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Chapter 9
BGP Scalability and Advanced Features
When multi-homing two networks, three classifications are possible:
Basic
Medium
Full
Basic multi-homing is the simplest and requires only that the AS attach to
the ISP. The ISP will offer only default routes to the Internet. The internal AS
(your AS) decides the ISP connection to use, similar to the one portrayed in
Figure 9.5. This method uses the least amount of CPU time and the least
amount of RAM on the router.
Medium multi-homing uses default routes and BGP. In this case, the inter-
nal AS gets more routing information and can select the best ISP to use based
on that information.
Full multi-homing uses only BGP, and all the routes are learned using the
AS-PATH attribute information to make routing decisions. This is the most
processor-intensive type and requires a lot of RAM.
Summary
T
his was one of the most complex chapters that we have ever written.
Not only was it the most complex, it was the longest to write and we used
more equipment and cabling than we have ever used writing a chapter. If you
implement BGP incorrectly, you can not only screw up advertising your own
internal network, but you can also wreak havoc on other organizations' net-
works as well, and this includes your own ISP's network. If you do not know
how to configure or maintain BGP in a network, you should not implement it.
For the exam, you need a firm understanding of route maps, prefix lists,
communities, peer groups, multi-homing, static routes, access lists, distrib-
ute lists, and confederations. We covered all of these in detail in this chapter.
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