Within the United States, major NSPs peer at both public and private NAPS, with most tending toward the latter. The NAPs in San Jose, Washington, Pensauken, and Chicago are still important exchange points for the major NSPs—particularly for the middle-size NSPs and large federal networks. Peering always takes place via BGP.
Usually, the networks fall into the three-layer hierarchical model, consisting of acore, a distribution, and an access network:
• Peers usually connect their core routers via high-speed links and use BGP for route exchange. Core network links generally range between DS3 and OC12 speeds, traditionally using IP over HSSI, then ATM, and more recently directly over Sonet at OC12 and OC48.
Core routing is usually achieved via internal BGP and either IS-IS or OSPF. Many of the larger providers prefer IS-IS, simply because of its demonstrated stability. Most large networks minimize the number of routes carried in the link-state backbone: In many cases, only the routes for links between routers themselves are necessary. This is sufficient to enable internal BGP as the workhorse that carries the bulk of the routes injected from the access layer (customers).
• Within the distribution layer, geographical regions may be divided into OSPF areas or ISIS level 1 domains. Again, internal BGP is pervasive through the distribution layer. Due to its ease of administration and its capability of filtering routes, smaller networks may use RIP within the distribution layer, with subsequent redistribution into the link-state core. Wide-area technologies typically include ATM, Frame Relay, and T1.
• At the access level, several routing protocols may be found. In most cases, these are distance-vector protocols, such as BGP, RIP, IGRP, or even static routes because they can be readily filtered. Here, the widest range of Layer 2 technologies can be found, including Frame Relay, ATM, T1, SMDS, ISDN, ASDL, and, of course, POTS.
The Internet has come a long way since its four hosts in 1969. Figure 1-9 shows the astronomical growth of the network's hosts, networks, and domains.
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