Administrator Guide

Border Gateway Protocol (BGP)
Border Gateway Protocol (BGP) is an interdomain routing protocol that manages routing between edge routers.
BGP uses an algorithm to exchange routing information between switches enabled with BGP. BGP determines a path to reach a particular
destination using certain attributes while avoiding routing loops. BGP selects a single path as the best path to a destination network or
host. You can also inuence BGP to select dierent path by altering some of the BGP attributes.
Topics:
• Border Gateway Protocol version 4 (BGPv4)
• Conguring a basic BGP network
• Advanced BGP conguration tasks
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 rst (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.
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 identies 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), dened 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 trac 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).
IBGP provides routers inside the AS with the knowledge to reach routers external to the AS. EBGP routers exchange information with other
EBGP routers as well as IBGP routers to maintain connectivity and accessibility.
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