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Questions about BGP

Hi Guys,

i have many questions about some concepts at BGP

1- what the meaning of Autonomous Systems .... and i know the official definition of it but i still don`t understand exactly what it`s

    and is AS related to physical location or not ...... mean if we have one international company ( single administrative entity ) but this company have many branches worldwide       then all these networks will be under one AS or not.

2- why we create AS and BGP protocol ...... i mean why we need them .....

    why we not use the normal routing protocols like ospf or eigrp and use them to spread networks between different sites

    why we divide them into AS`s and connect between them by BGP .... why ?

3- why we use all these attributes at BGP .... why we don`t use only one metric like hop count with RIP or cost with OSPF ...... why we make BGP so complex like that

2 Replies 2

InayathUlla Sharieff
Cisco Employee
Cisco Employee


i have many questions about some concepts at BGP

1- what the meaning of Autonomous Systems .... and i know the official definition of it but i still don`t understand exactly what it`s

and is AS related to physical location or not ...... mean if we have one international company ( single administrative entity ) but this company have many branches worldwide then all these networks will be under one AS or not.

Answer:- Autonomous system is a collection of routers under the same domain. ( Example in simple words:- I and my family lives in a single home hence I am in one AS..were in my brother/sister/parents belongs to one AS. If I want to talk to neighbor or other relatives I need to go out of my home and talk to them right..which means I have to go out of my Autonomous system and talk to them. Now if one more person gets added to my family they belong to my own AS and not the other) hope this clarifies.

2- why we create AS and BGP protocol ...... i mean why we need them .....
why we not use the normal routing protocols like ospf or eigrp and use them to spread networks between different sites
why we divide them into AS`s and connect between them by BGP .... why ?

Answer:- If you read the BGP protocol in depth you would understand what is the need of running the same. IGP protocols doesnt much scale or handle so much of routes which run at the ISP level. BGP is scalable and have the capability to handle the routes.BGP is used for multiple Interconnections, especially with different ISPs, the BGP with them can be useful. The most common reason, is to "know" best path to same destination, between ISPs. But BGP can also be use to dynamically pass policy information.

Why do we divide them into AS:- Before for the management it would be easy.

3- why we use all these attributes at BGP .... why we don`t use only one metric like hop count with RIP or cost with OSPF ...... why we make BGP so complex like that.

Answer:- BGP is known for its attributes which differs it from any other protocol. It has very rich metric were you can influence the routes coming in to take a different path or the update the routes going out which none of the protocols has it. 

Hope it clarifies. If you still have any queries please do feel free to update us.

REgards

Inayath

**Please do not forget to rate the post if helpfull***

Joseph W. Doherty
Hall of Fame
Hall of Fame

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#1 It provides a demarcation on for BGP operation.  Between ASs it's eBGP, within an AS, it's iBGP.

AS is a logical group, it might be used to for physical locations, or might not too.

#2 BGP is designed to deal with very, very large route tables (e.g. the Internet) and BGP is designed to support routing policies.

IGPs, like OSPF and EIGRP generally don't scale or support policy routing like BGP can.

We create BGP ASs for multiple reasons, one reason also being a demarcation for routing policies.  Another reason is for path selection, as BGP uses ASs much like RIP uses router hops.

#3 We can use the attributes of BGP for routing policy implementation.

BTW, BGP isn't always complex, i.e. use all its capabilities, just as RIP isn't always simple (for example using offset lists).  It depends one you're trying to accomplish and what your needs are.

Earlier you mentioned OSPF (and EIGRP), but an OSPF implementation might be "simple", or complex.  Do you always need to use OSPF areas, or different types of areas, or adjust interface costs, or summarize address blocks?  No, you don't but those OSPF capabilites are there, if needed, and sometimes they are need.  Likewise for BGP.

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