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Chapter 7

BGP's Basic Components

A router can be a member of only one autonomous system and must

appear to other autonomous systems to have one routing plan. All the des-
tinations must be reachable through that plan.

There are 65,535 available autonomous system numbers that can be

assigned, from 1 to 65,535. Autonomous system numbers (ASN) are 16-bit
integers. Of those 65,535 ASNs, the numbers 64,512 to 65,535 are reserved
for private use. This obviously means that there must be some authority
available to assign these numbers to those who need them, right? Yes, there is.

RFC 1930 provides guidelines for assigning BGP autonomous system

numbers. When you request a BGP ASN, you will be required to provide the
following information:

All administrative contacts in the company.

The Internet address of your routers.

A preferred autonomous system name.

A hardware profile of your routing hardware and the software being
used. In the case of Cisco, this would be the model and IOS version.

Expected deployment schedule when you will begin using two or more
upstream providers.

All networks in your organization connected by the routers.

The Internet Assigned Numbers Authority (IANA) is the organization

that assigns BGP autonomous system numbers. The IANA allows the Amer-
ican Registry for Internet Numbers (ARIN) to assign autonomous system
numbers for North America, South America, the Caribbean, and Africa.
Reseaux IP Eurpeennes-Network Information Center (RIPE-NIC) assigns
the AS numbers for Europe, and the Asia Pacific-NIC (AP-NIC) assigns the
numbers for Asia.

Stub AS

A stub AS is a single-homed network with only one entry and exit point, as
shown in Figure 7.2. In this type of network, the stub network does not need
to learn Internet routes. The reason? The local service provider or Internet
Service Provider is the next hop, and all the traffic is sent to one exit interface
to the provider. The provider then can have the responsibility for advertising
its customers' static routes. This type of situation works well if there are rel-
atively few static routes to manually configure and advertise. If there are

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