variable Bit Rate-Real Time (VBR-RT) Not widely used or implemented 62 ATM QoS Categories Other Categories For Connection-Oriented Data ATM: Class C ATM Forum Service Category: Variable Bit Rate-Not Real Time (VBR-NRT) Most data not connection-oriented Not widely used, implemented 63 ATM Switches: Virtual Circuits Often Arranged in a Mesh But all traffic between two stations still is consigned to a path called a virtual circuit that is set up before the first frame transmission ATM Cell Virtual Circuit 64 65 ATM Switches Virtual Circuits Mean that there is Only a Single Possible Path between Any Two Stations Virtual circuits simplify switch operation and so lower switch cost ATM Cell Virtual Circuit ATM Switches 66 Permanent Virtual Circuits (PVCs) Set up once, for each pair of sites Simplest and least expensive administratively because rarely changed Most widely used form of virtual circuit Switched Virtual Circuit (SVC) Set up at time of use Flexible but expensive 67 ATM Switches ATM Frame Header Does NOT have a destination address field Instead, has two fields that together contain a hierarchical virtual circuit number Like a route number on a bus--names the route, not the destination ATM Header Virtual Circuit Number 68 ATM Switches Hierarchical Virtual Circuit Number Virtual Path Identifier Higher-level number; Often specifies a site Virtual Channel Identifier Lower-level number; Often specifies a computer at a site ATM Header Virtual Circuit Number ATM Switches 69 Virtual Circuit All traffic between two sites can be given the same VPI number But difference VCI values Switch needs only one VPI table entry for all this traffic Dramatically reduces number of table entries in switches between sites and therefore makes lookups very fast 70 ATM Switches ATM Reliability Virtual circuit reduces communication to a single path If a switch or trunk line along the path fails, communication stops But ATM switches also have addresses, which are used to set up a new virtual circuit fairly rapidly Not in Book Switches Versus Routers Switches Routers Fast Slow Inexpensive Expensive No benefits of alternative routing benefits of alternative routing “Switch where you can; route where you must” 71 72 Early Site Networks Organization LANs (subnets) based on hubs Routers link hubs Hierarchy of Routers Router Hub The Switching Revolution Switches Push Routers to the Edge Switches replace most routers in site networks Because switches are cheaper than routers Routing’s sophistication is still needed at the edge External Router Switch 73 The Switching Revolution 74 Layer 3 Switches Traditional switches operate at Layer 2; Switch based on MAC addresses Layer 3 switches switch based on internet layer IP addresses External Layer 3 Switch The Switching Revolution Layer 3 Switches Layer 3 switches are replacing many Layer 2 switches in site networks because of their ability to switch based on IP addresses External Layer 3 Switch 75 The Switching Revolution Layer 3 Switches versus Routers Layer 3 switches are much faster than routers Layer 3 switches cost less than routers External Layer 3 Switch 76 77 The Switching Revolution Layer 3 Switches versus Routers At the internet layer, Layer 3 switches normally only support IP and sometimes IPX; Routers route many more internet layer protocols, including those of AppleTalk, SNA, and others At the data link layer, Layer 3 switches normally support only Ethernet on LANs. Routers support many Layer 2 LAN protocols. Router Layer 3 Switch The Switching Revolution Layer 3 Switches versus Routers Layer 3 switches rarely support Layer 2 WAN protocols Routers usually are still needed at the edge of the site network, to communicate with external links External Layer 3 Switch 78 The Switching Revolution Routers 79 Layer 3 Switches Forward based on IP addresses and other internet layer addresses Forward based on IP addresses, sometimes IPX addresses Expensive and slow Inexpensive and Fast Handle multiple internet layer protocols Do not handle multiple internet layer protocols Handle multiple LAN and WAN subnet protocols Do not handle multiple LAN and WAN subnet protocols The Switching Revolution Layer 4 Switches Examine port fields in TCP and UDP These fields describe the application Therefore, can switch based on application (to give priority by application, etc.) Layer 4 Switch 80