BGP - Part 1 - :
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BGP (Border Gateway Protocol)

AS you know there are two types of routing protocols in general. These are IGP(Interor Gateway Protocols) and EGP(Exterior Gateway Protocols). In the previous posts we have talked about some of IGP protocols. In this post, we will talk on EGP.

EGP is the abbreviation of Exterior Gateway Protocol. In the first place there was only one protocol, EGP, as an EGP(again). But after years it retired and now BGP (Border Gateway Protocol) is the only EGP.

IGPs give us the ability to route the traffic within an Autonomous System. But EGP allow us to route between AS(Autonomous System)s. Because of the fact that BGP(Border Gataway Protocol) is the only EGP protocol, BGP makes routing between ASs. BGP is also known as Internet Protocol. Think about it, internet is the collection of connected ASs.

There are many corporates like Cisco, Juniper and Alcatel-Lucent on internet and they have their own AS numbers. And internet is collection of these Autonomous Systems.

bgp autonomous systems internet

BGP Autonomous Systems on the World, This is Internet
(much more points than this green points)

What is Autonomous System(AS) and AS Number(ASN)?

An autonomous system(AS) is a network or a collection of networks that are under a single entity or organization management. And the internet is the collection of ASs.

ASs has their own numbers for identification, ASNs. Every router used in BGP, can belong to only one AS.

As we say above, with BGP we route the traffic between ASs. Every AS has an AS Number between 1 and 65535, whose 64512 – 65535 range is reserved for private use. Here, you can think the private range, like private IP blocks, they are not on internet. But the rest of the numbers are belogns to specific ASs. These ASs are different vendors’, ISPs’, etc… (You can check the AS numbers for various companies, like Cisco).

Internet Assigned Numbers Authority (IANA) is the corporate who assigns AS Numbers(ASNs) to the Regional Internet Registries (RIRs) like RIPE, APNIC, etc.. all over the world. And they assigns this numbers to the AS owners.

You can check the assigned ASNs in the following link:IANA, Assigned AS Numbers

You can check the ASNs with the company which they are assigned
in the following link:Assigned AS Numbers and Companies

Other General Key Informations About BGP

BGP is a path vector protocol. So, BGP has both some link state and distance vector protocol characteristics.

The current BGP version is 4.

BGP uses TCP 179 port. BGP uses TCP, because the reliability is very important for BGP’s point of view.

In Cisco routers you can only run one BGP process.

The first step for BGP neighbourship, is a TCP session. After TCP session establishment, BGP messages are started to send.

Let’s continue with the BGP neighbourship(session establishment) and BGP messages in the next post ;)

BGP – Part 1
BGP – Part 2 (BGP Peers, BGP Sessions, BGP Messages)
BGP – Part 3 (IBGP, IBGP Topologies and EBGP)
BGP – Part 4 (BGP Administrative Distance and BGP Path Attributes)
BGP – Part 5 (Packet Tracer BGP Configuration Example)

About the Author
Gokhan Kosem is a telecommunation and network engineer. His ambition to IP networks and end-to-end system installation made him to prepare this web-site. By sharing his experiences about various networking protocols beside different system installation experiences and Cisco, Juniper, Alcatel-Lucent devices configurations, he is aimed to be helpful for his collegues in all over the world. He is currently lives in Istanbul, Turkey.

One comment for “BGP – Part 1”


Hi, Gokhan; I am studying for my CCNA. I completed my Cisco Networking Academy course from a technical school here in Georgia. Is BGP a CCNP topic that is part of the CCNP R&S at Cisco Networking Academy?

January 11th, 2016 at 02:38

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