The main advantages of SNMP include its de jure standard status, message-oriented operation, and its use of extensible MIBs. The command-line interface (CLI), on the other hand, is a de facto standard, ubiquitous, easy to understand, and can handle long-running processes such as file transfers. On balance, SNMP provides more flexibility as part of a full-featured network management system. These days, however, high-end devices such as the Cisco CRS-1 present important scalability issues to both SNMP and the CLIso much so that XML is increasingly being used for management on such devices.
Networks have long lifecycles, up to 30 years for telecom service providers. During this time, they typically experience a complex combination of growth (more users, sites, and services) and reconfigurations (more switches, routers, and links). Other events such as mergers and acquisitions result in substantial network changes. An acquired organization may even have much of its software application and network infrastructure replaced, perhaps because the two organizations might have different vendors and technology policies; for example, VLANs might be preferred over switched Ethernet.
Other network changes may be made to take advantage of new technologies and applications, such as migrating from FR/ATM to IP/MPLS and from TDM to VoIP. This sort of change can help in deploying new services, such as the following:
Layer 2/3 VPNs
Leased line replacement
Migration of layer 2 services into a layer 3 infrastructure
All such modifications and upgrades require substantial changes to the network. Similar considerations apply to the case in which an extranet is created that provides external companies with controlled access to a corporate intranet. The important point is that network configurations are not static.