Chapter 1: Planning for Complex Networks

1. For each route received, verify the next hop. If invalid, discard the route.

2. If multiple identical, valid routes are received by a routing protocol, choose the lowest metric.

3. Routes are identical if they advertise the same prefix and mask, so 192.168.0.0/16 and 192.168.0.0/24 are separate paths and are each placed into the routing table.

4. If more than one specific valid route is advertised by different routing protocols, choose the path with the lowest AD.

Choosing a Route

Routers look at the routing table to decide how to forward a packet. They look for a match to the destination IP address. Rarely will a route match the destination IP address exactly, so the router looks for the longest match. For instance, suppose a packet is bound for the IP address 10.1.1.1. The routing table has a route for 10.1.0.0/16, one for 10.1.1.0/24, and a default route of 0.0.0.0. The default route matches 0 bits of the destination address, the 10.1.0.0 route matches 16 bits of the destination address, and the 10.1.1.0 route matches 24 bits of the destination address. The 10.1.1.0 route is the longest match, so it will be used to forward the packet.

Planning a Routing Implementation

It is critical to take a structured approach to planning a routing implementation and to document thoroughly once you are done. Taking an ad-hoc approach could lead to network instability, suboptimal routing, or scalability problems.

Four commonly used models include

• Cisco Lifestyle Services: Uses the PPDIOO model (Prepare, Plan, Design, Implement, Operate, and Optimize.) Network engineers at the CCNP level are involved with the implementation planning during the Design phase, and the Implementation itself during the Implement phase.

• IT Infrastructure Library (ITIL): Emphasizes business requirements and processes as they relate to IT. Implementation and implementation planning are part of its best practices.

• Fault, Configuration, Accounting, Performance, and Security (FCAPS): Has five network management categories. Implementation and implementation planning are under the Configuration management category.

• Telecommunications Management Network (TMN): Based on the FCAPS model. Implementation and implementation planning are one of its building blocks.

Each approach includes identifying requirements, creating an implementation plan, implementing the changes, verifying your work, and then documenting it.

Creating an Implementation Plan

To create an implementation plan you need to know what the network looks like now, and what it should look like when you are done. This involves gathering information about the current network parameters such as IP addressing, physical connectivity, routing configuration, and equipment. Compare the current state to what is required. Be sure to include any site-specific requirements and any dependencies on the existing network.

An implementation plan includes most of the following, some of which might be site-specific:

• A checklist of tasks to be done

• Tools and resources needed

• The schedule of work, coordinated with all needed resources

• Device configurations

• Verification processes and tests

Creating Implementation Documentation

Documentation should be kept up-to-date, accurate, and accessible. It includes network information, tools and resources used, implementation tasks, verification methods, device configurations, performance measurements, and possibly screen shots or pictures.

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