Dell PowerSwitch S4820T Configuration Guide for the S4820T System 9.14.1.0 - Page 176

9 Border Gateway Protocol version 4 (BGPv4) This section provides a general description of BGPv4 as it is supported in the Dell EMC 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 operates with an internal gateway protocol (IGP) such as open shortest path first (OSPF) or router information protocol (RIP), allowing you to communicate to external ASs smoothly. BGP adds reliability to network connections by having multiple paths from one router to another. Topics: • Autonomous Systems (AS) • Sessions and Peers • Route Reflectors • BGP Attributes for selecting Best Path • Multiprotocol BGP • Implement BGP with Dell EMC Networking OS • Configuration Information • Enabling BGP • Enabling MBGP Configurations • BGP Regular Expression Optimization • Debugging BGP • Example-Configuring BGP peer groups 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 Assigned Numbers Authority (IANA) has reserved AS numbers 64512 through 65534 to be used for private purposes. IANA reserves ASNs 0 and 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 and operation. • multihomed AS - is one that maintains connections to more than one other AS. This group allows the AS to remain connected to the Internet in the event of a complete failure of one of their connections. However, this type of AS does not allow traffic from one 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 the customer 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 Internal Border Gateway Protocol). When BGP operates between ASs (AS1 and AS2), it is called External BGP (EBGP External Border Gateway Protocol). 176 Border Gateway Protocol version 4 (BGPv4)