The trend towards more complex environments is making internetwork design increasingly difficult.
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This means that careful planning by internetwork administrators is required to prevent inefficient use of resources.
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An internetwork that comprises 50 routers can pose complex problems that lead to unpredictable results.
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If you attempt to optimize internetworks that feature hundreds or thousands of routers, you run into even bigger problems.
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A hierarchical design helps to avoid some common operational problems.
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In a hierarchical design, routers are distributed by function.
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This minimizes the complexity of the internetwork and makes routers easier to manage.
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For example, you can accomplish phased upgrades to software and hardware more easily when you use a hierarchical design approach.
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A hierarchical design divides internetworks into three logical services:

• backbone
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• distribution
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• local area access
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Backbone (or core) services aim to optimize communication between routers at different sites or in different logical groups.
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Distribution services provide a way to implement internetwork traffic control.
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Local access services support communication between workstations and routers.
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Say host 3 and host 2 need to communicate.
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Such a transmission could be routed through router X to router D to router C to router B to host 2.
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The path from router X to router D travels through a portion of the backbone network.
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Communication between router D and router C (and subsequently router B) is controlled by traffic handling policy rules associated with distribution services.
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Router B and host 2 communicate using local access services.
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Controlling access to resources on this network is router B's primary goal.
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LANs are at the heart of most corporate internetworks.
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They form the backbone of most routed internetworks.
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The easiest way to witness growth in a routed internetwork is to study the technology employed in the backbone.
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Some networks consist of a single Ethernet segment that connects hosts to a mainframe.
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As the number of hosts within the building increases, routers link multiple Ethernet segments together.
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When the network evolves to a multiple building or campus topology, a fiber optic cable in a high-speed ring configuration connects the buildings, while Ethernet is kept in place in individual buildings.
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Many corporations use a FDDI/CDDI ring as the backbone in their current networks.
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As corporations become more global in nature, multiple campus networks require connectivity.
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New technologies, such as ATM, use high-speed fiber optic cable as a backbone to provide global connectivity for both private and public networks.
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Networks have differing design requirements.
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As your network grows, it needs to evolve to meet the needs of users.
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Once it has done this, you need to deal with the next set of management issues, such as controlling traffic flow and reducing overhead traffic.
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To identify a complex internetwork, you look for the following characteristics:

• more than 500 routes
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• global topology with multiple 
  remote connections
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• emphasis on security
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• routed and non-routed traffic 
  on the same medium
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Internetworks may be considered particularly complex when they approach or exceed:

• 500 IP routes
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• 300 IPX routes
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• 1000 IPX SAP services
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• 300 AppleTalk zones in one 
  area
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Complex internetworks force the router to maintain very large routing and service tables for each protocol.
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These tables have to be distributed or updated periodically.
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The updates can consume considerable bandwidth on all links.
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Internetworks with more than 500 routes, either in a local or remote topology, are subject to connectivity problems linked to metric limitations of the routing protocol.
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These internetworks may be slow to converge when topology changes occur.
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If your internetwork has 500 or more routes, you can manage router overhead by using

• route summarization
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• advanced routing 
  protocols
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The problems caused by maintaining more than 500 entries in a routing table can be managed by reducing the overall number of entries using route summarization.
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Route summarization occurs at major network boundaries for most routing protocols, consolidating address advertisement with a summary address.
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Some IP protocols, such as OSPF and Enhanced IGRP, allow manual summarization on arbitrary boundaries within the major network.
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Careful planning and address allocation are required for manual summarization to be most effective.
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Advanced routing protocols, such as OSPF and Enhanced IGRP, reduce bandwidth consumption by sending only topology changes rather than the entire routing table contents.
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These topology updates are sent at regular intervals.
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Enhanced IGRP further reduces overhead processing in the router because it uses a single algorithm to calculate IP, IPX, and AppleTalk routes.
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If you manage geographically remote networks (global topology), there is the challenge of tracing, logging, and accounting for resource usage.
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Keeping records of line usage, CPU time, and server time is a major challenge.
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Designing and managing internetworks with many remote links usually involves trading off cost against reliability.
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As internetworks become more geographically distributed, maintaining connectivity becomes more of a problem.
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Often, connections are maintained by service providers rather than by the individual company itself, and network segments are not always located in the same building or even in the same city.
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So you need to plan the management and repair of links.
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This requires strategic planning with fallback positions defined in advance.
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You can manage a global topology (incorporating different carriers and service providers) by using

• dial-on-demand routing (DDR)
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• data compression
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• packet switched networks
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You can maintain infrequent connectivity requirements using Cisco's dial-on-demand routing (DDR) feature.
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Active links are created only after traffic is detected by the router.
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This service replaces nailed-up circuits that accumulate costs even when the link is idle.
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Data compression of headers or entire frames is one way to reduce traffic.
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The router software accomplishes the compression before the frame is placed on the medium.
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Packet switched networks, such as X.25 and Frame Relay, offer you the advantage of global connectivity through a large number of service providers with established circuits to most major cities.
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As an internetwork administrator you need to be conscious of maintaining security in your internetwork.
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You need to block unauthorized access.
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You also need to verify authorized access, and even then access to certain connections may need to be restricted.
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An internetwork that delivers both routed and non-routed traffic has some unique problems.
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Most non-routable protocols lack a mechanism to provide flow control.
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So they are very sensitive to delays in delivery.
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Delivery delays, or packets arriving out of sequence, can result in an application session being lost.
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Some non-routable traffic, such as LAT and link protocols, for example IBM's SDLC, must be encapsulated inside a routable protocol.
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This allows delivery to remote hosts, but it also requires additional resources within the router.
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Non-routable (or bridged) traffic depends heavily on broadcasts for host delivery.
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In mixed media environments, such as Ethernet connected to Token Ring, address and header differences create problems that can be overcome only by additional CPU processing power in the router.
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Cisco's concurrent routing and bridging feature allows the same protocol to be both routed and bridged (albeit on different interfaces) within the router.
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This feature is very important during migration from bridged to routed environments.
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Network congestion occurs when the amount of data to be transmitted by a particular medium exceeds the bandwidth of the medium.
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A network has many sources of data traffic and overhead traffic.
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Data traffic is usually generated by user applications.
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These applications£R £rmay initiate file transfers using protocols£R £rsuch as FTP and TFTP.
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Client/server applications are another common source of data traffic.
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Overhead traffic is generated when the traffic is not directly related to user applications.
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Examples of overhead traffic include routing updates and broadcast requests, such as for a Domain Name server.
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Traffic generated by different protocols can cause problems for routers and other hardware devices.
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The routers have to maintain separate tables and caches for each protocol, wasting resources on what is effectively duplicated information.
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The physical presence of additional traffic from another protocol on the medium creates competition for the fixed bandwidth of that medium.
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Multiprotocol traffic presents problems for internetwork designers as well as for internetwork administrators.
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When you use only a single protocol, internetwork design can be tailored to suit the idiosyncrasies of that protocol suite.
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But when multiple protocol traffic is present, the internetwork must be balanced between the needs of each protocol.
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To resolve congestion, traffic must be either reduced or rescheduled.
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With Cisco routers you can control network congestion by

• filtering traffic
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• prioritizing traffic with queues
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• limiting broadcast/multicast traffic
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Reducing traffic is the most obvious method of solving congestion problems.
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Traffic filters prevent user traffic from accessing portions of the network.
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You use access lists to implement filters based on source and destination address characteristics, application type, and data size.
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So you can use traffic filters to keep certain traffic from reaching critical links.
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Service filters prevent access by hiding some services from designated groups of users.
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Of course, if you reduce the number of advertised services, you reduce bandwidth consumption on WAN links.
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You can use route redistribution filters to prevent routing information from reaching routing tables on certain routers.
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This limits communication with remote devices connected to those routers.
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You can reduce serial link congestion by establishing a corporate priority policy for user traffic.
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To do this on Cisco routers, you can use different types of traffic queuing systems:

• priority queuing
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• custom queuing
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• weighted fair queuing
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Priority queuing means that high-priority traffic gets through at the risk of losing lower-priority traffic.
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If you use custom queuing, each traffic type gets a share of the available bandwidth.
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Weighted fair queuing sorts and prioritizes traffic streams automatically.
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The weighted fair queue approach is acceptable on most networks, but it can be overridden by manual configuration of either priority or custom queuing.
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You can establish a queuing policy that gives priority to non-routable traffic.
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This can be useful, since applications that depend on non-routable protocols are very sensitive to traffic delays.
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Some periodic broadcasts have configurable transmission timers to lengthen the interval between broadcasts.
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If you lengthen the timer, you reduce overall traffic load on that link.
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You can use static entries in routing tables to eliminate the need to dynamically advertise routes across a link.
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This technique can be very effective for serial lines.
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