“Structuring and Modularizing the Network,” that the modules in the Cisco Enterprise Architecture correspond to places in the network. The choice of routing protocols depends on the network design goals. Therefore, the routing protocol decision should be made only after the network goals and topology are determined. Running multiple routing protocols might be necessary in large enterprise networks, for example, when a network upgrade is performed; the old routing protocol usually coexists with the new one during the transition period.

As discussed in previous sections of this chapter, routing protocols differ in many ways. For example, how routing information is exchanged, convergence times, metrics used for optimal route determination, required amount of processing power and memory, and availability of a routing protocol on various platforms can determine whether a routing protocol is more or less suitable for a network or parts of a network. The following sections explain why certain protocols are suitable for specific modules in the Enterprise Architecture, and the advantages and disadvantages of individual protocols.

Routing in the Campus Core

The Campus Core provides high-speed data transmission between Building Distribution devices. The Campus Core is critical for connectivity and, therefore, incorporates a high level of redundancy using redundant links and load sharing between equal-cost paths. In the event of a link failure, it must immediately converge, adapting quickly to change to provide a seamless transport service. EIGRP and OSPF both adapt quickly to changes and have short convergence times. Therefore, they are suitable for use in the Campus Core.

The decision of whether to use EIGRP or OSPF should be based on the underlying physical topology, IP addressing, equipment used, and possible issues related to the routing protocol in a particular situation. Figure 5.12 illustrates routing protocols in the Enterprise Architecture, including those recommended for the Campus Core.

The following are considerations for routing protocol use in the Campus Core:

■ OSPF imposes a strict hierarchical design. OSPF areas should map to the IP addressing plan, which cannot always be achieved.

■ EIGRP restricts vendor selection because it is a Cisco-proprietary protocol. One way to overcome this restriction is to use EIGRP in the Campus Core and other routing protocols in the non-Cisco parts of the network, and redistribute between the protocols.

The following are reasons that other routing protocols are not considered for the Campus Core:

■ Even if routing only IP, IS-IS requires detailed knowledge of the OSI protocol suite for proper configuration, and that knowledge is not widely available.

■ RIP is not recommended as a Campus Core routing protocol because of its periodic transmission of the entire routing table, which results in relatively slow convergence, and because the RIP metric is based on hop count.

■ Using static routing in the Campus Core is not an option because static routing requires administrative intervention for changes and on link failures.

Routing in the Building Distribution Layer

The Building Distribution layer is the intermediate point between the Campus Core and the Building Access layers. In addition to other issues (such as physical media and IP addressing), the choice of routing protocol depends on the routing protocols used in the Campus Core and Building Access.

Routing in the Enterprise Edge Modules

In the Enterprise Edge modules, the underlying physical topology, IP addressing, and the deployed equipment also drive the choice of routing protocol. The routing protocols in the Enterprise Edge modules are typically OSPF, EIGRP, BGP, and static routing. Routing protocols running in the enterprise edge module are referred to as edge routing protocols.