CCNP 1: Building Scalable Cisco InternetworksOverview Of Scalable Networks

The Hierarchical Network Design Model

Make sure that you visit the following link: Internetworking Design Basics

Core Layer

As the center of the network, the core layer is designed to be fast and reliable.

Access lists should be avoided in the core layer since they add latency and end users should not have access directly to the core.

In a hierarchical network, end user traffic should reach core routers only after those packets have passed through the distribution and access layers, where access lists may be implemented.

Core Layer (Continued) The most powerful Cisco routers serve the core because they have the

fastest switching technologies and the largest capacity for physical interfaces.

The Cisco 7000, 7200, and 7500 series routers are modular, allowing interface modules to be added providing scalability. The large chassis of this series can accommodate dozens of interfaces on multiple modules for virtually any media type, which makes these routers scalable and reliable core solutions.

Core routers achieve reliability through the use of redundant links, usually to all other core routers.

When possible, these redundant links should be symmetrical having equal throughput, so that equal-cost load balancing may be used.

Core routers need a relatively large number of interfaces to enable this configuration.

Core routers achieve reliability through redundant power supplies and usually feature two or more "hot-swappable" power supplies, which may be removed and replaced individually without shutting down the router.

Core Layer (Continued) With the high-end routers and WAN links involved, the

core can become a huge expense, even in a simple example such as this.

Some designers will choose not to use symmetrical links in the core to reduce cost. In place of redundant lines, packet-switched and dial-on-demand technologies, such as Frame Relay and ISDN, may be used as backup links.

The trade-off for saving money by using such technologies is performance. Using ISDN BRIs as backup links can eliminate the capability of equal-cost load balancing.

Core Layer Continued

The core of a network does not have to exist in the WAN. A LAN backbone may also be considered part of the core layer.

Campus networks, or large networks that span an office complex or adjacent buildings, might have a LAN-based core. Switched Fast Ethernet and Gigabit Ethernet are the most common core technologies, usually run over fiber.

Enterprise switches, such as the Catalyst 4000, 5000, and 6000 series, shoulder the load in LAN cores because they switch frames at Layer 2 much faster than routers can switch packets at Layer 3. In fact, as modular devices, these switches can be equipped with route switch modules (RSMs), adding Layer 3 routing functionality to the switch chassis.

Distribution Layer

The following rules will protect the core from unnecessary or unauthorized traffic.

Distribution layer routers need fewer interfaces and less switching speed than their counterparts in the core because they should handle less traffic. Nevertheless, a lightning fast core is useless if a bottleneck at the distribution layer prevents user traffic from accessing core links.

For this reason, Cisco offers robust, powerful distribution routers, such as the 4000, 4500, and the 3600 series router. These routers are modular allowing interfaces to be added and removed depending on need. However, the smaller chassis of these series are much more limiting than those of the 7000, 7200, and 7500 series.

Distribution Layer Continued

Distribution layer routers bring policy to the network by using a combination of access lists, route summarization, distribution lists, route maps, and other rules to define how a router should deal with traffic and routing updates

Distribution Layer Continued The figure shows two 3620 routers

have been added at Core A, in the same wiring closet as the 7507. This means that the high-speed LAN links may be used to make the connections between the distribution routers and the core router. Depending on the size of the network, these links may be part of the campus backbone and will most likely be fiber running 100 or 1000 Mbps.

In this example, Dist-1 and Dist-2 are part of the Core A campus backbone. Dist-1 serves remote sites, while Dist-2, serves access routers at Site A. If Site A employs VLANs throughout the campus, Dist-2 may be responsible for routing between them.

Distribution Layer Continued

Both Dist-1 and Dist-2 use access lists to prevent unwanted traffic from reaching the core. In addition, these routers summarize their routing tables in updates to Core A, keeping the Core A routing table as small and efficient as possible

Access Layer

Routers at the access layer are deployed to permit users at Site A and remote sites Y and Z to access the network.

Access routers generally offer fewer physical interfaces than distribution and core routers. For this reason, Cisco access routers, which include the 1600, 1700, 2500, and 2600 series, feature a small, streamlined chassis that may or may not support modular interfaces.

Access Layer Continued Each remote site in the example

requires only one Ethernet interface for the LAN side and one serial interface for the WAN side.

The WAN interface connects by way of Frame Relay or ISDN to the distribution router in the wiring closet of Site A.

For this application, the 2610 router provides a single 10-Mbps Ethernet port and will work well at these locations. These remote sites, Y and Z, are small branch offices that must access the core through Site A.

Therefore, Dist-1 A is acting as a WAN hub for the organization. As the network scales, dozens of remote sites may access the core by connection to distribution routers at the WAN hubs, Site A, Site B, and Site C.  

5 Characteristics of Scalable Networks Reliable and available – A reliable network should

be dependable and available 24 hours a day, seven days a week. In addition, failures need to be isolated, and recovery must be invisible to the end user.

Responsive – A responsive network should provide Quality of Service (QoS) for various applications and protocols without affecting a response at the desktop.

Adaptable – An adaptable network is capable of accommodating different protocols, applications, and hardware technologies.

5 Characteristics of Scalable Networks Efficient – Large internetworks must optimize the

use of resources, especially bandwidth. Reducing the amount of overhead traffic, such as unnecessary broadcasts, service location, and routing updates, resulting in an increase in data throughput without increasing the cost of hardware or the need for additional WAN services.

Accessible but secure – An accessible network allows for connections using dedicated, dialup, and switched services while maintaining network integrity.

Making The Network Reliable and Available Scalable Routing Protocols

Routers in the core of a network should converge rapidly and maintain reachability to all networks and subnetworks within an Autonomous System (AS). A scalable protocol such as Open Shortest Path First (OSPF) or Enhanced Interior Gateway Routing Protocol (EIGRP) should be implemented in the core layer.

A network that consists of multiple links and redundant routers will contain several paths to a given destination.

Making The Network Reliable and Available Load Balancing

Redundant links do not necessarily remain idle until a link fails. Routers can distribute the traffic load across multiple links to the same destination. This process is called load balancing. Load balancing can be implemented using alternate paths with the same cost or metric, (equal-cost load balancing.), or implemented over alternate paths with different metrics, (unequal-cost load balancing). When routing IP, the Cisco IOS offers two methods of load balancing, per packet and per destination load balancing. If process switching is enabled, the router will alternate paths on a per packet basis. If fast switching is enabled, only one of the alternate routes will be cached for the destination address and all packets in the packet stream bound for a specific host will take the same path.

Protocol Tunnels The administrator can configure a point-to-point link through the core between the two routers using IP. When this link is configured, IPX packets can be encapsulated inside IP packets. IPX can then traverse the core over IP links and the core can be spared the additional burden of routing IPX. Using tunnels, the administrator increases the availability of network services.

Making The Network Reliable and Available Dial Backup

Sometimes two redundant WAN links are not enough or a single link needs to be fault tolerant, however a full-time redundant link is too expensive. In these cases a backup link can be configured over a dialup technology, such as ISDN, or even an ordinary analog phone line. These relatively low-bandwidth links remain idle until the primary link fails.

Dial backup can be a cost-effective insurance policy, but it is not a substitute for redundant links that can effectively double throughput by using equal-cost load balancing.

Making the network responsive The IOS addresses priority and responsiveness

issues through queuing. The question of priority is most important on routers that maintain a slow WAN connection and therefore experience frequent congestion. Queuing refers to the process that the router uses to schedule packets for transmission during periods of congestion. By using the queuing feature, a congested router may be configured to reorder packets so that mission-critical and delay sensitive traffic is processed first. These higher priority packets are sent first even if other low priority packets arrive ahead of them.

Making The Network Efficient An efficient network should not waste bandwidth, especially over

costly WAN links. To be efficient, routers should prevent unnecessary traffic from traversing the WAN and should minimize the size and frequency of routing updates. The IOS includes several features designed to optimize a WAN connection:

Access lists Snapshot routing Compression over WANs Dial-on-demand routing (DDR) Route summarization Incremental updates

Making The Network Adaptable EIGRP is an exceptionally adaptable protocol

because it supports routing information for three routed protocols: IP, IPX, and AppleTalk.

The IOS also supports route redistribution. Mixing Routable and none routable protocols

Making the Network Accessible But Secured Dialup and dedicated access –

Cisco routers can be directly connected to basic telephone service or digital services such as T1/E1. Dialup links can be used for backup or remote sites that need occasional WAN access, while dedicated leased lines provide a high-speed, high capacity WAN core between key sites.

Packet switched – Cisco routers support Frame Relay, X.25, Switched Multi-megabit Data Service (SMDS), and ATM. With this variety of support, the WAN service, or combination of WAN services, to deploy can be determined based on cost, location, and need.

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Web-Based Curriculum

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Labs Lab1.4.3:  Access Control Lists basic and

extended Ping Lab 1.4.2:  Capturing HyperTerminal and

Telnet Sessions Lab 1.4.4:  Implementing Quality of Service

with Priority Queuing Lab 1.5.2:  Unequal-Coast Load Balancing

with IGRP