Decision Tables in Network Design

Decision tables are used for making systematic decisions when there are multiple solutions or options to a network issue or problem. Decision tables facilitate the selection of the most appropriate option from many possibilities and can be helpful for justifying why a certain solution was chosen. Options are usually selected based on the highest level of compliance with given requirements. Basic guidelines for creating a network design decision table include the following:

Step 1 Determine the network building block about which decisions will be made (the physical topology, routing protocol, security implementation, and so on).

Step 2 Collect possible options for each decision. Be certain to include all options (or as many as possible) to obtain maximum value from the decision table. A thorough survey of the existing state of technology and considerable knowledge are needed to include all options.

Step 3 Create a table of the possible options and the given requirements. Include the relevant parameters or properties.

Step 4 Match the given requirements with the specific properties of the given options.

Step 5 Select the most appropriate option—the option with the most matches—if all requirements are treated equally. However, if some requirements are considered more important than others, implement a weighting system such that each of the requirements is assigned a weight that is proportional to its importance in the decision-making process.

Figure 2-22 is an example of a decision table for selecting a routing protocol based on multiple criteria. In this example, several routing protocols are considered as possible options: OSPF,

Intermediate System-to-Intermediate System (IS-IS), Enhanced Interior Gateway Routing Protocol (EIGRP), and BGP. Five required parameters are listed, along with an indication of how well the routing protocols comply with these parameters. As indicated in the figure, the chosen protocol should include the following properties:

■ It should support a large network. All the protocols being considered meet this requirement.

■ It must be Enterprise-focused, rather than Internet service provider-focused. BGP was designed to support interconnecting networks of autonomous systems; it is not optimized for use in the enterprise. IS-IS is typically deployed in service provider environments, rather than in enterprises.

■ Support for variable-length subnet mask (VLSM) is required. All the protocols being considered support VLSM.

■ It must be supported on Cisco routers, which is the case for all the protocols being considered.

■ Network support staff should have a good knowledge of the chosen protocol to enable them to troubleshoot the network. In this case, the network support staff are knowledgeable about EIGRP, but not about OSPF, IS-IS, or BGP.

NOTE All requirements in this example have the same level of importance, so no weights are used.

Based on the stated requirements, EIGRP is the routing protocol of choice in this example.

Figure 2-22 Sample Decision Table for Routing Protocol Selection

Options

Parameters

EIGRP

OSPF

IS-IS

BGP

Required Network Parameters

Size of Network (Small/Medium/ Large/Very Large)

Large

Large

Very Large

Very Large

Large

Enterprise-Focused (Yes/No)

Yes

Yes

No

No

Yes

Support for VLSM (Yes/No)

Yes

Yes

Yes

Yes

Routers

(Yes/No)

Yes

Yes

Yes

Yes

Yes

Network Support Staff Knowledge (Good/Fair/Poor)

Good

Fair

Poor

Poor

Good

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