Switching in an Enterprise Network Introducing Routing and Switching in the Enterprise– Chapter 3
Objectives • Compare the types of switches used in an enterprise network. • Explain how Spanning Tree Protocol prevents switching loops. • Describe and configure VLANs on a Cisco switch. • Describe and configure trunking and Inter-VLAN routing. • Maintain VLANs in an enterprise network.
Introducing switching and network segmentation •Topics: • The reliance on switches in network design • The switch as an adaptable Layer 2 device that moves traffic based on MAC addresses • Content addressable memory (CAM) as the technology for maintaining the MAC address table • The role of switches in micro-segmenting domains to a single port • Multilayer switching that combines hardware-based switching and routing in the same device • The two major methods for switching: store and forward, and cut-through • The need for securing switches
Compare the Types of Switches Used in an Enterprise Network • Switching and network segmentation • Content addressable memory (CAM) • Virtual circuits
Discussion 01 • You have probably seen the advertisements for Internet service, “with up to a blazing-fast 12 Mbps” and then the fine print “Many factors affect speeds. Actual speeds may vary and are not guaranteed.”2 Advertised network speeds reflect a “best case scenario”. • Under some circumstances, wire speed represents the best-case scenario for a switched network. Wire speed represents the hypothetical maximum data transmission rate of a cable or other transmission medium. Wire speed is rarely achieved outside of a network device. CPU limitations, disk read/write overhead, or contention for resources can reduce the speed of transmission over a network.3 2) Comcast. (2007). Comcast High Speed Internet. Retrieved on September 10, 2007 from http://www.comcast.com/highspeedoffer-s/?CMP=KNC 1TO1Q3GOOGLE30&s_kwcid=comcast%20internet|751518367. 3) http://en.wikipedia.org/wiki/Wire_speed
How do you find a MAC address? • What other devices in this room also have MAC addresses? • How do we discover the MAC addresses of other devices on the network?
What about CAM? • DEF: A switch moves traffic based on MAC addresses. Each switch maintains a MAC address table in high-speed memory, called content addressable memory • What makes CAM different from RAM? • In RAM, the user (application) supplies a memory address and the RAM returns the data word stored at that address. • CAM functions as the reverse of RAM. • In CAM, the user supplies the data word and the CAM searches its memory to see if it has the data word. • Thinking about network hardware, what kind of device might have CAM and what data might it contain?”
Investigations into CAM • Why does it make sense to remove (delete) entries from the MAC address table if they are not used within a certain period of time? • How does a switch handle a broadcast frame? Reflection #1, Investigations into CAM
Forward or Flood • The larger the collision and broadcast domains the more likely that network traffic will be affected. • Simply put – the more devices participating in a collision domain the more collisions occur. • This is similar to what happens to drivers at a rotary or roundabout
Microsegmentation • How does a switch process traffic differently than a hub?
ASICs • Application-Specific Integrated Circuit • Taking A Look At The Basics Of ASICs • Smith, Michael. (June 1997) Application-Specific Integrated Circuits. Retrieved on September 16, 2007 from. ASICs can consolidate the work of many chips into a single, smaller, faster package, reducing manufacturing and support costs while boosting the speed of the device built with them. ASIC technology is now so advanced that many functions traditionally implemented in software can be migrated to ASICs.
Routing with a Level 3 Switch • A Layer 3 switch is a high-performance device for network routing. • Layer 3 switches actually differ very little from routers. • A Layer 3 switch can support the same routing protocols as network routers do. Both inspect incoming packets and make dynamic routing decisions based on the source and destination addresses inside. Both types of boxes share a similar appearance
Compare the Types of Switches Used in an Enterprise Network • Hardware-based Layer 2 switching • Software-based Layer-3 (multilayer) switching
Which is faster? • Routing has become much faster and often finds a route to an unknown host faster than the techniques used by standard Layer 2 switches. • Layer 2 switches have wire speed performance, and Layer 3 routers have higher latency. It would seem that switches should always be faster… hint - unknown host
Compare the Types of Switches Used in an Enterprise Network • Store and forward switching • Cut-through switching • Fast-forward • Fragment-free
Today, most Cisco LAN switches rely on the store-and-forward method for switching.
Compare the Types of Switches Used in an Enterprise Network • Switch physical security • Switch access security Complete the lab in packet tracer
Redundancy in a Switched Network • Redundancy is crucial in many areas of business and health care. • Few people would want to undergo open-heart surgery if there was only one heart/lung machine keeping them alive while their heart was stopped, nor would a multi-national publicly traded company have only one set of financial records. • Skydivers have reserve chutes in case the main chute does not open; amusement park rides have manual and automatic seatbelts on the same rides to protect against human error. • Think of your favorite sports team. Does every player on the team get to participate on every play? Why is it important for a team to have ‘depth’ at certain positions?
Redundancy in a Switched Network • Networks require redundancy as well. • In the first quarter of 2007, Amazon.com generated a daily profit of $1.22 million per day, which equals $50,833 an hour or nearly a $1,000 a minute.¹ • If the network goes down for an hour, once a week every week for a year, the total loss of profit is $2,643,316. • Do you think that Amazon.com has redundant networks in place?
E-Bay example • One company that did not, but now does, is eBay. • “Prior to June 10, 1999, eBay experienced significant network failures and has since suffered additional outages, which together totaled more than 70 hours of outages in the first seven months of the year.¹ • During the two day June crisis, eBay's stock crashed to $47 from $135, wiping out $5.7 billion of market capitalization, and dipped below $80 in early August before rising again to the $130 range.¹ • Experts assessing the cause of the disaster cite eBay's failure to build redundant, scalable web architecture.”²
E-Bay example - references • 1) CNN Money.com. (September 14, 2007). Amazon.com Inc. Retrieved on September 8, 2007 from http://money.cnn.com/quote/financials/financials.html?symb=AMZN. • 2) Cuomo, Andrew. (n.d.). Online Brokerage Industry Report. Retrieved on September 8, 2007 from http://www.oag.state.ny.us/investors/1999_online_brokers/points_reference.html. Reflection #2, Redundancy Failures
Explain How Spanning Tree Protocol Prevents Switching Loops • Redundancy in network equipment • Redundant network links • Dangers of switching loops • Broadcast storms
Lets get paid double-checks • The module mentions the problems within the network caused by multiple frame transmissions. • Imagine the real world problems caused by multiple frame transmissions – duplicate paychecks, duplicate invoices for the same purchase, online banking with duplicate deposits or withdrawals, stock market transactions, etc. It is not only wasted bandwidth or CPU time we have to be concerned with – it is the very real chance that important transactions may be duplicated if multiple frames are sent.” • MAC database instability can also result from a switched loop network. Ask students, "What are the results of the MAC database being incorrect?"
MAC Database Instability If two switches on the same network can cause so many problems is there any way to support redundancy?
Explain How Spanning Tree Protocol Prevents Switching Loops • Create a loop-free logical topology • Potential loop detection and port blocking • Redundancy without switching loops
Explain How Spanning Tree Protocol Prevents Switching Loops • Determining a root bridge • Bridge ID (BID) • Root ports, designated ports, and blocked ports
BPDUs • BPDUs are frames that multicast every 2 seconds to all other switches. BPDUs contain information such as: • Identity of the source switch • Identity of the source port • Cumulative cost of path to root bridge • Value of aging timers • Value of the hello timer
STP port states – 4 - Forwarding A fifth state, disabled, indicates that the administrator has shut down the switch port.
Root Bridges • Determining a root bridge • Bridge ID (BID) • Root ports, designated ports, and blocked ports
Selection of root bridge • The root bridge does not need to be the most “powerful”; rather, it needs to be centrally located • The root bridge is based on the lowest BID value. Since switches typically use the same default priority value <32768>, the switch with the lowest MAC address becomes the root bridge. We can force selection by changing the priority value.
Changing the priority to FIX the election • To set priority: • S3(config)#spanning-tree vlan 1 priority 4096 • To restore priority to default: • S3(config)#no spanning-tree vlan 1 priority
STP Recalculations take time • If a link failure occurs, STP recalculates by: • Changing some blocked ports to forwarding ports • Changing some forwarding ports to blocked ports • Forming a new STP tree to maintain the loop-free integrity of the network STP is not instantaneous This calculation and transition period takes about 30 to 50 seconds on each switch. During this recalculation, no user data passes through the recalculating ports.
How Spanning Tree Protocol Prevents Switching Loops • STP recalculations • Minimizing downtime • PortFast • UplinkFast • BackboneFast
STP Enhancements • STP PortFast causes an access port to enter the forwarding state immediately, bypassing the listening and learning states. • Using PortFast on access ports that are connected to a single workstation or server allows those devices to connect to the network immediately, instead of waiting for STP to converge. • STP UplinkFast accelerates the choice of a new root port when a link or switch fails or when STP reconfigures itself. • The root port transitions to the forwarding state immediately without going through the listening and learning states, as it would do with normal STP procedures.
STP Enhancements • BackboneFast provides fast convergence after a spanning tree topology change occurs. • It quickly restores backbone connectivity. BackboneFast is used at the Distribution and Core Layers, where multiple switches connect. • Limitation of all three • All the enhancements are Cisco proprietary. • All the switches in the network must be running Cisco IOS
Discussion • What type of host or server would you connect with PortFast? • Could every host on a network be connected using PortFast? • Could you connect another switch to a network using PortFast? • Understanding and Configuring the Cisco Uplink Fast Feature, http://www.cisco.com/warp/public/473/51.html
How Spanning Tree Protocol Prevents Switching Loops • Spanning-tree verification commands
Show spanning-tree commands • show spanning-tree - Displays root ID, bridge ID, and port states • show spanning-tree summary - Displays a summary of port states • show spanning-tree root - Displays the status and configuration of the root bridge • show spanning-tree detail - Displays detailed port information • show spanning-tree interface - Displays STP interface status and configuration • show spanning-tree blockedports - Displays blocked ports
Spanning tree poetry • AlgorhymeBy Radia Perlman(Adapted from "Trees", by Joyce Kilmer)I think that I shall never seeA graph more lovely than a tree.A tree whose crucial propertyIs loop-free connectivity.A tree which must be sure to spanSo packets can reach every LAN.First the Root must be selectedBy ID it is elected.Least cost paths from Root are tracedIn the tree these paths are placed.A mesh is made by folks like meThen bridges find a spanning tree.
RSTP • Rapid Spanning Tree Protocol (RSTP), defined in IEEE 802.1w, significantly speeds the recalculation of the spanning tree. Unlike PortFast, UplinkFast, and BackboneFast, RSTP is not proprietary. • RSTP requires a full-duplex, point-to-point connection between switches to achieve the highest reconfiguration speed. Reconfiguration of the spanning tree by RSTP occurs in less than 1 second, as compared to 50 seconds in STP.
RSTP • RSTP eliminates the requirements for features such as PortFast and UplinkFast. RSTP can revert to STP to provide services for legacy equipment. • To speed up the recalculation process, RSTP reduces the number of port states to three: discarding, learning and forwarding. The discarding state is similar to three of the original STP states: blocking, listening, and disabled. • RSTP also introduces the concept of active topology. All ports that are not discarding are part of the active topology and will immediately transition to the forwarding state.
Explain How Spanning Tree Protocol Prevents Switching Loops • Rapid Spanning Tree Protocol • Discarding • Active topology