Cisco devices fall into three main categories: bridges and switches, routers, and access servers. We discuss bridges and switches first.
Bridges and Switches
A bridge is a network device that operates at the data link layer. A bridge connects multiple data link layer network segments into a single logical network segment. There are many different types of bridges:
Although the Cisco IOS implements each of these types of bridging, we discuss only the first three types of bridging in this book. Source-route and source-route translational bridging are used in Token Ring environments.
Bridging allows for physical and logical separation of traffic when necessary to reduce traffic loads on a network segment. The main advantage of bridging is to ensure network reliability, availability, scalability, and manageability by segmenting a logical network into multiple physical pieces. We examine bridging as it relates to routing throughout this text.
A bridge performs its function by examining the data link layer information in each packet and forwarding the packet to other physical segments only if necessary. The information concerning which packets to forward to which network segments is learned by the bridge and kept in a forwarding table. The forwarding table includes a list of known data link layer addresses and the associated network segment where these devices are believed to exist, as shown in Figure 1-4.
Figure 1-4. The Forwarding Table Maps Data Link Addresses to Physical Network Segments
Bridges communicate with one another to determine the best method of forwarding packets to a given data link layer destination using a Spanning Tree Protocol. This protocol allows bridges to build a loop-free topology over which to forward packets. A loop-free topology, a topology that guarantees that a packet reaches every segment of a network exactly once, is needed in a bridging environment to avoid broadcast storms and to avoid multiple parallel bridges forwarding a packet multiple times to a given segment. A broadcast storm is a network segment event in which a broadcast packet ”that is, a packet meant for every station on the segment ”is sent in a continual loop until the segment is overloaded with traffic.
The simplest form of a bridge, a transparent bridge, can handle the connection of only like data link layer protocols. Encapsulation and translational bridges can be considered transparent bridges, with the additional functionality of enabling different data link layer protocols to interoperate .
An encapsulation bridge encapsulates an entire data link layer frame in another data link layer, which allows transparent bridging between like data link layers to occur when they are physically separated by a second, different data link layer. For example, two encapsulation bridges, each with one Ethernet port and one serial port, can bridge Ethernet network segments when they are connected by a serial link. The serial link is a different Layer 2 medium than is Ethernet. Encapsulation bridging allows the entire Ethernet frame to be bridged from one segment to another when separated by the serial link because the bridge encapsulates the Ethernet frame in the serial link data link protocol. The result is that the devices on the two Ethernet segments that are joined by the encapsulation bridges believe that all the devices are attached to a single, logical Ethernet segment.
Another type of bridge is a translational bridge. A translational bridge performs the function of a transparent bridge between different types of data link layer protocols. For example, a translational bridge may translate Ethernet frames into Token Ring frames on the data link layer. If two devices are on different mediums connected by a translational bridge, they appear to be on one logical network segment. The transparent interconnection of two different mediums can provide the necessary connectivity for two devices that need to communicate solely at the data link layer.
A Cisco switch is essentially a multiport bridge that runs the IOS. A switch, which functions at the data link layer, performs the same basic functions as a bridge. The essential difference between a bridge and a switch is not technical, but packaging.
A switch may have more ports than a bridge, cost less per port than a bridge, and possess embedded management functions that a bridge does not have. Yet, when you examine the functionality of bridges and switches within the context of the OSI reference model, they do not differ . Many switches have multiple ports supporting a single data link layer protocol, such as Ethernet, and a smaller number of high-speed data link layer ports used to connect to faster mediums, such as ATM or Fast Ethernet. If a switch has two or more different interfaces to two or more data link layer protocols, it can be considered a translational bridge. Many switches today have interfaces that operate at multiple speeds, such as Ethernet, Fast Ethernet, and Gigabit Ethernet.
Figure 1-5 shows a small switched internetwork.
Figure 1-5. A Small Switched Internetwork
A router is a device that directs packets through the network based on network layer information. We focus on three network layer protocols in this book: IP, IPX, and AppleTalk. A router understands the network layer addressing in a packet and it has algorithms, called routing protocols, that build tables to determine the route that a packet should take to reach its final destination. For a multiprotocol routers ”one that understands multiple network layer addressing formats and routing protocols, such as a Cisco router ”the router keeps a separate routing table for each network layer protocol that is being routed, as shown in Figure 1-6.
Figure 1-6. A Multiprotocol Router Keeps a Routing Table for Each of Its Network Layer Protocols
A bridge or switch connects two or more physical networks into a single logical network, while a router connects two or more logical networks and routes between them using information that is built by routing protocols and kept in routing tables. The advantages of a router (as compared to using any type of bridge) are that it physically and logically breaks a network into multiple manageable pieces, allows for control of routed packets, and routes many different network layer protocols at the same time. In this book, we discuss many router configuration options in the Cisco IOS.
An access server, also called a communications server, is a device that connects asyn-chronous devices to a network. A common application of an access server is to connect a computer communicating over a modem to the Internet. The access server combines the functions of a router with the functions of an asynchronous protocol.
If a machine connects to an access server via an asynchronous interface, the access server provides the software that allows the machine to appear to be on the network. For example, an access server may have 16 asynchronous ports and a single Ethernet port. Any device that connects to an asynchronous port appears to be on the Ethernet where the access server resides, which allows people running IP, IPX, or AppleTalk to work from a remote machine, just as they would if they were on the local network. We discuss the configuration and functions of access servers throughout this book.