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This chapter is from the book

This chapter is from the book

Fully Redundant Layer 2 and Layer 3 Designs with Services

After discussing the build-out of a fully redundant Layer 2 and Layer 3 topology and considering the foundation of the Data Center, the focus becomes the design issues related to other Data Center services. These services are aimed at improving security and scaling the performance of application services by offloading processing away from the server farm to the network. These services include security, load balancing, SSL offloading, and caching; they are supported by a number of networking devices that must be integrated into the infrastructure following the design requirements.

Additionally, this section discusses application environment trends brought about by technology advancements in either applications, the application infrastructure, or the network infrastructure.

Additional Services

At the aggregation layer, in addition to Layer 2 and Layer 3, the Data Center might need to support the following devices:

  • Firewalls

  • Intrusion Detection Systems (IDSs)

  • Load balancers

  • SSL offloaders

  • Caches

It is important to discuss design issues when supporting some of these devices.

Service devices bring their own requirements that could change certain aspects of the design—for instance, the exchange state or status information, the NAT function that they perform on the source or destination IP addresses that forces them to be in the inbound and outbound path, and so on.

Service devices can be deployed using service modules integrated in the aggregation switches or as appliances connected to the aggregation switches. Both deployments require network connectivity and forethought about the actual traffic path.

Firewalls and load balancers may support the default gateway function on behalf of the server farms. Default gateway support traditionally has been provided by the router, so with two additional alternatives, you need to decide which is the default gateway and in which order traffic is processed through the multiple devices. Firewalls and load balancers are capable of providing stateful failover, which is supported by specific redundancy protocols. The protocols, which are specific to the firewalls or load balancers, must be supported by the design. SSL offloaders are typically used with load balancers and require the same considerations, with one exception: They do not support default gateway services.

IDSs are transparent to the design, which means that they integrate well with any existing design. The main consideration with regard to IDSs is their placement, which depends on selecting the location to analyze traffic and the traffic types to be monitored.

Caches, on the other hand, are deployed in reverse proxy cache mode. The placement of the caches and the mechanism for directing traffic to them impact the Data Center design. The options for traffic redirection are the Web Cache Communication Protocol (WCCP) on the Layer 2 or Layer 3 switches, and load balancers to distribute the load among the cache cluster. In either case, the cache or cache cluster changes the basic traffic path to the server farm when in use.

The following section presents the multiple deployment options.

Service Deployment Options

Two options exist when deploying Data Center services: using service modules integrated into the aggregation switch and using appliances connected to the aggregation switch. Figure 4-18 shows the two options.

Figure 18Figure 4-18 Service Deployment Options

Option a shows the integrated design. The aggregation switch is represented by a router (Layer 3) and a switch (Layer 2) as the key components of the foundation (shown to the left) and by a firewall, load balancer, SSL module, and IDS module (shown to the right as add-on services). The service modules communicate with the routing and switching components in the chassis through the backplane.

Option b shows the appliance-based design. The aggregation switch provides the routing and switching functions. Other services are provided by appliances that are connected directly to the aggregation switches.


Designs that use both modules and appliances are also possible. The most common case is when using caches, which are appliances, in both design options. Current trends on Data Center services lean toward integrated services. Evidence of this integration trend is the proliferation of services modules in the Catalyst 6500 family and the use of blade servers and blade chassis to collapse multiple services in one device.

A thoughtful approach to the design issues in selecting the traffic flow across different devices is required whether you are considering option a, option b, or any combination of the options in Figure 4-18. This means that you should explicitly select the default gateway and the order in which the packets from the client to the server are processed. The designs that use appliances require more care because you must be concerned with physical connectivity issues, interoperability, and the compatibility of protocols.

Design Considerations with Service Devices

Up to this point, several issues related to integrating service devices in the Data Center design have been mentioned. They are related to whether you run Layer 2 or Layer 3 at the access layer, whether you use appliance or modules, whether they are stateful or stateless, and whether they require you to change the default gateway location away from the router. Changing the default gateway location forces you to determine the order in which the packet needs to be processed through the aggregation switch and service devices.

Figure 4-19 presents the possible alternatives for default gateway support using service modules. The design implications of each alternative are discussed next.

Figure 4-19 shows the aggregation switch, a Catalyst 6500 using a firewall service module, and a content-switching module, in addition to the routing and switching functions provided by the Multilayer Switch Feature Card (MSFC) and the Supervisor Module.

The one constant factor in the design is the location of the switch providing server connectivity; it is adjacent to the server farm.

Figure 19Figure 4-19 Service Module Interoperability Alternatives

Option a presents the router facing the core IP network, the content-switching module facing the server farm, and the firewall module between them firewalling all server farms. If the content switch operates as a router (route mode), it becomes the default gateway for the server farm. However, if it operates as a bridge (bridge mode), the default gateway would be the firewall. This configuration facilitates the creation of multiple instances of the firewall and content switch combination for the segregation and load balancing of each server farm independently.

Option b has the firewall facing the server farm and the content switch between the router and the firewall. Whether operating in router mode or bridge mode, the firewall configuration must enable server health-management (health probes) traffic from the content-switching module to the server farm; this adds management and configuration tasks to the design. Note that, in this design, the firewall provides the default gateway support for the server farm.

Option c shows the firewall facing the core IP network, the content switch facing the server farm, and the firewall module between the router and the content-switching module. Placing a firewall at the edge of the intranet server farms requires the firewall to have "router-like" routing capabilities, to ease the integration with the routed network while segregating all the server farms concurrently. This makes the capability to secure each server farm independently more difficult because the content switch and the router could route packets between the server farm without going through the firewall. Depending on whether the content-switching module operates in router or bridge mode, the default gateway could be the content switch or the router, respectively.

Option d displays the firewall module facing the core IP network, the router facing the server farm, and the content-switching module in between. This option presents some of the same challenges as option c in terms of the firewall supporting IGPs and the inability to segregate each server farm independently. The design, however, has one key advantage: The router is the default gateway for the server farm. Using the router as the default gateway allows the server farms to take advantage of some key protocols, such as HSRP, and features, such as HSRP tracking, QoS, the DHCP relay function, and so on, that are only available on routers.

All the previous design options are possible—some are more flexible, some are more secure, and some are more complex. The choice should be based on knowing the requirements as well as the advantages and restrictions of each. The different design issues associated with the viable options are discussed in the different chapters in Part V. Chapter 21, "Integrating Security into the Infrastructure," addresses the network design in the context of firewalls.

Application Environment Trends

Undoubtedly, the most critical trends are those related to how applications are being developed and are expected to work on the network. These trends can be classified arbitrarily into two major areas:

  • Application architectures

  • Network infrastructure

Application Architecture Trends

Application architecture trends include the evolution of the classic client/server model to the more specialized n-tier model, web services, specific application architectures, the server and client software (operating systems), application clients, the server and client hardware, and middleware used to integrate distributed applications in heterogeneous environments.

The more visible trends of application architectures are the wide adoption of web technology in conjunction with the use of the n-tier model to functionally segment distinct server types. Currently, web, application, and database servers are the basic types, yet they are combined in many ways (depending on the vendor of the application and how the buyer wants to implement it).

This functional partitioning demands that the network be smarter about securing and scaling the tiers independently. For instance, the n-tier model's web tier layer created the need for smaller and faster servers used to scale up the front-end function. This resulted in 1RU (rack unit) servers, which offer adequate performance for web servers at a low cost and minimal infrastructure requirements (power and rack space).

Web services are bringing a service-oriented approach to the use of different and distinct distributed applications that are accessible using standard messages over Internet protocols. Web services rely initially on the transport functions of the network and eventually on using the network as an extension to provide computing capacity to the distributed application environments by offloading tasks to network hardware.


The World Wide Web consortium (W3C) defines a web service as "a software application identified by a URI, whose interfaces and binding are capable of being defined, described, and discovered by XML artifacts and supports direct interactions with other software applications using XML-based messages via Internet-based protocols." For more information on web services and its architecture, consult the W3C at http://www.w3.org.

Grid computing is another trend that actually brings the applications and the network closer together by treating the servers as a network of CPUs in which the applications use the most available CPU on the network. Other trends related to grid computing include blade servers as an alternative to 1RU servers, to provide higher CPU density per RU, lower power consumption per server, and an additional benefit of lower cabling requirements. Blade servers are servers on blades (or modules) that are inserted into a chassis, much like network modules or line cards are inserted on a switch chassis. Using blade servers in blade chassis enables you to centralize the server-management functions (one chassis instead of however many servers are in the chassis), requires less cables (one set per chassis instead of one set per server), and provides higher computing and memory capacity per rack unit.

However, the blade server technology is still young, which explains the variety of flavors, architectures, connectivity options, and features.

An instance of middleware is the software used in the management and control of distributed CPUs in a grid of computers that can be 1RU or blade servers. This specific middleware virtualizes the use of CPUs so that the applications are given a CPU cycle from CPUs on the network instead of through the traditional manner.

Network Infrastructure Trends

The network infrastructure is growing smarter and more application-aware, and it thereby supports application environments both by offloading some computationally intense tasks to the network (typically hardware-based) and by replacing some functions performed by servers that could be better handled by networking devices.

Load balancing is a good example of a function performed by the network that replaces clustering protocols used by servers for high availability. Clustering protocols tend to be software-based, hard to manage, and not very scalable in providing a function that the network performs well using hardware.

Trends such as blade servers bring new design considerations. Most blade server chassis (blade chassis, for short) in the market support both an option to provide redundant Ethernet switches inside the chassis and as an option to connect the blade servers to the network using pass-through links, with the chassis simply providing at least twice as many uplinks as servers in the chassis, to allow dual-homing.

Figure 4-20 presents both connectivity alternatives for a blade chassis.

Figure 20Figure 4-20 Blade Server Chassis Server Connectivity

Option a in Figure 4-20 shows a blade server chassis in which each blade server is connected to each of the blade chassis's redundant Layer 2 Ethernet switches. Each blade chassis's Ethernet switch provides a number of uplinks that can be channeled to the IP network. The number of uplinks is typically smaller than the combined number of links per server, which requires planning for oversubscription, particularly if the servers are Gigabit Ethernet–attached. The midplane is the fabric used for management tasks, that is, control plane traffic such as switch status.

Option b in Figure 4-20 presents the pass-through option in which the servers are dual-homed and preconnected to a pass-through fabric that provides the connectivity to the IP network. This option does not use Ethernet switches inside the chassis. The pass-through fabric is as simple as a patch panel that conserves the properties of the server NICs, but it also could become a more intelligent fabric, adding new features and allowing blade server vendors to differentiate their products. Either approach you take to connect blade servers to your network requires careful consideration on short- and long-term design implications.

For instance, if the choice is to utilize the redundant Ethernet switches in the blade chassis, you have the following design alternatives to consider:

  • How to use the redundant Ethernet switches' uplinks for connectivity

  • Whether to connect the blade chassis to the access or aggregation switches

  • What level of oversubscription is tolerable

Figure 4-21 displays two connectivity choices utilizing the uplinks on the redundant Ethernet switches. For redundancy, two switches are used to connect the uplinks from the blade chassis. Switches A and B, the small clouds in the IP network cloud, provide a redundant network fabric to the blade chassis to avoid single point of failure issues.

Figure 21Figure 4-21 Blade Chassis Uplink Connectivity

Option a in Figure 4-21 shows all the uplinks from both blade chassis' Ethernet switches connected to a single switch in the IP network. This allows the uplinks to be channeled. In contrast, option b in Figure 4-21 shows each blade chassis Ethernet switch connected to each IP network switch, also avoiding a single point of failure. This presents the advantage of having a direct link to either switch A or switch B, thus avoiding unnecessary hops. Additionally, if each blade chassis Ethernet switch supports more than two uplinks, they can also be channeled to switches A and B for greater redundancy and higher bandwidth.

The next step is to determine whether to connect the blade chassis to the access-layer switches, as is traditionally done with servers, or to the aggregation layer switches. Figure 4-22 displays the connectivity options for the next-hop switches from the blade chassis.

Figure 22Figure 4-22 Blade Chassis Next-Hop Switch

Option a in Figure 4-22 shows the blade chassis connected to the access-layer switches. This particular design choice is equivalent to connecting Layer 2 access switches to Layer 2 access switches. The design must take into account spanning tree recommendations, which, based on the topology of option a in Figure 4-22, are aimed at determining a loop-free topology given the number of Layer 2 switches and the amount of available paths to the STP root and the secondary root from each leaf node. If the blade chassis Ethernet switches support 802.1w, the convergence time stays within two to three seconds; however, if the support is strictly 802.1d, the convergence time goes back to the typical range of 30 to 50 seconds. Other design considerations have to do with whether the midplane is used for more than management and switch-to-switch control traffic communication functions. If for some reason the midplane also is used to bridge VLANs (forward Bridge Protocol Data Units, or BPDUs) the STP topology needs to be considered carefully. The design goals remain making the topology predictable and deterministic. This implies that you need to explicitly set up root and bridge priorities and analyze the possible failure scenarios to make sure they support the requirements of the applications.

Option b in Figure 4-22 shows the blade chassis Ethernet switches directly connected to the aggregation switches. This is the preferred alternative because it lends itself to being more deterministic and supporting lower convergence times. Much like in the previous option, if the blade chassis Ethernet switches do not support 802.1w or some of the STP enhancements such as Uplinkfast and Loopguard, the convergence time would be in the range of 30 to 50 seconds. The topology still needs to be made deterministic and predictable by explicitly setting up root and bridge priorities and testing the failures scenarios.

How to scale the blade server farm is another consideration. Scalability on server environments is done simply by adding pairs of access switches for redundancy and connecting them to the aggregation switches, as shown in option a in Figure 4-23.

Figure 23Figure 4-23 Server Farm Scalability

If a single scalable server module supports X servers (limited by port density), higher scalability is achieved by replicating the scalable module Y times (limited by slot density in the aggregation switch). The total number of servers could be X * Y. Depending on the access switch port density and the aggregation switch slot density, this could grow to thousands of servers. Scaling the number of blade servers might require a slightly different strategy. Because blade chassis with Ethernet switches are the access layer, the amount of blade server is limited to the number of slots and ports per slot at the aggregation switches. Option b in Figure 4-23 shows this alternative.

Notice that the scalable module is now the aggregation switch along with a set number of blade chassis. This is because the aggregation switch has a limit to the number of slots that can be used for blade chassis. In addition, line cards used to support blade server uplinks now receive aggregate server traffic, thus requiring less oversubscription. This leads to fewer ports used per line card. So, the total number of blade servers is limited somewhat by the slot and port density. Even though this design alternative is likely to support hundreds of blade servers and satisfy the requirements for a fast-growing server farm environment, you must have a plan for what to do if you need to increase your server farm beyond what the current design supports. Figure 4-24 shows this alternative.

Figure 24Figure 4-24 Core Layer Within the Data Center

Figure 4-24 introduces a new layer in the Data Center: the core layer. The core layer is used to aggregate as many server blade modules as needed, but the number is limited to the port and slot capacity to the aggregation switches. The pass-through option might not require as much planning because the blade chassis do not have redundant Ethernet switches. The uplinks are connected to the access layer, which is equivalent to current designs in which servers are dual-homed to a redundant set of access switches.

Setting aside the connectivity, port density, slot density, and scalability considerations, other areas, such as oversubscription, uplink capacity, and service deployment options, might require design and testing before the Data Center architecture is established.

Additional trends include the dual-homing of servers, the migration from Fast Ethernet to Gigabit Ethernet, application firewalls, and the use of transparent network service devices. Application firewalls are firewalls that are more in tune with application behavior than ordinary firewalls, thus making the firewalling process more granular to application information in addition to just network or transport layer information. For instance, an application firewall might be capable of identifying not only that a packet is TCP and that the information in the TCP payload is HTTP, but also that the request comes from a specific high-priority user and is a SQL request for sensitive payroll information, which requires a higher security service level.

Transparent network services include firewalling, load balancing, SSL offloading, and so on. These services are provided by network devices with minimal interoperability issues that leave the existing designs unchanged. These transparent services could apply to traditional network services such as load balancing and firewalling, yet they are implemented to minimize disruption and changes in the application environment. This approach might include using physical devices as if they were distinct logical entities providing services to different server farms concurrently. This implies that the administration of those services, such as configuration changes or troubleshooting efforts, is isolated to the specific logical service. Think of it as a single physical firewall that is deployed to support many server farms concurrently where access to the CLI and configuration commands is available only to users who have been granted access to the specific server farm firewall service. This would appear to the user as a completely separate firewall.

Some of these trends are ongoing, and some are barely starting. Some will require special design and architectural considerations, and some will be adopted seamlessly. Others will not exist long enough for concern.

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