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Cisco s enterprise campus hierarchical model, shown in Figure 14-1, contains three layers: core, distribution, and access A well-designed campus network typically follows this topology The core layer, as its name suggests, is the backbone of the network It provides a high-speed connection between the different distribution layer devices Because of the need for high-speed connections, the core consists of high-speed switches and will not, typically, perform any type of packet or frame manipulations, such as filtering Layer 2 or layer 3 switches (more commonly the latter) are used at the core with Gigabit connectivity, sometimes using EtherChannels (discussed at the end of the chapter) The traffic that traverses the core is typically intending to access enterprise corporate resources: connections to the Internet, WAN connections, and applications on servers in the campus server farm
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Layer 2 Redundancy
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FIGURE 14-1 Core layer
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Cisco s hierarchical model
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Distribution layer
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A layer 3 switch is basically a router that performs its switching using application-specific integrated circuits (ASICs) instead of a central CPU (ASICs are discussed in 4) Some of Cisco s switches support this feature, from the Catalyst 3500s and higher with the right IOS image Layer 3 switches are most commonly used at the distribution layer to connect the access layers to the core Fast Ethernet, but more commonly Gigabit Ethernet, connections are used for this connectivity For smaller networks, sometimes layer 2 switches are used The responsibilities of the distribution layer include the following:
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Containing broadcasts between the model layers via layer 3 separation, such
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as VLANs
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Securing traffic between subnets using access control lists (ACLs are
discussed in 22)
Providing a hierarchy through layer 3 logical addressing and route summarization Translating between different media types
Layer 3 separation is provided between the access layers and the core; however, between the distribution layer switches and the access layer switches, layer 2 is used, creating a layer 2 loop
14: Switches and Redundancy
The bottom layer of the three-layer hierarchical model is the access layer Actually, the access layer is at the periphery of your campus network, separated from the core layer by the distribution layer The main function of the access layer is to provide the user s initial connection to your network Typically, this connection is provided by a layer 2 switch or sometimes a wireless access point Cisco has online design tools to help you choose the appropriate devices and uplink connections in your three-layer hierarchical network In a basic design, you should not oversubscribe your access-to-distribution layer links by more than 20-to-1 Oversubscription occurs when too much traffic comes from your users access connections and not enough bandwidth is available from the access layer switches to the distribution layer (oversubscription is discussed in more depth in 26) You should also not oversubscribe your distribution-to-core links by more than 4-to-1 However, every situation is unique when it comes to designing the appropriate solution Rate limiting is commonly used to prevent users from oversubscribing and creating problems for other users or applications
Layer 2 Issues
As you can see in Figure 14-1, a redundant design was implemented: two links exist from the access layer switch up to the distribution layer, with multiple layer 2 connections (in most cases an EtherChannel) between the distribution layer switches This redundancy commonly introduces layer 2 loops into a network design, which can create these layer 2 problems:
Multiple frame copies Broadcast storms Mislearning MAC addresses
The following three sections will discuss these problems, using the simple example in Figure 14-2 to illustrate the issues these problems create
Multiple Frame Copies and Broadcast Storms
Recall from 4 that a switch will flood three kinds of frames: broadcast, multicast, and unknown destination unicast frames In the case of broadcast and multicast traffic, this can create serious performance issues with a layer 2 network that has loops For example, imagine that PC-A in Figure 14-2 performs an ARP
Layer 2 Redundancy
FIGURE 14-2
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