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This chapter describes the following three technologies that network designers can use to design switched LAN internetworks:
In the past, network designers had only a limited number of hardware options when purchasing a technology for their campus networks. Hubs were for wiring closets and routers were for the data center or main telecommunications operations. The increasing power of desktop processors and the requirements of client-server and multimedia applications, however, have driven the need for greater bandwidth in traditional shared-media environments. These requirements are prompting network designers to replace hubs in their wiring closets with switches, as shown in Figure 12-1.
This strategy allows network managers to protect their existing wiring investments and boost network performance with dedicated bandwidth to the desktop for each user. Coinciding with the wiring closet evolution is a similar trend in the network backbone. Here, the role of Asynchronous Transfer Mode (ATM) is increasing as a result of standardizing protocols, such as LAN emulation (LANE), that enable ATM devices to coexist with existing LAN technologies. Network designers are collapsing their router backbones with ATM switches, which offer the greater backbone bandwidth required by high-throughput data services.
With the advent of such technologies as Layer 3 switching, LAN switching, and VLANs, building campus LANs is becoming more complex than in the past. Today, the following three technologies are required to build successful campus networks:
ATM switching offers high-speed switching technology for voice, video, and data. Its operation is similar to LAN switching technologies for data operations. ATM, however, offers superior voice, video, and data integration today.
Routing is a key technology for connecting LANs in a campus network. It can be either Layer 3 switching or more traditional routing with Layer 3 switching features and enhanced Layer 3 software features.
Most network designers are beginning to integrate switching devices into their existing shared- media networks to achieve the following goals:
Segmenting shared-media LANs divides the users into two or more separate LAN segments, reducing the number of users contending for bandwidth. LAN switching technology, which builds upon this trend, employs microsegmentation, which further segments the LAN to fewer users and ultimately to a single user with a dedicated LAN segment. Each switch port provides a dedicated, 10MB Ethernet segment, or dedicated 4/16MB Token Ring segment.
Segments are interconnected by internetworking devices that enable communication between LANs while blocking other types of traffic. Switches have the intelligence to monitor traffic and compile address tables, which then allows them to forward packets directly to specific ports in the LAN. Switches also usually provide nonblocking service, which allows multiple conversations (traffic between two ports) to occur simultaneously.
Switching technology is quickly becoming the preferred solution for improving LAN traffic for the following reasons:
Network designers are discovering, however, that many products offered as switched internetwork solutions are inadequate. Some offer a limited number of hardware platforms with little or no system integration with the current infrastructure. Others require complete abandonment of all investments in the current network infrastructure. To be successful, a switched internetwork solution must accomplish the following:
The key to achieving these benefits is to understand the role of the internetworking software infrastructure within the switched internetworks. Within today's networks, routers allow for the interconnection of disparate LAN and WAN technologies, while also implementing security filters and logical firewalls. It is these capabilities that have allowed current internetworks to scale globally while remaining stable and robust.
As networks evolve toward switched internetworks, similar logical internetworking capabilities are required for stability and scalability. Although LAN and ATM switches provide great performance improvements, they also raise new internetworking challenges. Switched internetworks must integrate with existing LAN and WAN networks. Such services as VLANs, which will be deployed with switched internetworks, also have particular internetworking requirements.
A true switched internetwork, therefore, is more than a collection of boxes. Rather, it consists of a system of devices integrated and supported by an intelligent internetworking software infrastructure. Presently, this network intelligence is centralized within routers. However, with the advent of switched internetworks, the intelligence will often be dispersed throughout the network, reflecting the decentralized nature of switching systems. The need for an internetworking infrastructure, however, will remain.
A switched internetwork is composed of the following three basic components:
Cisco provides network designers with a complete, end-to-end solution for implementing and managing scalable, robust, switched internetworks.
The first component of the switched internetworking model is the physical switching platform. This can be an ATM switch, a LAN switch, or a router.
Although switched internetworks can be built with a variety of technologies, many network designers will deploy ATM in order to utilize its unique characteristics. ATM provides scalable bandwidth that spans both LANs and WANs. It also promises Quality of Service (QoS) guarantees—bandwidth on demand—that can map into and support higher-level protocol infrastructures for emerging multimedia applications and provide a common, multiservice network infrastructure.
ATM switches are one of the key components of ATM technology. All ATM switches, however, are not alike. Even though all ATM switches perform cell relay, ATM switches differ markedly in the following capabilities:
Just as there are routers and LAN switches available at various price/performance points with different levels of functionality, ATM switches can be segmented into the following four distinct types that reflect the needs of particular applications and markets:
As Figure 12-2 shows, Cisco offers a complete range of ATM switches.
Workgroup ATM switches are optimized for deploying ATM to the desktop over low-cost ATM desktop interfaces, with ATM signaling interoperability for ATM adapters and QoS support for multimedia applications.
Enterprise ATM switches are sophisticated multiservice devices that are designed to form the core backbones of large, enterprise networks. They are intended to complement the role played by today's high-end multiprotocol routers. Enterprise ATM switches, much as campus ATM switches, are used to interconnect workgroup ATM switches and other ATM-connected devices, such as LAN switches. Enterprise-class switches, however, can act not only as ATM backbones but can serve as the single point of integration for all of the disparate services and technology found in enterprise backbones today. By integrating all of these services onto a common platform and a common ATM transport infrastructure, network designers can gain greater manageability while eliminating the need for multiple overlay networks.
A LAN switch is a device that typically consists of many ports that connect LAN segments (Ethernet and Token Ring) and a high-speed port (such as 100-Mbps Ethernet, Fiber Distributed Data Interface [FDDI], or 155-Mbps ATM). The high-speed port, in turn, connects the LAN switch to other devices in the network.
A LAN switch has dedicated bandwidth per port, and each port represents a different segment. For best performance, network designers often assign just one host to a port, giving that host dedicated bandwidth of 10 Mbps, as shown in Figure 12-3, or 16 Mbps for Token Ring networks.
When a LAN switch first starts up and as the devices that are connected to it request services from other devices, the switch builds a table that associates the MAC address of each local device with the port number through which that device is reachable. That way, when Host A on Port 1 needs to transmit to Host B on Port 2, the LAN switch forwards frames from Port 1 to Port 2, thus sparing other hosts on Port 3 from responding to frames destined for Host B. If Host C needs to send data to Host D at the same time that Host A sends data to Host B, it can do so because the LAN switch can forward frames from Port 3 to Port 4 at the same time it forwards frames from Port 1 to Port 2.
Whenever a device connected to the LAN switch sends a packet to an address that is not in the LAN switch's table (for example, to a device that is beyond the LAN switch), or whenever the device sends a broadcast or multicast packet, the LAN switch sends the packet out all ports (except for the port from which the packet originated)—a technique known as flooding.
Because they work like traditional "transparent" bridges, LAN switches dissolve previously well-defined workgroup or department boundaries. A network built and designed only with LAN switches appears as a flat network topology consisting of a single broadcast domain. Consequently, these networks are liable to suffer the problems inherent in flat (or bridged) networks—that is, they do not scale well. Note, however, that LAN switches that support VLANs are more scalable than traditional bridges.
Beyond private networks, ATM platforms will also be widely deployed by service providers both as customer premises equipment (CPE) and within public networks. Such equipment will be used to support multiple MAN and WAN services—for example, Frame Relay switching, LAN interconnect, or public ATM services—on a common ATM infrastructure. Enterprise ATM switches will often be used in these public network applications because of their emphasis on high availability and redundancy, and their support of multiple interfaces.
In addition to LAN switches and ATM switches, typically network designers use routers as one of the components in a switched internetwork infrastructure. While LAN switches are being added to wiring closets to increase bandwidth and to reduce congestion in existing shared-media hubs, high-speed backbone technologies, such as ATM switching and ATM routers are being deployed in the backbone. Within a switched internetwork, routing platforms also allow for the interconnection of disparate LAN and WAN technologies while also implementing broadcast filters and logical firewalls. In general, if you need advanced internetworking services, such as broadcast firewalling and communication between dissimilar LANs, routers are necessary.
The second level of a switched internetworking model is a common software infrastructure. The function of this software infrastructure is to unify the variety of physical switching platforms: LAN switches, ATM switches, and multiprotocol routers. Specifically, the software infrastructure should perform the following tasks:
First-generation VLANs are based on various OSI Layer 2 bridging and multiplexing mechanisms, such as IEEE 802.10, LAN Emulation (LANE), and Inter-Switch Link (ISL), that allow the formation of multiple, disjointed, overlaid broadcast groups on a single network infrastructure. Figure 12-4 shows an example of a switched LAN network that uses VLANs. Layer 2 of the OSI reference model provides reliable transit of data across a physical link. The data link layer is concerned with physical addressing, network topology, line discipline, error notification, ordered delivery frames, and flow control. The IEEE has divided this layer into two sublayers: the MAC sublayer and the LLC sublayer, sometimes simply called link layer.
In Figure 12-4, 10-Mbps Ethernet connects the hosts on each floor to switches A, B, C, and D. 100-Mbps Fast Ethernet connects these to Switch E. VLAN 10 consists of those hosts on Ports 6 and 8 of Switch A and Port 2 on Switch B. VLAN 20 consists of those hosts that are on Port 1 of Switch A and Ports 1 and 3 of Switch B.
VLANs can be used to group a set of related users, regardless of their physical connectivity. They can be located across a campus environment or even across geographically dispersed locations. The users might be assigned to a VLAN because they belong to the same department or functional team, or because data flow patterns among them is such that it makes sense to group them together. Note, however, that without a router, hosts in one VLAN cannot communicate with hosts in another VLAN.
The third and last component of a switched internetworking model consists of network management tools and applications. As switching is integrated throughout the network, network management becomes crucial at both the workgroup and backbone levels. Managing a switch-based network requires a radically different approach than managing traditional hub and router-based LANs.
As part of designing a switched internetwork, network designers must ensure that their design takes into account network management applications needed to monitor, configure, plan, and analyze switched internetwork devices and services. Cisco offers such tools for emerging switched internetworks.
Cisco offers the following products that meet the needs of a switched internetwork, all discussed in the following sections:
In order to support the bursty, best-effort traffic generated by LAN switches and routes, the LightStream 1010 provides advanced traffic management mechanisms. The LightStream 1010's intelligent early packet discard mechanism allows it to discard entire packets rather than individual cells when necessary, which greatly increases performance for current protocols, such as TCP/IP and IPX. It also supports the latest ATM Forum Available Bit Rate (ABR) congestion control specifications, which allows the LightStream 1010 to slow traffic sources before congestion becomes excessive. Because of its support for the ATM Forum private network-network interface (PNNI) protocols, networks of LightStream 1010s can scale to hundreds of nodes.
In addition, the LightStream 1010 offers a high degree of manageability. Advanced port snooping and connection-steering capabilities allow the connections on any port to be directed to a monitor port for analysis by an external ATM analyzer. This capability is critical for the monitoring and troubleshooting of ATM switching systems, which unlike shared-media LANs, cannot be monitored easily with external devices. Simple Network Management Protocol (SNMP) monitoring and configuration invoked through the CiscoView graphical user interface (GUI) device configuration applications and the AtmDirector CiscoWorks ATM system management application, allow for comprehensive network management.
By building on the Cisco IOS software, the LightStream 1010 switch also shares the advanced serviceability capabilities found today on Cisco's multiprotocol routers. As with all Cisco routers, the LightStream 1010 switch supports such protocols as BOOTP, DHCP, Telnet, and Trivial File Transfer Protocol (TFTP) for remote access and autoconfiguration. It also offers the access protections of the Cisco IOS software, from multiple password levels to TACACS for remote access validation, to preclude unauthorized changes to the switch configuration. These capabilities are clearly essential to safeguard the operation of the mission-critical campus backbones in which the LightStream 1010 will typically be deployed.
The Cisco/StrataCom BPX/AXIS is a powerful broadband 9.6-Gbps ATM switch designed to meet the demanding, high-traffic needs of a large private enterprise or public service provider. The Cisco/StrataCom IGX is a 1.2-Gbps ATM-based enterprise WAN switch that can be used to provide enterprise WAN features in your internetwork. For more information on these enterprise ATM switches, see "Designing ATM Internetworks."
Cisco's Catalyst family is a comprehensive line of high-performance switches designed to help network managers easily migrate from traditional shared LANs to fully switched internetworks. The Catalyst family delivers the varying levels of flexibility and cost-effectiveness required for today's desktop, workgroup, and backbone applications while enabling enterprise-wide switched internetworks. Using these LAN switches instead of traditional shared hubs increase performance and provides new capabilities, such as VLANs.
Figure 12-5 shows an example of switches that can be used in a campus backbone. In this example, the Cisco switches are used to interconnect the four buildings that comprise the campus network.
Table 12-1 summarizes the LAN switches that Cisco offers.
| Cisco LAN Switch | Description |
|---|---|
Catalyst 5500 switching system | The Catalyst 5500 switch chassis has 13 slots. Slot 1 is for the Supervisor Engine II model which provides switching, local and remote management, and dual Fast Ethernet interfaces. Slot 2 contains an additional redundant Supervisor Engine II in case the first module fails. |
Route switch module | The Cisco Catalyst 5000 series route switch module builds upon the Route Switch Processor (RSP) featured in Cisco's 7500 routing platform. The route switch module provides high-performance multilayer switching and routing services between switched Virtual LANs (VLANs), emulated LANs (ELANs) within an Asynchronous Transfer Mode (ATM) fabric, or across mixed media via an optional Versatile Interface Processor (VIP) and port adapters. |
Catalyst 5000 switching system | A modular switching platform that meets high performance needs, bandwidth-intensive networking switching applications. It offers five slots that can be populated with any combination of 10BaseT, 10BaseFL modules, switched 10-Mbps Fast Ethernet, FDDI, or ATM modules. It delivers high performance both for client and server connections as well as for backbone connections. Its switching backplane operates at 1.2 Gbps and provides nonblocking performance for all switched 10-Mbps Ethernet interfaces. It supports enterprise-wide VLAN communications across Ethernet, Fast Ethernet, CDDI/FDDI, and ATM connections via the following protocols: ISL for Fast Ethernet interfaces, 802.10 for FDDI interfaces, and LANE v1.0 for ATM. |
Catalyst 3000 stackable | A 16-port 10BaseT switch that has two open expansion bays that can be populated with 100BaseTX/FX, 10BaseFL, 10BaseT, 100VG-AnyLAN, or ATM. With the Matrix module, up to eight Catalyst 3000 switches can be stacked together as one logical switching system. The Catalyst 3000 system can also be populated with the Catalyst 3011 WAN router module. A fully loaded Catalyst 3000 system can support up to 192 10BaseT ports, or 128 10BaseT ports with 16 high-speed ports. Supports up to 64 VLANs within the stack. Also supports ISL for Fast Ethernet, and ATM LANE. |
Catalyst 2900 Fast | A 14-port, fixed-configuration, Fast Ethernet switch that provides media-rate Fast Ethernet switching in backbone, server cluster, and high-performance workgroup applications. Its software architecture combines superior traffic throughput, complete VLAN solutions, traffic management, and fault tolerance. |
Catalyst 1800 Token Ring switch | A Token Ring switch that has 16 dedicated or shared ports in the base unit plus two feature-card slots that is designed for the workgroup switching environment. Using the four-port Token Ring unshielded twisted-pair/shielded twisted-pair (UTP/STP) feature cards, it supports eight additional Token Ring ports. |
Catalyst 1900 and Catalyst 2820 | Ideally suited to replace shared 10BaseT hubs in the wiring closet with feature-rich switched Ethernet capability to the desktop. The Catalyst 1900 Ethernet switch features 25 switched Ethernet ports providing attachment to individual workstations and 10BaseT hubs. It also has two 100BaseT ports for high speed connectivity to servers and backbones. The Catalyst 2820 Ethernet switch has 25 switched Ethernet ports and two high-speed expansion slots. Field-installable modules provide configuration, wiring, and backbone flexibility with a choice of 100BaseT, FDDI, and future ATM modules available, which support Category 5 UTP or fiber-optic cabling. |
Catalyst 1200 workgroup switch | A multilayer switch for workgroup applications that can benefit from OSI Layer 3 as well as Layer 2 capabilities. It offers eight 10BaseT or 10BaseFL ports and one expansion slot. The expansion slot can be populated with either one A/B CDDI interface or one A/B FDDI interface. IP routing only supports 802.10 over FDDI VLANs. In addition to meeting a wide range of performance needs for Ethernet and FDDI, it offers such unique features as embedded Remote Monitoring (RMON) functionality, which helps network managers monitor and control the growth and changes of client-server workgroups. |
Both the Cisco 7000 and Cisco 4000 family of multiprotocol routers are particularly well suited for switched internetworking. In particular, the first native-mode ATM router interface, the ATM Interface Processor (AIP) for the Cisco 7000 family of routers, is a key enabler for integrating existing LAN and WAN networks with evolving, ATM-based switched internetworks.
The sophisticated ATM signaling and traffic management capabilities of the AIP also allows it to play a crucial role in the deployment of new services such as VLANs. The AIP, a key enabler for the production deployment of switched internetworks, allows VLANs to internetwork either with each other or with external networks. The Cisco 4000 family of multiprotocol routers also support such capabilities, thereby providing network designers with a wide choice of price/performance points for ATM-capable routers.
Because the Cisco 7000 and Cisco 4000 families support FDDI, Fast Ethernet, and ATM, they provide network designers with a full set of options for high-speed connectivity. Both router families also support routing between VLANs on all media for ease of migration.
In general, incorporating switches in campus network designs results in the following benefits:
If you need advanced internetworking services, however, routers are necessary. Routers offer the following services:
Some of these router services will be offered by switches in the future. For example, support for multimedia often requires a protocol, such as Internet Group Management Protocol (IGMP), that allows workstations to join a group that receives multimedia multicast packets. In the future, Cisco will allow switches to participate in this process by using the Cisco Group Management Protocol (CGMP). One router will still be necessary, but you will not need a router in each department because CGMP switches can communicate with the router to determine whether any of their attached users are part of a multicast group.
Switching and bridging sometimes can result in nonoptimal routing of packets. This is because every packet must go through the root bridge of the spanning tree. When routers are used, the routing of packets can be controlled and designed for optimal paths. Cisco now provides support for improved routing and redundancy in switched environments by supporting one instance of the spanning tree per VLAN.
When designing switched LAN networks, you should consider the following:
The fundamental difference between a LAN switch and a router is that the LAN switch operates at Layer 2 of the OSI model and the router operates at Layer 3. This difference affects the way that LAN switches and routers respond to network traffic. This section compares LAN switches and routers with regard to the following network design issues:
Switched LAN topologies are susceptible to loops, as shown in Figure 12-6.
In Figure 12-6, it is possible for packets from Client X to be switched by Switch A and then for Switch B to put the same packet back on to LAN 1. In this situation, packets loop and undergo multiple replications. To prevent looping and replication, topologies that may contain loops need to run the Spanning-Tree Protocol. The Spanning-Tree Protocol uses the spanning-tree algorithm to construct topologies that do not contain any loops. Because the spanning-tree algorithm places certain connections in blocking mode, only a subset of the network topology is used for forwarding data. In contrast, routers provide freedom from loops and make use of optimal paths.
In transparent switching, neighboring switches make topology decisions locally based on the exchange of Bridge Protocol Data Units (BPDUs). This method of making topology decisions means that convergence on an alternative path can take an order of magnitude longer than in a routed environment.
In a routed environment, sophisticated routing protocols, such as Open Shortest Path First (OSPF) and Enhanced Interior Gateway Routing Protocol (Enhanced IGRP), maintain concurrent topological databases of the network and allow the network to converge quickly.
In some cases, the circulation of broadcasts can saturate the network so that there is no bandwidth left for application data. In this case, new network connections cannot be established, and existing connections may be dropped (a situation known as a broadcast storm). The probability of broadcast storms increases as the switched internetwork grows. Routers do not forward broadcasts, and, therefore, are not subject to broadcast storms. For more information about the impact of broadcasts, see "Broadcasts in Switched LAN Internetworks."
Transparently switched internetworks are composed of physically separate segments, but are logically considered to be one large network (for example, one IP subnet). This behavior is inherent to the way that LAN switches work—they operate at OSI Layer 2 and have to provide connectivity to hosts as if each host were on the same cable. Layer 2 addressing assumes a flat address space with universally unique addresses.
Routers operate at OSI Layer 3, so can formulate and adhere to a hierarchical addressing structure. Routed networks can associate a logical addressing structure to a physical infrastructure so that each network segment has, for example, a TCP/IP subnet or IPX network. Traffic flow on routed networks is inherently different from traffic flow on switched networks. Routed networks have more flexible traffic flow because they can use the hierarchy to determine optimal paths depending on dynamic factors such as network congestion.
Information is available to routers and switches that can be used to create more secure networks. LAN switches may use custom filters to provide access control based on destination address, source address, protocol type, packet length, and offset bits within the frame. Routers can filter on logical network addresses and provide control based on options available in Layer 3 protocols. For example, routers can permit or deny traffic based on specific TCP/IP socket information for a range of network addresses.
Two factors need to be considered with regard to mixed-media internetworks. First, the maximum transfer unit (MTU) differs for various network media. Table 12-2 lists the maximum frame size for various network media.
| Media | Minimum Valid Frame | Maximum Valid Size |
|---|---|---|
Ethernet | 64 bytes | 1518 bytes |
Token Ring | 32 bytes | 16 KB theoretical, 4 KB normal |
Fast Ethernet | 64 bytes | 1518 bytes |
FDDI | 32 bytes | 4400 bytes |
ATM LANE | 64 bytes | 1518 bytes |
ATM Classical IP | 64 bytes | 9180 bytes |
Serial HDLC | 14 bytes | No limit, 4.5 KB normal |
When LANs of dissimilar media are switched, hosts must use the MTU that is the lowest common denominator of all the switched LANs that make up the internetwork. This requirement limits throughput and can seriously compromise performance over a relatively fast link, such as FDDI or ATM. Most Layer 3 protocols can fragment and reassemble packets that are too large for a particular subnetwork, so routed networks can accommodate different MTUs, which maximizes throughput.
By working at Layer 3, routers are essentially independent of the properties of any physical media and can use a simple address resolution algorithm (such as Novell-node-address = MAC-address) or a protocol, such as the Address Resolution Protocol (ARP), to resolve differences between Layer 2 and Layer 3 addresses.
An individual Layer 2 switch might offer some or all of the following benefits:
Routers control broadcasts and multicasts in the following ways:
Successful network designs contain a mix of appropriately scaled switching and routing. Given the effects of broadcast radiation on CPU performance, well-managed switched LAN designs must include routers for broadcast and multicast management.
In addition to preventing broadcasts from radiating throughout the network, routers are also responsible for generating services to each LAN segment. The following are examples of services that the router provides to the network for a variety of protocols:
In a flat virtual network, a single router would be bombarded by myriad requests needing replies, severely taxing its processor. Therefore, the network designer needs to consider the number of routers that can provide reliable services to a given subset of VLANs. Some type of hierarchical design needs to be considered.
In the past, routers have been used to connect networks of different media types, taking care of the OSI Layer 3 address translations and fragmentation requirements. Routers continue to perform this function in switched LAN designs. Most switching is done within like media (such as Ethernet, Token Ring, and FDDI switches) with some capability of connecting to another media type. However, if a requirement for a switched campus network design is to provide high-speed connectivity between unlike media, routers play a significant part in the design.
In a flat, bridged network all broadcast packets generated by any node in the network are sent to and received by all other network nodes. The ambient level of broadcasts generated by the higher layer protocols in the network—known as broadcast radiation—will typically restrict the total number of nodes that the network can support. In extreme cases, the effects of broadcast radiation can be so severe that an end station spends all of its CPU power on processing broadcasts.
VLANs have been designed to address the following problems inherent in a flat, bridged network:
VLANs solve some of the scalability problems of large flat networks by breaking a single bridged domain into several smaller bridged domains, each of which is a virtual LAN. Note that each virtual LAN is itself constrained by the scalability issues described in "Broadcasts in Switched LAN Internetworks." It is insufficient to solve the broadcast problems inherent to a flat switched network by superimposing VLANs and reducing broadcast domains. VLANs without routers do not scale to large campus environments. Routing is instrumental in the building of scalable VLANs and is the only way to impose hierarchy on the switched VLAN internetwork.
VLANs offer the following features:
This section describes the different methods of creating the logical groupings (or broadcast domains) that make up various types of VLANs. There are three ways of defining a VLAN:
Cisco's initial method of implementing VLANs on routers and Catalyst switches is by port. To efficiently operate and manage protocols, such as IP, IPX, and AppleTalk, all nodes in a VLAN should be in the same subnet or network.
Cisco uses three technologies to implement VLANs:
The three technologies are similar in that they are based on OSI Layer 2 bridge multiplexing mechanisms.
In the switched backbone topology shown in Figure 12-7, you want to ensure that intra-VLAN traffic goes only between Segment A and Segment D (both in VLAN 10) and Segment B and Segment C (both in VLAN 20).
In Figure 12-7, all Ethernet ports on Switches X, Y, and Z are in a VLAN and are to be VLAN interfaces. All FDDI interfaces in Switches X, Y, and Z are called VLAN trunk interfaces. To ensure that traffic from Segment A destined for Segment D on Switch Z is forwarded onto Ethernet 3 and not onto Ethernet 2, it is colored when it leaves Switch X. Switch Z recognizes the color and knows that it must forward these frames onto Ethernet 3 and not onto Ethernet 2.
The coloring of traffic across the FDDI backbone is achieved by inserting a 16-byte header between the source MAC address and the Link Service Access Point (LSAP) of frames leaving a switch. This header contains a 4-byte VLAN ID or "color." The receiving switch removes the header and forwards the frame to interfaces that match that VLAN color.
It is important that a single VLAN use only one subnet. In Figure 12-8, VLAN 10 (subnet 10) is "split" and therefore must be "glued" together by maintaining a bridged path for it through the network. For Switch X and nodes in VLAN 20 (subnet 20), traffic is switched locally if appropriate. If traffic is destined for a node in VLAN 30 (subnet 30) from a node in VLAN 20, Router Y routes it through the backbone to Router Z. If traffic from Segment D on VLAN 10 is destined for a node in VLAN 20, Router Y routes it back out the FDDI interface.
The difference between these two strategies is subtle. Table 12-3 compares the advantages and disadvantages of the two strategies.
| Switched Backbone | Routed Backbone | ||
|---|---|---|---|
| Advantages | Disadvantages | Advantages | Disadvantages |
Propagates color information across entire network. | Backbone is running bridging. | No bridging in backbone. | Color information is not propagated across backbone and must be configured manually. |
Allows greater scalability by extending bridge domains. | Broadcast traffic increases drastically on the backbone. | Easy to integrate into existing internetwork. | If subnets are split, a bridged path has to be set up between switches. |
|
| Can run native protocols in the backbone. |
|
A VLAN interface can have only one VLAN ID, and VLAN trunk interfaces support multiple VLANs across them.
ISL is a Cisco-proprietary protocol for interconnecting multiple switches and maintaining VLAN information as traffic goes between switches. This technology is similar to IEEE 802.10 in that it is a method of multiplexing bridge groups over a high-speed backbone. It is defined only on Fast Ethernet. The discussion of routing and switching in the backbone in the section "IEEE 802.10," earlier in this chapter, also applies to ISL.
With ISL, an Ethernet frame is encapsulated with a header that maintains VLAN IDs between switches. A 30-byte header is prepended to the Ethernet frame, and it contains a two-byte VLAN ID. In Figure 12-10, Switch Y switches VLAN 20 traffic between segments A and B if appropriate. Otherwise, it encapsulates traffic with an ISL header that identifies it as traffic for VLAN 20 and sends it through the interim switch to Router X.
Router X routes the packet to the appropriate interface, which could be through a routed network beyond Router X (as in this case) out the Fast Ethernet interface to Switch Z. Switch Z receives the packet, examines the ISL header noting that this packet is destined for VLAN 20, and switches it to all ports in VLAN 20 (if the packet is a broadcast or multicast) or the appropriate port (if the packet is a unicast).
LAN Emulation (LANE) is a service that provides interoperability between ATM-based workstations and devices connected to existing legacy LAN technology. The ATM Forum has defined a standard for LANE that provides to workstations attached via ATM the same capabilities that they are used to obtaining from legacy LANs.
LANE uses MAC encapsulation (OSI Layer 2) because this approach supports the largest number of existing OSI Layer 3 protocols. The end result is that all devices attached to an emulated LAN appear to be on one bridged segment. In this way, AppleTalk, IPX, and other protocols should have similar performance characteristics as in a traditional bridged environment. In ATM LANE environments, the ATM switch handles traffic that belongs to the same emulated LAN (ELAN), and routers handle inter-ELAN traffic. For more information about LANE, see "Designing ATM Internetworks."
In traditional networks, there are usually several well-known servers, such as e-mail and corporate servers, that almost everyone in an enterprise needs to access. If these servers are located in only one VLAN, the benefits of VLANs will be lost because all of the different workgroups will be forced to route to access this common information source.
This problem can be solved with LANE and virtual multihomed servers, as shown in Figure 12-11. Network interface cards (NICs) allow workstations and servers to join up to eight different VLANs. This means that the server will appear in eight different ELANs and that to other members of each ELAN, the server appears to be like any other member. This capability greatly increases the performance of the network as a whole because common information is available directly through the optimal Data Direct VCC and does not need to be routed. This also means that the server must process all broadcast traffic in each VLAN that it belongs to, which can decrease performance.
To multihome servers in non-ATM environments, there are two possible choices:
The Catalyst 5000 switch implements Cisco's Virtual Trunk Protocol (VTP). VTP is the industry's first protocol implementation specifically designed for large VLAN deployments. VTP enhances VLAN deployment by providing the following:
Good network design is based on many concepts that are summarized by the following key principles:
Figure 12-12 shows a high-level view of the various aspects of a hierarchical network design. A hierarchical network design presents three layers—core, distribution, and access—with each layer providing different functionality.
In the non-campus environment, the distribution layer can be a redistribution point between routing domains or the demarcation between static and dynamic routing protocols. It can also be the point at which remote sites access the corporate network. The distribution layer can be summarized as the layer that provides policy-based connectivity.
The access layer is the point at which local end users are allowed into the network. This layer may also use access lists or filters to further optimize the needs of a particular set of users. In the campus environment, access-layer functions can include the following:
In the non-campus environment, the access layer can give remote sites access to the corporate network via some wide-area technology, such as Frame Relay, ISDN, or leased lines.
It is sometimes mistakenly thought that the three layers (core, distribution, and access) must exist in clear and distinct physical entities, but this does not have to be the case. The layers are defined to aid successful network design and to represent functionality that must exist in a network. The instantiation of each layer can be in distinct routers or switches, can be represented by a physical media, can be combined in a single device, or can be omitted altogether. The way the layers are implemented depends on the needs of the network being designed. Note, however, that for a network to function optimally, hierarchy must be maintained.
With respect to the hierarchical model, traditional campus LANs have followed one of two designs—single router and distributed backbone—as shown in Figure 12-13.
In the single-router design, the core and distribution layers are present in a single entity—the router. Core functionality is represented by the backplane of the router and distribution is represented by the router. Access for end users is through individual- or chassis-based hubs. This design suffers from scalability constraints because the router can be only be in one physical location, so all segments end at the same location—the router. The single router is responsible for all distribution functionality, which can cause CPU overload.
The distributed backbone design uses a high-speed backbone media, typically FDDI, to spread routing functionality among several routers. This also allows the backbone to traverse floors, a building, or a campus.
When designing switched LAN campus networks, the following factors must be considered:
Campus network designs are evolving rapidly with the deployment of switching at all levels of the network—from the desktop to the backbone. Three topologies have emerged as generic network designs:
The scaled switching design shown in Figure 12-14 deploys switching at all levels of the network without the use of routers. In this design, each layer consists of switches, with switches in the access layer providing 10-Mbps Ethernet or 16-Mbps Token Ring to end users.
Scaled switching is a low-cost and easy-to-install solution for a small campus network. It does not require knowledge of address structure, is easy to manage, and allows all users to communicate with one another. However, this network comprises a single broadcast domain. If a scaled switched network needs to grow beyond the broadcast domain, it can use VLANs to create multiple broadcast domains. Note that when VLANs are used, end users in one VLAN cannot communicate with end users in another VLAN unless routers are deployed.
In the case of ATM in the distribution layer, the following key issues are relevant:
In the case of LAN switching in the distribution layer, the following key issues are relevant:
To scale the large switched/minimal routing design, a logical hierarchy must be imposed. The logical hierarchy consists of VLANs and routers that enable inter-VLAN communication. In this topology, routing is used only in the distribution layer, and the access layer depends on bandwidth through the distribution layer to gain access to high-speed switching functionality in the core layer.
The distributed routing/switching design follows the classic hierarchical network model both physically and logically. Because it provides high bandwidth for access to routing functionality, this design scales very well. This design is optimized for networks that do not have the 80/20 pattern rule. If servers are centralized, most traffic is inter-VLAN; therefore, high routing content is needed.
Campus LAN designs use switches to replace traditional hubs and use an appropriate mix of routers to minimize broadcast radiation. With the appropriate pieces of software and hardware in place, and adhering to good network design, it is possible to build topologies, such as the examples described in the section "Switched LAN Network Designs" earlier in this chapter.
Posted: Wed Apr 10 10:43:51 PDT 2002
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