Who Needs BGP? IN CISCO NETWORKING

Not as many internetworks need BGP as you might think. A common misconception is that whenever an internetwork must be
broken into multiple routing domains, BGP should be run between the domains. BGP is certainly an option, but why complicate
matters by unnecessarily adding another routing protocol to the mix?
Take, for example, a multinational corporate network consisting of 3000 routers and perhaps 150,000 users. Figure 2-9 shows
how such a huge internetwork might be constructed. The entire network is routed with OSPF and is divided into eight
geographic OSPF routing domains for easier manageability. Although the illustration shows only the backbone areas for each
OSPF domain, each of the domains is divided into multiple OSPF areas that also correspond to geographic subreg Even a Very Large Internetwork Can Be Built Using Only Multiple IGP Domains
BGP can be used to provide connectivity between the multiple OSPF domains, but it is unnecessary. Instead, each of the eight
OSPF backbone areas redistributes into a single global backbone. The global backbone is another OSPF domain, consisting of a
single OSPF area. Although this core consists of high-end routers to handle the packet-switching load, the load on these routers
from routing tables and OSPF processing is actually very small. Because of the way the entire internetwork is addressed, each
of the eight OSPF domains advertises only a single aggregate route to the global backbone. In fact, aggregation is fundamental
to making this design work. There are, presumably, such a large number of subnets in such an internetwork that without
aggregation OSPF would "choke" trying to process them all. The result would be very poor performance and possible router
failures.