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Network architectures and design methodologies help you manage the complexities of networks. This chapter also describes steps in design methodology and contents of design documents. This architecture model separates network design into more manage- able modules. This chapter also addresses the use of device, media, and route redundancy to improve network availability.

In the hierarchical network model, which layer is responsible for fast transport? Network layer b. Core layer c. Distribution layer d. Access layer 2. Which Enterprise Architecture model component interfaces with the service provider SP? Campus infrastructure b. Access layer c. Enterprise edge d. Edge distribution 3. In the hierarchical network model, at which layer do security filtering, address aggre- gation, and media translation occur?

Which of the following is are a method methods of workstation-to-router redun- dancy in the access layer? Answers b and c e. Answers a, b, and c 5. The network-management module has tie-ins to which component s? Server farm c. SP edge e. Answers a and b f. Answers a, b, and c g.

Answers a, b, c, and d 6. Public switched telephone network PSTN service b. Edge distribution c. Server farm d. Core layer 7. Campus core b. E-commerce c. Edge distribution farm 8. High availability, port security, and rate limiting are functions of which hierarchical layer? The hierarchical network model was one of the first Cisco models that divided the network into core, distribution, and access layers.

In addition to a hierarchy, modules are used to organize server farms, network man- agement, campus networks, WANs, and the Internet. A modular approach to network design allows for higher scalability, better resiliency, and easier fault isolation of the network.

Hierarchical Network Models Hierarchical models enable you to design internetworks that use specialization of function combined with a hierarchical organization. Such a design simplifies the tasks required to build a network that meets current requirements and can grow to meet future requirements. Hierarchical models use layers to simplify the tasks for internetworking.

Each layer can focus on specific functions, allowing you to choose the right systems and features for each layer.

Keeping each design element simple and functionally focused facilitates ease of understand- ing, which helps control training and staff costs. You can distribute network monitoring and management reporting systems to the different layers of modular network architectures, which also helps control management costs.

Hierarchical design facilitates changes and growth. In a network design, modularity lets you create design elements that you can replicate as the network grows—allowing maximum scalability. As each element in the network design requires change, the cost and complex- ity of making the upgrade are contained to a small subset of the overall network. In large, flat network architectures, changes tend to impact a large number of systems.

Limited mesh topologies within a layer or component, such as the campus core or backbone connecting central sites, retain value even in the hierarchical design models. Network managers can easily understand the transition points in the network, which helps identify failure points. It is more difficult to troubleshoot if hierarchical design is not used because the network is not divided into segments.

To control the impact of routing-protocol processing and bandwidth consumption, you must use modular hierarchical topologies with protocols designed with these controls in mind, such as the Open Shortest Path First OSPF routing protocol.

Hierarchical network design facilitates route summarization. Route summarization reduces routing-protocol overhead on links in the network and reduces routing-protocol processing within the routers.

It is less possible to provide route summarization if the network is not hierarchical. To Enterprise Edge Modules Core Distribution Access Figure Hierarchical network design has three layers: core, distribution, and access Each layer provides necessary functionality to the enterprise campus network.

You do not need to implement the layers as distinct physical entities. You can implement each layer in one or more devices or as cooperating interface components sharing a common chassis. Maintaining an explicit awareness of hierarchy is useful as the network grows.

It is also referred as the backbone. As noted, it is considered good practice to design for a consistent diameter within a hierarchical network. The trip from any end station to another end station across the back- bone should have the same number of hops. The distance from any end station to a server on the backbone should also be consistent.

Use of a block implementation isolates existing end stations from most effects of network growth. Access Layer The access layer provides user access to local segments on the network. The access layer is characterized by switched LAN segments in a campus environment.

Microsegmentation using LAN switches provides high bandwidth to workgroups by reducing the number of devices on Ethernet segments. The LAN switch in the access layer can con- trol access to the port and limit the rate at which traffic is sent to and from the port. Other chapters of this book cover the other functions in the list. Remote access can include virtual private network VPN technology. Table summarizes the hierarchical layers.

Figure is an example of a switched hierarchical design in the enterprise campus. In this design, the core provides high-speed transport between the distribution layers.

The building distribution layer provides redundancy and allows policies to be applied to the building access layer. Layer 3 links between the core and distribution switches are recommended to allow the routing protocol to take care of load balancing and fast route redundancy in the event of a link failure.

The distribution layer is the boundary between the Layer 2 domains and the Layer 3 routed network. Inter-VLAN communica- tions are routed in the distribution layer. Route summarization is configured under the routing protocol on interfaces towards the core layer. The drawback with this design is that Spanning Tree Protocol STP allows only one of the redundant links between the access switch and the distribution switch to be active.

In the event of a failure, the second link becomes active, but at no point does load balancing occur. Figure shows examples of a routed hierarchical design. In this design, the Layer 3 boundary is pushed toward the access layer.

Layer 3 switching occurs in access, distribu- tion, and core layers. Route filtering is configured on interfaces toward the access layer. Route summarization is configured on interfaces toward the core layer. The benefit of this design is that load balancing occurs from the access layer since the links to the distribution switches are routed.

Another solution for providing redundancy between the access and distribution switching is the Virtual Switching System VSS.

VSS solves the STP looping problem by converting the distribution switching pair into a logical single switch. As shown in Figure , the two switches are connected via 10GE links called virtual switch links VSLs , which makes them seem as a single switch. The hub-and-spoke design, illustrated in Figure , also scales better and is easier to manage than ring or mesh topologies. For example, implementing security policies in a full mesh topology would become unmanageable because you would have to configure policies at each point location.

Hub-and-Spoke Topology allows for more Ring Topology adds more delay scalability and easier management. Mesh Topology requires a network connection to all other devices. It is a two-layer hierarchy used with smaller networks. It is commonly used on sites with a single build- ing with just multiple floors. As shown in Figure , the core and distribution layers are merged, providing all the services needed for those layers.

Design parameters to decide if 2 you need to migrate to the three-layer hierarchy include not enough capacity and through- put at the distribution layer, network resiliency, and geographic dispersion. As networks become more sophisticated, it is necessary to use a more modular approach to design than just WAN and LAN core, distribution, and access layers.

The architecture divides the network into functional network areas and modules. The modular approach in design should be a guide to the network architect. In smaller networks, the layers can collapse into a single layer, even a single device, but the functions remain. Figure shows the Cisco Enterprise Architecture model.

The enterprise campus area contains a campus infrastructure that consists of core, building distribution, and building access layers, with a data center module.

The enterprise edge connects to the edge-distribution module of the enterprise campus. In small and medium sites, the edge distribution can collapse into the campus backbone component.

It provides connectivity to outbound services that are further described in later sections. The campus infrastructure consists of the campus core, building distribution, and building access layers. The campus core provides a high-speed switched backbone between buildings, to the server farm, and towards the enterprise edge.

This segment consists of redundant and fast-convergence connectivity. The building distribution layer aggregates all the closet access switches and performs access control, QoS, route redundancy, and load balancing. Network management servers are located in the server farm, but these servers link to each module in the campus to provide network moni- toring, logging, trending, and configuration management.

An enterprise campus infrastructure can apply to small, medium, and large locations. In most instances, large campus locations have a three-tier design with a wiring-closet component building access layer , a building distribution layer, and a campus core layer.

Small campus locations likely have a two-tier design with a wiring-closet component Ethernet access layer and a backbone core collapsed core and distribution layers. It is also possible to configure distribution functions in a multilayer building access device to maintain the focus of the campus backbone on fast transport. Medium-sized campus network designs sometimes use a three-tier implementation or a two-tier implementation, depending on the number of ports, service requirements, manageability, performance, and availability required.

It uses the high availability designs of the server farm module with the Internet connectivity of the Internet module. Design techniques are the same as those described for these modules. Connectivity to one or several Internet service providers ISPs is also pro- vided. The simplest form is to have a single circuit between the enterprise and the SP, as shown in Figure The drawback is that you have no redundancy or failover if the circuit fails. Option 2 provides link and ISP redundancy but does not provide redundancy for a local router failure.

Option 3 provides link and local router redundancy but does not provide for an ISP failure. Option 4 provides for full redundancy of the local router, links, and ISPs.

VPNs reduce communication expenses by leveraging the infrastructure of SPs. For critical applications, the cost savings might be offset by a reduction in enterprise control and the loss of deterministic service. Branch offices obtain local Internet access from an ISP.

Teleworkers also obtain local Internet access. These connections are assigned to the Internet connectivity module. Implement the security policy and configure authentication and authorization parameters. ISPs offer enterprises access to the Internet. It is common now for the SP to have their ISP router at the customer site and provide Ethernet access to the customer. For the enterprise network, the PSTN lets dialup users access the enterprise via analog or cellular wireless technologies.

These and other WAN technologies are described in Chapter 6. Enterprise Branch Module The enterprise branch normally consists of remote offices or sales offices.

These branch offices rely on the WAN to use the services and applications provided in the main campus. The offsite data center provides disaster recovery and business continuance services for the enterprise.

Highly available WAN services are used to connect the enterprise campus to the remote enterprise data center. As shown in Figure , mobile users connect from their homes, hotels, or other locations using dialup or Internet access lines.

VPN clients are used to allow mobile users to securely access enterprise applications. IP phone capabilities are also provided in the Cisco Virtual Office solution, providing corporate voice services for mobile users. Table Cisco Enterprise Architecture Model Enterprise Description Area or Module Enterprise The enterprise campus module includes the building access and building campus area distribution components and the shared campus backbone component or campus core.

Edge distribution provides connectivity to the enterprise edge. High availability is implemented in the server farm, and network management monitors the enterprise campus and enterprise edge. WAN module Enterprise remote The enterprise branch normally consists of remote offices, small offices, branch module or sales offices.

Enterprise data The enterprise data center consists of using the network to enhance the center module server, storage, and application servers. Enterprise The enterprise teleworker module supports a small office, mobile users, teleworker or home users providing access to corporate systems via VPN tunnels. High Availability Network Services This section covers designs for high availability network services in the access layer.

When designing a network topology for a customer who has critical systems, services, or network paths, you should determine the likelihood that these components will fail and then design redundancy where necessary.

VSS is covered earlier in the chapter. A router running proxy ARP can respond with its data link layer address. Cisco routers run proxy ARP by default. Explicit Configuration Most IP workstations must be configured with the IP address of a default router, which is sometimes called the default gateway.

In an IP environment, the most common method for a workstation to find a server is via explicit configuration a default router. Some IP stacks enable you to configure multiple default routers, but many other IP implementations support only one default router. You should use RIP in passive mode rather than active mode. Active mode means that the station sends RIP frames every 30 seconds.

The work- stations use this virtual IP address as their default router. The active router sends periodic hello messages.

The other HSRP routers listen for the hello messages. If the active router fails and the other HSRP routers stop receiving hello messages, the standby router takes over and becomes the active router.

If the router that originally sent the ARP reply later loses its con- nection, the new active router can still deliver the traffic. Figure shows a sample implementation of HSRP. The workstation is configured to use the phantom router The active router does the work for the HSRP phantom.

The change is transparent to the workstation. The VRRP router controlling the IP addresses associated with a virtual router is called the master, and it forwards packets sent to these IP addresses. The election process provides dynamic failover in the forwarding responsibility should the master become unavailable.

This allows any of the virtual router IP addresses on the LAN to be used as the default first-hop router by end hosts. The virtual router backup assumes the forwarding responsibility for the virtual router should the master fail. Each host is configured with the same virtual IP address, and all routers in the virtual router group participate in forwarding packets.

By default, all routers within a group forward traffic and load-balance automati- cally. GLBP members communicate between each other through hello messages sent every three seconds to the multicast address Server Redundancy Some environments need fully redundant mirrored file and application servers.

For example, in a brokerage firm where traders must access data to buy and sell stocks, two or more redundant servers can replicate the data.

Route Redundancy Designing redundant routes has two purposes: balancing loads and increasing availability. Load Balancing Most IP routing protocols can balance loads across parallel links that have equal cost. Use the maximum-paths command to change the number of links that the router will balance over for IP; the default is four, and the maximum is six.

To support load balancing, keep the bandwidth consistent within a layer of the hierarchical model so that all paths have the same cost. Cisco Enhanced Interior Gateway Routing Protocol [EIGRP] is an exception because it can load-balance traffic across multiple routes that have different metrics by using a fea- ture called variance. A hop-based routing protocol does load balancing over unequal-bandwidth paths as long as the hop count is equal.

After the slower link becomes saturated, packet loss at the saturated link prevents full utilization of the higher-capacity links; this scenario is called pinhole con- gestion.

You can avoid pinhole congestion by designing and provisioning equal-bandwidth links within one layer of the hierarchy or by using a routing protocol that takes bandwidth into account. IP load balancing in a Cisco router depends on which switching mode the router uses. Process switching load balances on a packet-by-packet basis. Fast, autonomous, silicon, optimum, distributed, and NetFlow switching load balances on a destination-by-destination basis because the processor caches information used to encapsulate the packets based on the destination for these types of switching modes.

Increasing Availability In addition to facilitating load balancing, redundant routes increase network availability. You should keep bandwidth consistent within a given design component to facilitate load bal- ancing. Another reason to keep bandwidth consistent within a layer of a hierarchy is that rout- ing protocols converge much faster on multiple equal-cost paths to a destination network.

By using redundant, meshed network designs, you can minimize the effect of link failures. Depending on the convergence time of the routing protocols, a single link failure cannot have a catastrophic effect. You can design redundant network links to provide a full mesh or a well-connected partial mesh. A full-mesh network provides complete redundancy and also provides good performance because there is just a single-hop delay between any two sites. Each router is connected to every other router.

A well-connected partial-mesh network provides every router with links to at least two other routing devices in the network. Figure Full-mesh network: Every router has a link to every other router in the network. A full-mesh network can be expensive to implement in WANs because of the required number of links. In addition, groups of routers that broadcast routing updates or service advertisements have practical limits to scaling.

As the number of routing peers increases, the amount of bandwidth and CPU resources devoted to processing broadcasts increases. A suggested guideline is to keep broadcast traffic at less than 20 percent of the bandwidth of each link; this amount limits the number of peer routers that can exchange routing tables or service advertisements.

When designing for link bandwidth, reserve 80 percent of it for data, voice, and video traffic so that the rest can be used for routing and other link traffic. When planning redundancy, follow guidelines for simple, hierarchical design. Figure illustrates a classic hierarchical and redundant enterprise design that uses a partial-mesh rath- er than a full-mesh topology. For LAN designs, links between the access and distribution layers can be Fast Ethernet, with links to the core at Gigabit Ethernet speeds.

Headquarters 1. In switched networks, switches can have redundant links to each other. This redundancy is good because it minimizes downtime, but it can result in broadcasts continuously circling the network, which is called a broadcast storm. The spanning-tree algorithm guarantees that only one path is active between two network stations.

The algorithm permits redundant paths that are automatically activated when the active path experiences problems. STP has a design limitation of only allowing one of the redundant paths to be active. VSS can be used with Catalyst switches to overcome this limitation.

You can use EtherChannel to bundle links for load balancing. Links are bundled in pow- ers of 2 2, 4, 8 groups. It aggregates the bandwidth of the links. Hence, two 10GE ports become 20 Gbps of bandwidth when they are bundled. For more granular load balancing, use a combination of source and destination per-port load balancing if available on the switch. LACP helps protect against Layer 2 loops that are caused by misconfigura- tion.

One downside is that it introduces overhead and delay when setting up the bundle. As shown in Figure , you can provision backup links so that they become active when a primary link goes down or becomes congested.

Often, backup links use a different technology. By using floating static routes, you can specify that the backup route must have a higher administrative distance used by Cisco routers to select routing information so that it is not normally used unless the pri- mary route goes down. Different carriers sometimes use the same facilities, meaning that your backup path might be susceptible to the same failures as your primary path.

Do some investigative work to ensure that your backup really is acting as a backup. It bonds multiple WAN links into a single logical channel. MPPP does not specify how a router should accomplish the decision-making process to bring up extra channels. Instead, it seeks to ensure that packets arrive in sequence at the receiving router.

Then, the data is encapsulated within PPP and the datagram is given a sequence number. At the receiving router, PPP uses this sequence number to re-create the original data stream. Multiple channels appear as one logical link to upper-layer protocols.

Table summarizes the four main redundancy models. Medianet at a Glance, www. Application Performance white paper, www. Virtual Switching System, www. Table lists a reference of these key topics and the page numbers on which each is found.

True or false: The core layer of the hierarchical model does security filtering and media translation. True or false: The access layer provides high availability and port security. In which submodule of the Enterprise Architecture model should you place Communications Manager?

True or false: HSRP provides router redundancy. Chapter 2: Network Design Models 69 5. Which enterprise edge submodule connects to an ISP? List the six modules of the Cisco Enterprise Architecture model for network design. True or false: In the Cisco Enterprise Architecture model, the network management submodule does not manage the SP edge. How many links are required for a full mesh of six sites?

List and describe four options for multihoming to the SP between the enterprise edge and the SP edge. Which option provides the most redundancy? To what enterprise edge submodule does the SP edge Internet submodule connect? What are four benefits of hierarchical network design? Match the redundant model with its description: i. Workstation-router redundancy ii. Server redundancy iii. Route redundancy iv. Media redundancy a. Provides load balancing. Host has multiple gateways. Data is replicated.

True or false: Small-to-medium campus networks must always implement three layers of hierarchical design. How many full-mesh links do you need for a network with ten routers?

Which layer provides routing between VLANs and security filtering? Access layer b. Distribution layer c. WAN module List the four modules of the enterprise edge area. List the three submodules of the SP edge. List the components of the Internet edge. WAN b. Internet d. Server farm Which of the following describe the access layer? Select two. High-speed data transport b. Applies network policies c. Performs network aggregation d. Concentrates user access e.

Provides PoE f. Avoids data manipulation Which of the following describe the distribution layer? Which of the following describe the core layer? Trust the best selling Official Cert Guide series from Cisco Press to help you learn, prepare, and practice for exam success. To see what your friends thought of this book, please sign up. Want to Read Currently Reading Read.

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