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Chapter 7
BGP's Basic Components
Obtaining an AS number for a stub network may be somewhat difficult
when using an ISP. The ISP considers your AS an extension of their AS, and
it must abide by the ISP's AS policies. What we have seen in most cases is that
the ISP assigns the customer a number out of the private pool discussed ear-
lier. The private pool of numbers runs from 64,512 to 65,535.
Transit AS
A transit AS is an AS through which data from one AS must travel to get to
another AS. A non-transit AS is an AS that does not pass data through
to another AS. A non-transit AS can be used to pass data from two service
providers but never between them, as shown in Figure 7.4.
F I G U R E 7 . 4
A non-transit AS connected to three ISPs
An enterprise network can have a transit AS if the network uses multiple
ASes. In this situation, we would look at this as a backbone of backbones in
the enterprise network. A good example of a transit AS is a local service pro-
vider. Local service providers carry traffic for many other ASes as this is the
local service provider's primary business.
Now let's take a more in-depth look at eBGP and iBGP.
BGP Peers
I
n BGP, the word peer is somewhat confusing because it has two mean-
ings. It can be used at both the protocol level and the policy level. The first
usage is simple: Two BGP routers that have a BGP session running between
them over a TCP connection are called peers or neighbors.
The second usage occurs at the BGP policy level and refers to a relation-
ship within an entire AS. Peering is used to pair two ASes of the same status
or an AS at one level with an AS at a higher level. If two ASes decide that they
AS
ISP1
ISP2
ISP3
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