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BGP Concepts Brief

Latest reply: Aug 17, 2018 07:15:05 761 7 3 0 0

Hello everyone,

Now I’d like to share with you BGP

What is an Autonomous System?
An Autonomous System (AS) is a group of Internet Protocol (IP) networks that are controlled by one entity, typically an Internet service provider (ISP), and have the same routing policy. Each AS is assigned a unique AS number, which identifies an AS on a BGP network. Two types of AS numbers are available: 2-byte AS numbers and 4-byte AS numbers. A 2-byte AS number ranges from 1 to 65535, and a 4-byte AS number ranges from 1 to 4294967295. Devices supporting 4-byte AS numbers are compatible with devices supporting 2-byte AS numbers.

EBGP runs between ASs. To prevent routing loops between ASs, a BGP device discards theroutes with the local AS number when receiving the routes from EBGP peers.
IBGP runs within an AS. To prevent routing loops within an AS, a BGP device does not advertise the routes learned from an IBGP peer to the other IBGP peers and establishes full-mesh connections with all IBGP peers. To address the problem of too many IBGP connections between IBGP peers, BGP uses Route Reflector and BGP Confederation.
Devices Roles in BGP

There are two device roles in BGP message exchange:


The device thatsends BGP messages is a BGP speaker. A BGP speaker receives and generates newroutes, and advertises the routes to other BGP speakers.


BGP speakers that exchange messages witheach other are BGP peers. A group of BGP peers sharing the same policies canform a peer group.

BGP RouterID

The BGP routerID is a 32-bit value that is often represented by an IPv4 address to identify aBGP device. It is carried in the Open message sent during the establishment ofa BGP session. When two BGP peers need to establish a BGP session, they eachrequire a unique router ID. Otherwise, the two peers cannot establish a BGPsession.

The BGP router ID of a device must be unique on a BGP network.It can be manually configured or selected from IPv4 addresses on the device. Bydefault, an IPv4 address of a loopback interface on a device is used as the BGProuter ID. If no loopback interface is configured on the device, the systemselects the largest IPv4 address from all IPv4 addresses of interfaces as theBGP router ID. Once the BGP router ID is selected, the system retains thisrouter ID even if a larger IPv4 address is configured on the device later. Thesystem changes the BGP router ID only when the corresponding IPv4 address isdeleted.

This is what I want to share with you today, thank you!

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Created Aug 16, 2018 15:04:26

Important to mention that each AS numbers are assigned in blocks by the Internet Assigned Numbers Authority (IANA) to regional Internet registries (RIRs).
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Created Aug 16, 2018 15:25:19

a complex item however in these days BGP networks has become more and more common in the customer networks
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Created Aug 16, 2018 16:16:41

i'll back and read it thoroughly
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Created Aug 16, 2018 17:19:27

I am just stdying this for my certification exam, this helped me to understand it better, thanks a lot.
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Created Aug 16, 2018 22:17:28

There are not much of available BGP AS numbers left.
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Created Aug 17, 2018 07:12:42

this helped me to understand it better, thanks a lot.
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Created Aug 17, 2018 07:15:05

it is very useful to help understand BGP, thanks!
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