Service Manual

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 influence BGP to select different path by altering some of the BGP attributes.
Topics:
• BGP IP version 4 (BGPv4) Overview
• Basic BGP configuration tasks
• Advanced BGP configuration tasks
BGP IP version 4 (BGPv4) Overview
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 (EGP) that transmits interdomain routing information within and between autonomous
systems (AS). An AS is an network managed by a single entity or administration. BGPv4 includes support for 4-byte autonomous
system numbers and multiprotocol extensions to allow BGP to exchange information. BGP also support multiple Layer 3 protocol
address families including IPv4 and IPv6. 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.
BGP uses a path-vector routing algorithm to communicate network information with other BGP configured routers. The network
information is exchanged as updates between BGP peers, which contains information about the network, attributes and AS
that is used to reach destination network. BGP selects a single path as a best path to a destination using best path selection
algorithm.
BGP Autonomous Systems
BGP autonomous systems (ASs) are a collection of nodes under common administration with common network routing policies.
Devices in the AS use IGP to communicate with one another. For devices in different AS to communicate, they need to use
EGP. BGP is the EGP which allows the devices to communicate.
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.
The devices within an AS (AS1 or AS2, as seen in the following illustration) exchange routing information using Internal BGP
(IBGP), whereas the devices in different AS communicate using External BGP (EBGP). IBGP provides routers inside the AS with
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