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.
Note Switched LAN internetworks are also referred to as campus LANs.
Role of LAN Switching Technology in Campus Networks
Most network designers are beginning to integrate switching devices into their existing shared- media networks to achieve the following goals:
- Increase the bandwidth that is available to each user, thereby alleviating congestion in their shared-media networks.
- Employ the manageability of VLANs by organizing network users into logical workgroups that are independent of the physical topology of wiring closet hubs. This, in turn, can reduce the cost of moves, adds, and changes while increasing the flexibility of the network.
- Deploy emerging multimedia applications across different switching platforms and technologies, making them available to a variety of users.
- Provide a smooth evolution path to high-performance switching solutions, such as Fast Ethernet and ATM.
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:
- Unlike hubs and repeaters, switches allow multiple data streams to pass simultaneously.
- Switches have the capability through microsegmentation to support the increased speed and bandwidth requirements of emerging technologies.
- Switches deliver dedicated bandwidth to users through high-density group switched and switched 10BaseT or 100BaseT Ethernet, flexible 10/100 BaseT Ethernet, fiber-based Fast Ethernet, Fast EtherChannel, Token Ring, CDDI/FDDI, and ATM LAN Emulation (LANE).
Switched Internetwork Solutions
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:
- Leverage strategic investments in the existing communications infrastructure while increasing available bandwidth.
- Reduce the costs of managing network operations.
- Offer options to support multimedia applications and other high-demand traffic across a variety of platforms.
- Provide scalability, traffic control, and security that is at least as good or better than that of today's router-based internetworks.
- Provide support for embedded remote monitoring (RMON) agent.
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.
Components of the Switched Internetworking Model
A switched internetwork is composed of the following three basic components:
- Physical switching platforms
- A common software infrastructure
- Network management tools and applications
Cisco provides network designers with a complete, end-to-end solution for implementing and managing scalable, robust, switched internetworks.
Scalable Switching Platforms
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.
ATM Switches
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:
- Variety of interfaces and services that are supported
- Redundancy
- Depth of ATM internetworking software
- Sophistication of traffic management mechanism
- Blocking and non-blocking switching fabrics
- SVC and PVC support
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:
- Workgroup ATM switches
- Campus ATM switches
- Enterprise ATM switches
- Multiservice access switches
As Figure 12-2 shows, Cisco offers a complete range of ATM switches.
Figure 12-2: Different types of ATM switches.
Workgroup and Campus 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.
Campus ATM switches are generally used for small-scale ATM backbones (for example, to link ATM routers or LAN switches). This use of ATM switches can alleviate current backbone congestion while enabling the deployment of such new services as VLANs. Campus switches need to support a wide variety of both local backbone and WAN types but be price/performance optimized for the local backbone function. In this class of switches, ATM routing capabilities that allow multiple switches to be tied together is very important. Congestion control mechanisms for optimizing backbone performance is also important.
Enterprise ATM Switches
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.
LAN Switches
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.
Figure 12-3: Sample LAN switch configuration.
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.