Switching Inter-VLAN Routing Architectures, ebook's network
[ Pobierz całość w formacie PDF ]
//-->CCNP Practical Studies: Switching | Inter-VLAN Routing Arc...Account Sign In!View Your Cartthe trusted technology learning sourceSearch�½��ExploreTopicsStoreCCNP PracticalStudies: SwitchingBy Justin MengaAuthorsSafari Books OnlineImprintsInter-VLAN Routing ArchitecturesWithin a LAN topology, inter-VLAN routing is used to route packets between different VLANs. Three commoninter-VLAN routing architectures are used in modern LAN networks today:Router-on-a-stickRouter-on-a-stick using trunksLayer 3 switchingThis section examines each of these in detail, outlining any restrictions or issues associated with each.Table of ContentsCopyrightAbout the AuthorAbout the TechnicalReviewersIntroductionChapter 1. SwitchingConnectivityChapter 2. VLANOperationsChapter 3. Trunking andBandwidth AggregationChapter 4. Spanning TreeChapter 5. Inter-VLANRoutingInter-VLAN RoutingArchitecturesMultilayer LAN DesignScenario 5-1:Configuring Basic IPRoutingScenario 5-2:Configuring Layer 3SwitchingScenario PrerequisitesSummaryChapter 6. Layer 3SwitchingChapter 7. MulticastRouting and SwitchingChapter 8. Traffic Filteringand SecurityChapter 9. Quality ofServiceChapter 10. Maintenance,Monitoring, andTroubleshootingChapter 11.Comprehensive SwitchingSelf-Study LabAppendix A.Comprehensive SwitchingSelf-Study Lab Part ISolutionAppendix B.Comprehensive SwitchingSelf-Study Lab Part IISolutionRouter-on-a-StickTherouter-on–a-stickarchitecture is the most basic method of inter-VLAN routing. In this architecture, a router issimply connected to each VLAN and forwards inter-VLAN traffic between the appropriate VLANs.Figure 5-1shows thisarchitecture.Figure 5-1Router-on–a-StickAs you can see inFigure 5-1,the router has a physical Ethernet interface dedicated for each VLAN. If IP hosts on VLAN100 need to communicate with hosts of VLAN 200, IP packets with the appropriate source and destination IP addressesare sent to the router, which looks up the destination IP address and forwards to the appropriate host on thedestination VLAN. The router-on-a-stick architecture is simple to understand because the Layer 2 functions (providedby a switch) and Layer 3 functions (provided by a router) are physically separated.The major issue with this architecture is performance. Because routers are software-based, they cannot route packetsas fast as switches (hardware-based) can switch frames. Even if you are using high-performance routers, the physicalinterface connecting each VLAN to the router is a bottleneck because it can transmit packets only at 10 Mbps, 100Mbps, or 1 Gbps depending on the interface type. This restriction means that the router becomes a performancebottleneck when routing between high-speed VLANs.Another issue with this architecture is the number of routers and physical interfaces required to support multipleVLANs. A dedicated Ethernet interface is required per VLAN. Routers are low-density devices, meaning that there is ahigh cost per port and multiple routing devices might be required to support multiple VLANs, increasing the complexityof the network.Finally, all inter-VLAN traffic must travel via the router. InFigure 5-1,even though the PCs in VLAN 100 and VLAN 200are connected to the same switch, all inter-VLAN traffic between the PCs must be sent through the router, which isinefficient.Router-on-a-Stick Using TrunksAs discussed in the last section, the router-on-a-stick architecture has physical limitations based upon a dedicatedphysical interface being required for each VLAN. This limitation can be removed by using trunk interfaces, wheremultiple VLANs are supported on a single physical interface by using tagging technologies such as 802.1Q or ISL.Rather than using physical interfaces to attach the router to each VLAN, virtual or logical interfaces are used to attachthe router to each VLAN.Figure 5-2shows this architecture.Figure 5-2Router-on-a-Stick Using TrunksInFigure 5-2,virtual interfaces (rather than physical interfaces) are used to connect the router to each VLAN. A singlephysical trunk interface transports tagged VLAN traffic to the router, with the tag determining to which virtual interfacea frame should be forwarded for routing. Apart from the differences between using physical interfaces per VLAN asopposed to virtual interfaces per VLAN, this architecture is essentially identical to the traditional router-on-a-stickarchitecture and suffers the same performance limitations, because the routing engine is still software-based and thetrunk interface is limited to 10 Mbps, 100 Mbps, or 1 Gbps.Routing Using Layer 3 SwitchesThe architectures discussed thus far represent the traditional inter-VLAN routing architectures. The major issue withthese architectures is performance—if gigabit speed routing is required between VLANs, extremely high performanceand costly routers are required. A new form of inter-VLAN routing on the LAN has emerged in recent years calledLayer3 switching.With a Layer 3 switch, the traditionally separated Layer 2 and Layer 3 functions are combined into a single1z323.10.2013, 23:11CCNP Practical Studies: Switching | Inter-VLAN Routing Arc...device, eliminating the bottleneck associated with the cable between a router and switch by replacing the cable with ahigh-speed backplane connection. Layer 3 switches also typically perform routing in specially designed hardwarecircuitry rather than software, using specialized hardware that can perform routing functions at high speed. This meansthat the performance of Layer 3 switches is much higher than traditional router-on-a-stick architectures. For example,if you use a Cisco 3640 series router in the router-on-a-stick architecture, you can achieve routing speeds of up to40,000 packets per second. If you compare this with a Cisco Catalyst 3550-24-EMI Layer 3 switch, which is actuallycheaper than a Cisco 3640 router, you can route packets at up to 6.6 million packets per second. This is obviouslyquite a difference and highlights the limitations of using router-on-a-stick architectures for inter-VLAN routing on theLAN. Of course, the Cisco 3640 router still has a place in the network; it supports a wide variety of diverse media,including serial and ATM connections for WAN connectivity; also supports advanced features such as firewalling,encryption, and so on—all of which are not supported on Cisco Catalyst switches.The Layer 3 switch uses application-specific integrated circuits (ASICs), which are hardware chips that can route trafficat very high speeds. These ASICs are installed on the switching engine of a Layer 3 switch, which traditionally switchesframes at Layer 2. The ASICs allow the switching engine to also switch frames that contain packets sent betweendifferent VLANs. Each ASIC is programmed with the information required to route traffic from one VLAN to another,without having to pass the traffic through the CPU of the routing engine. This information includes the egress port,egress VLAN, and new destination MAC address that should be written for the frame that is sent. Some form of routecache is normally used to store such information, with the ASIC searching the cache for routing information for thedestination IP address of packets as they are received. How this information is programmed into the route cachedepends on the Layer 3 switch architecture used; however, the end result is essentially the same.In addition to the high-speed routing feature, these ASICs also can apply security access control list (ACL) filtering andLayer 3 quality of service (QoS) classification, all at wire-speed, meaning these useful features can be turned on withoutaffecting performance.NOTEThe internal mechanics of Layer 3 switching are covered in more detail in Chapter 6, "Layer 3 Switching."When examining the architecture of a Layer 3 switch, it is important to understand that several different approaches toLayer 3 switching implemented by Cisco exist:Router-on-a-stick—Some chassis-based Catalyst switches (e.g., the Catalyst 4000 and 5000) support routingmodules, which are effectively routers on a blade. Apart from having a high-speed connection to the switchbackplane, the routing module is essentially a router-on-a-stick, with all routed traffic requiring processing throughthe routing module. This architecture is not really Layer 3 switching at all because the switch hardware has nospecial ASICs for Layer 3 switching; instead, it is a high-speed, router-on-a-stick architecture.Multilayer switching (MLS)—In this architecture, hardware-based ASICs on the switching component of the Layer 3switch refer to a cache that is populated with the required information to route a packet received on one VLAN toanother VLAN, without having to pass the packet through the routing engine. With MLS, the Layer 3 switching cacheis populated after the first packet of a particular flow (connection) is received and the route processor is queried forrouting information.Cisco Express Forwarding (CEF)—This architecture is identical to MLS in terms of the hardware-based ASICsreferring to a Layer 3 cache for information as to how to route packets between VLANs without involving the routerprocessor. CEF differs from MLS in terms of the way the Layer 3 cache is populated. CEF pre-populates the cacheswith full routing information, which means the route processor never needs to be queried for the initial routinginformation that is required in a MLS architecture.In this chapter, you learn how to configure the Catalyst 4000 using the Layer 3 routing module in a router-on-a-stickarchitecture. You also learn how to configure Layer 3 switching on the Catalyst 3550, which is based upon a CEFarchitecture. In Chapter 6, you learn about MLS and CEF on the Catalyst 6000/6500 family of switches.Previous SectionMultilayer LAN Design |Next SectionShare This2z323.10.2013, 23:11CCNP Practical Studies: Switching | Inter-VLAN Routing Arc...AboutAffiliatesContact UsJobsLegal NoticePrivacy PolicyPressPromotionsSite MapWrite for Us© 2013 Pearson Education, Informit. All rights reserved.800 East 96th Street, Indianapolis, Indiana 462403z323.10.2013, 23:11
[ Pobierz całość w formacie PDF ]