Information Technology Reference
In-Depth Information
The solution is not to use private addressing in ISP networks. This is considered bad
practice and is frowned on by the industry.
Customer Addressing
The standard model for handling customer prefixes is to carry them in BGP and not the IGP.
The result is that there is no benefit to be had by internally aggregating or assigning address
space on a regional basis. There is also the additional complication that customers often
multihome or sometimes move and need to be rehomed. Maintaining a strict regional
addressing scheme can very quickly become an administrative burden that is not supported
by technical benefits.
Customer Connectivity
There are two common methods of handling prefix information from customer connec-
tions. The first method is BGP peering with the customer. The second method is static route
configuration on the ISP edge router, redistributing the prefix into BGP. The later section,
“Identifying Customer Prefixes,” describes a method of identifying the source of BGP
prefixes.
Customer BGP Peering
Customer BGP peering is used when the customer is either multihomed or requires the
capability to dynamically advertise prefixes. In the case where the customer is multihomed
to different ISPs, the customer must have a unique public autonomous system number
(ASN) from an assigning authority. If the customer is multihomed to a single ISP or is not
multihomed but requires the ability to dynamically advertise prefixes, two methods do not
require the customer to obtain a public ASN. They are described next.
Generic Customer ASN
The ISP can obtain an ASN from an assigning authority to use as a generic customer ASN.
This means that the ISP asks all customers to use this customer ASN. For example, the ISP
has its own primary ASN of 100 and a second ASN of 101 for customer peering. ASN 101
is shared by all customers that require the capability to dynamically advertise prefixes to
the ISP.
The main caveat is that prefix information sent by one customer using ASN 101 is not
accepted by another customer running ASN 101. Initially, this might appear to present a
problem; however, this method is used for customers that are not multihomed to multiple
ISPs. The ISP only needs to originate the default to the customer to ensure full connectivity.
