10Border Gateway Protocol IPv4 (BGPv4)This chapter provides a general description of BGPv4 as it is supported in the Dell Networking Operating System (OS).BGP protocol standards are listed in the Standards Compliance chapter.BGP is an external gateway protocol that transmits interdomain routing information within and between autonomous systems (AS).The primary function of the BGP is to exchange network reachability information with other BGP systems. BGP generally operateswith an internal gateway protocol (IGP) such as open shortest path first (OSPF) or router information protocol (RIP), allowing you tocommunicate to external ASs smoothly. BGP adds reliability to network connections by having multiple paths from one router toanother.Autonomous Systems (AS)BGP autonomous systems (ASs) are a collection of nodes under common administration with common network routing policies.Each AS has a number, which an internet authority already assigns. You do not assign the BGP number.AS numbers (ASNs) are important because the ASN uniquely identifies each network on the internet. The Internet AssignedNumbers Authority (IANA) has reserved AS numbers 64512 through 65534 to be used for private purposes. IANA reserves ASNs 0and 65535 and must not be used in a live environment.You can group autonomous systems into three categories (multihomed, stub, and transit), defined by their connections andoperation.• multihomed AS — is one that maintains connections to more than one other AS. This group allows the AS to remain connectedto the Internet in the event of a complete failure of one of their connections. However, this type of AS does not allow traffic fromone AS to pass through on its way to another AS. A simple example of this group is seen in the following illustration.• stub AS — is one that is connected to only one other AS.• transit AS — is one that provides connections through itself to separate networks. For example, in the following illustration,Router 1 can use Router 2 (the transit AS) to connect to Router 4. Internet service providers (ISPs) are always transit ASs,because they provide connections from one network to another. The ISP is considered to be “selling transit service” to thecustomer network, so thus the term Transit AS.When BGP operates inside an AS (AS1 or AS2, as seen in the following illustration), it is referred to as Internal BGP (IBGP InternalBorder Gateway Protocol). When BGP operates between ASs (AS1 and AS2), it is called External BGP (EBGP External BorderGateway Protocol). IBGP provides routers inside the AS with the knowledge to reach routers external to the AS. EBGP routersexchange information with other EBGP routers as well as IBGP routers to maintain connectivity and accessibility.Border Gateway Protocol IPv4 (BGPv4) 169
