This section covers Border Gateway Protocol (BGP) theory and design concepts. The current version of BGP, version 4, is defined in RFC 4271. BGP is an interdomain routing protocol, which means it is used to exchange routing information between autonomous systems. (It is used for inter-autonomous system routing.) The primary function of BGP is to provide and exchange network-reachability information between domains or autonomous systems. BGP is a path vector protocol. BGP is best suited for setting routing policies between autonomous systems. In the enterprise campus architecture, BGP is used for Internet connectivity.

BGP is the de facto standard for routing between service providers on the Internet because of its rich features. You can also use it to exchange routes in large internal networks. The Internet Assigned Numbers Authority (IANA) reserved TCP port 179 to identify BGP. BGPv4 was created to provide CIDR, a feature that was not present in the earlier versions of BGP. BGP is a path vector routing protocol; it is neither a distance vector nor link-state routing protocol.

Note

RFC 1519 describes CIDR, which provides the capability to forward packets based on IP prefixes only, with no concern for IP address class boundaries. CIDR was created as a means to constrain the growth of the routing tables in the Internet core through the 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 different autonomous systems. Each autonomous system is under different technical administration. BGP is frequently used to connect the enterprise to service providers and to interconnect service providers, as shown in Figure 4-9. The routing protocol within the enterprise could be any interior gateway protocol (IGP). Common IGP choices include RIPv2, EIGRP, OSPF, and IS-IS. BGPv4 is the only deployed exterior gateway protocol (EGP).

Figure 4-9 BGP Neighbors

BGP is an interdomain routing protocol that allows BGP speakers residing in different autonomous systems to exchange routing network layer reachability information. An autonomous system is a collection of devices under common administration. BGP autonomous systems range from 1 through 65,535. Autonomous system numbers (ASNs) 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 with the local RIR and be approved before being assigned an ASN. ASNs 64,512 through 65,535 are considered private ASNs. These ASNs can be used by any organization, but, like RFC 1918 addresses, they cannot be used on the Internet.

Before two BGP routers can exchange routing updates, they must become established neighbors. After BGP routers establish a TCP connection, exchange information, and accept the information, they become established neighbors and start exchanging routing updates. If the neighbors do not reach an established state, they do not exchange BGP updates. The information exchanged before the neighbors are established includes the BGP version number, ASN, BGP router ID, and BGP capabilities.