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 Designing Switched LAN Internetworks 12-5
Components of the Switched Internetworking Model
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.
Host A
Host B
Ethernet
Port 1
Ethernet
Port 2
High-speed port
Ethernet
Port 3
Ethernet
Port 4
Port n
Host C
Host D
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.
Multiservice Access Switches
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
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Cisco CCIE Fundamentals: Network Design
Common Software Infrastructure
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.
Routing Platforms
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.
Common Software Infrastructure
The second level of a switched internetworking model is a common software infrastructure. The