Information Technology Reference
In-Depth Information
This method identifies customer prefixes by setting the originating ASN to 101.
Private ASN
The second method is to use an ASN in the range of 64512 to 65535. These ASN have been
reserved for private use by the Internet Assigned Numbers Authority (IANA). These ASNs
should not be advertised to the general Internet, which means that the ISP needs to remove
the private ASN before propagating prefix information to the public Internet.
Static Route Redistribution
If a customer does not require the ability to dynamically advertise prefix information and
is not multihomed, overhead for a BGP peering session is not needed. The customers
aggregated onto a single edge router can number into the thousands. If a BGP peering
session is used for every customer connection, this can place significant processing load on
the router.
The most common way to provide customer connectivity is static route configuration on the
ISP edge router. The customer router is configured with a static default route to the ISP. The
static routes for the customer prefixes are then redistributed directly into BGP on the ISP
edge router. When routes are injected into BGP through redistribution, the origin is set to
Incomplete. ISPs often redistribute the routes through a route map to manually set the ori-
gin to IGP and perform any other BGP attribute manipulation, such as adding communities.
The use of route maps to filter redistribution also helps reduce configuration mistakes.
Identifying Customer Prefixes
Chapter 6, “Internet Connectivity for Enterprise Networks,” introduced the concept of
advertising partial routes to a multihomed customer. Partial routes consist of the ISP's local
routes and direct customers. An ISP must be able to identify what routes specifically are
customer routes, as opposed to transit and peering routes, if it wants to offer partial routes.
The use of static redistribution does not inherently provide a distinguisher like the generic
customer ASN. Furthermore, attempting to filter based on a private ASN, a generic custom-
er ASN, redistributed static routes, and customers with their own assigned ASNs can be
cumbersome.
The solution is to define a specific BGP community for customer prefixes. This community
is assigned to customer prefixes received via BGP and is added to static customer routes
redistributed into BGP. Prefix advertisements to customers requesting partial routes can
then be filtered based on community to block all routes except those with the ISP customer
routes community.
