Larger Site Networks Part 1

2 Small Site –Single-hub or Single- Switch Ethernet LANs Large Site –Multi-hub Ethernet LANs –Ethernet Switched Site Networks –Congestion, Latency, and solutions –ATM LANs –Routers, Layer 3, and Layer 4 Switches

Multi-hub LANs Multiple hubs Multiple hubs in 10Base-T Multiple hubs in 100Base-TX Multiple hubs in Gigabit Ethernet

4 Hubs Small LANs –Single-hub or single-switch LAN –200 meter maximum distance span between farthest two stations with UTP 100 m X Y 200 m

5 Hubs Large LANs –Multiple-hub LANs –To increase maximum distance span 100 m

6 Two Hubs in Base-T 1. Station X transmits bit to Hub A –Hub operates at the physical layer (one bit at a time) 2. Hub A broadcasts bit out all ports A B X Y

7 Two Hubs in Base-T 3. Uplink Port sends bit to Hub B –Uplink ports are marked by an “X” 4. Hub B broadcasts bit to all attached stations, including Y Note that all stations on both hubs receive the bit broadcast almost simultaneously A B X Y 3 Uplink Port

8 Multiple Hubs in 10Base-T Farthest stations in 10Base-T can be five segments (500 meters apart) –100 meters per segment –Separated by four hubs 100m 500m, 4 hubs 10Base-T hubs

9 Multiple Hubs in 10Base-T No loops allowed –Only one possible path between any two stations No Loops A B C AB=1,2,3,4,5 AC=1,2,3,4,6 BC=5,4,6 First two have too many hubs No!

10 Multiple Hubs in 10Base-T No loops allowed –If hub or link fails, network is divided No Loops A B C No!

11 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

12 Multiple Hubs in 100Base-TX Limit of Two Hubs in 100Base-TX –Must be within a few meters of each other –Maximum span is 200 meters –Shorter maximum span than 10Base-T 100m 2 Collocated Hubs 100Base-TX Hubs ~200 m

13 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

14 Multiple Hubs in Perspective 10Base-T Hubs –500 meter maximum distance span with UTP –Farther with some optical fiber links –However 10Base-T is limited by the number of stations it can support –So the maximum practical distance span is really much smaller 100Base-TX Hubs and Gigabit Ethernet Hubs –200 meter maximum distance span

Switched Ethernet Site Networks No Maximum Distance Spans Hierarchies and Single Possible Paths High Speeds and Low Prices

16 Ethernet Switched Networks There are Distance Limits Between Pairs of Switches –100 meters with UTP –Longer with optical fiber But There is No Limit on the Number of Switches Between the Farthest Stations –So there is no maximum distance span Maximum Separation 100 m with UTP Longer with optical fiber Ethernet Switch

17 Hierarchies Ethernet Switches Must be Arranged in a Hierarchy –Root is the top-level Usually, Fastest Switches are at the Top (Root) –Sizes given are only examples Gigabit Ethernet Campus Switch 100Base-X Building Switch 10Base -T Workgroup Switch Root

18 Hierarchies Only a Single Possible Path (2,1,3,4) Between Any Two Stations Single Possible Path Ethernet Switch A B 2

19 Hierarchies Vulnerable to Single Points of Failure –Switch or Link (trunk line between switches) –Divide the network into pieces X X Ethernet Switch

20 Hierarchies 802.1D Spanning Tree Allows Redundant Links –Automatically deactivated to prevent loops Deactivated Redundant Link Ethernet Switch

21 X Hierarchies 802.1D Spanning Tree Allows Redundant Links –Automatically reactivated in case of failure –Slow and not completely effective Reactivated Redundant Link Ethernet Switch Failure

22 Hierarchies Link Aggregation Protocol Allows Multiple Links Between Stations –If one link fails, others continue –Switch failures or cuts of all links still fatal Multiple Links Ethernet Switch

23 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

24 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 A3.. B2.. Port 3 5

25 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 A3.. B2.. Port 3 5

26 Hierarchies Overall, then, Ethernet switch forwarding is much simpler than router forwarding –So Ethernet switches are both cheaper and faster than routers Simple Forwarding Decision Ethernet Switch

27 Hierarchies Router networks are meshes, allowing multiple alternative routes to the destination host –Each alternative route is represented by a row in the router forwarding table –Router must evaluate each row for each packet –For each row, may have to compute match length, and metric –After looking at all rows, must choose the best alternative route

More on Switched Ethernet Switch Learning Purchase Considerations VLANs Intelligent Switched Network Design

29 Switch Learning Switch Forwarding Table has Address- Port Pairs Manual Entry is Too Time Consuming –Many addresses –Addresses change Solution: Learn addresses automatically Address A3.. B2.. Port 3 5

30 Switch Learning Situation: Switch with –NIC A1-33-B6-47-DD-65 (A1) on Port 1 –NIC BF-78-C F4 (BF) on Port 2 –NIC C AB-DF-96 (C9) on Port 5 Switch Forwarding Table is Initially Empty AddressPort A1BFC9 Ethernet Switch At Start

31 Switch Learning 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 Address A1 Port 1 A1BFC9 Ethernet Switch After Transmission

32 Switch Learning C9 on Port 5 Sends to A1 on Port 1 –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 Address A1 C9 Port 1 5 A1BFC9 Ethernet Switch After Transmission

33 Switch Learning Every Few Minutes, Switch Erases Switch Forwarding Table –To eliminate obsolete information –Relearning is very fast AddressPort A1BFC9 Ethernet Switch Erased

34 Switch Learning Switches Can be in Hierarchy –Switches only learn that stations are out certain ports –Do not Learn of switch in Between A1BFC9 Address A1 BF C9 Port 1 Port 1 Switch A Switch B

35 Switch Purchasing Decision Hub Purchases are Simple –Number of Ports and Port Speeds Switch Purchases are More Complex –Port speed –Number of ports –Maximum number of MAC-Port pairs in forwarding table –Queue sizes –Switching matrix aggregate throughput Blocking or nonblocking –Reliability –Manageability