Information Technology Reference
In-Depth Information
summarization of IP addresses across network class boundaries. The early 1990s saw an
increase in the growth of Internet routing tables and a reduction in Class B address space.
CIDR provides a way for service providers to assign address blocks smaller than a Class B
network but larger than a Class C network.
BGP Neighbors
BGP is usually configured between two directly connected routers that belong to differ-
ent autonomous systems. Each autonomous system is under different technical administra-
tion. BGP is frequently used to connect the enterprise to service providers and to
interconnect service providers, as shown in Figure 11-8. The routing protocol within the
enterprise could be any Interior Gateway Protocol (IGP). Common IGP choices include
RIPv2, EIGRP, OSPF, IS-IS. BGPv4 is the only deployed Exterior Gateway Protocol (EGP).
Service Provider 1
AS 100
Service Provider 3
AS 300
Service Provider 2
AS 400
Router A
1.1.1.1
1.1.1.2
Router B
Enterprise
Autonomous System
AS 200
Figure 11-8
BGP Neighbors
BGP is an interdomain routing protocol that allows BGP speakers residing in different au-
tonomous systems to exchange routing (NLRI) information. An autonomous system is a
collection of devices under common administration. BGP autonomous systems range from
1 through 65,535. Autonomous system numbers (ASN) 1 through 64,511 are considered
public ASNs. These are allocated by IANA to Regional Internet Registries (RIR). Entities
wanting to receive an ASN must complete the application process of their local RIR and
be approved before being assigned an ASN. ASNs 65,512 through 65,535 are considered
private ASNs. These ASNs can be used by any organization, but,
dresses, cannot be used on the Internet.
