Understanding BGP - Looking Glass

Can someone give me a quick high level overview of BGP, as it relates to Internet routing tables, and looking glass sites?

Specifically, my small site will be moving ISPs, from MCI to AT&T.  Our /20 network is being advertised by MCI right now.  The routers we manage have nothing to do with it, we just static route to an MCI router, that handles the BGP.  But it's our address space (asigned to us), not MCI.  So when we move to AT&T, they will start to advertise that address space.

The ISPs assure us that they will work together to handle the routing transition.  Which is fine.  But I'd like to at least understand a little bit about how it works, and how I can verify (using Looking Glass sites I think?) who is advertising it?

Thanks
Shane
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shanepresleyAsked:
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rafael_accConnect With a Mentor Commented:
Hi.

First of all, I'd say it's hard to give a "high level overview" of such a complex routing protocol as BGP. And if you get one, then it wouldn't be enough for the "overview" level you want. However, here's my try.

Now, a definition: On the Internet, an autonomous system (AS) is the unit of router policy, either a single network or a group of networks that is controlled by a common network administrator (or group of administrators) on behalf of a single administrative entity (such as a university, a business enterprise, or a business division). An autonomous system is also sometimes referred to as a routing domain. An autonomous system is assigned a globally unique number, sometimes called an Autonomous System Number (ASN). [Definition taken from http://searchnetworking.techtarget.com/sDefinition/0,,sid7_gci213662,00.html]

BGP is based on AS decisions. It can function in intra-AS decisions and inter-AS decisions. Regarding ISPs, usually it works as inter-AS routing protocol. The advantage of the BGP is it's flexible configuration - hence its complexity. BGP configuration is based on attributes (properties) - these properties or attributes, when properly combined it forms what it's called as "routing policies".

Concluding, BGP is a routing protocol based that allows policies based decisions in a very flexible and granular manner.



Here are some links as well:

http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/bgp.htm
http://www.cse.ucsc.edu/research/ccrg/publications/brad.globalinternet96.pdf
http://24.237.160.4/files/networking/Infocom%20stuff/Infocom2001%20CD/DATA00/03D_1.PDF

Cheers
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shanepresleyAuthor Commented:
Thanks, that helps. The Cisco link was especially useful.

Perhaps more specific to my situation is...

Is it possible to tell who (what company/network/or router) is advertising a given address space?  Currently our space xxx.xxx.0.0 /20 is being advertised by MCI.  Or so I am told.  How could I verify that, and then confirm that after the change, it is now being advertised by AT&T?

Shane
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rafael_accCommented:
You might be able ... or not! It depends. Anyway, you can deploy some "filtering" policies if you want to restrict routing updates you might get from the ISP. However, again, in order to achieve that, might not be a straight forward process! And sometimes, you might have to work along with the ISP.

Regarding your specific question, I guess your new ISP would be AT&T, right? So you must ask them, I guess!

Cheers.
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rafael_accCommented:
Refining ...

If you advertise (say) the route R1 to your ISP, then it's up to them to advertise it further. Or maybe I'm not clear enough with the question !?

Cheers
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PennGwynCommented:
If you attempt a traceroute to one of your addresses (from somewhere remote on the Internet -- check out traceroute.org), it will transit various networks trying to reach you.  Odds are good that the last big ISP it reaches before you (most ISPs provide rDNS info for their routers) is advertising your route to the rest of the Internet, either because you advertise to it or because it is statically configured to do so.  

There are cases where it might not be the ONLY ISP advertising your route, in which case it's probably the one advertising the lowest route cost.

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pseudocyberCommented:
If you're advertising your network through BGP, then if you posted it, engineers here with BGP routers could look in their routers to see where you AS is being advertised.  Also, if you told us your AS it would help.
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sciwriterCommented:
Currently, or last I checked, BGPs need a fully connected ARS to work correctly  --

http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/bgp.htm

and the high-level sites need a widely connected network to advertise you.  You might be facing host provider limitations (marketing hype >>> reality?) -- but PennGwyn or another might correct me on this
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shanepresleyAuthor Commented:
We are not actually advertising the network.  I assume our ISP is.

Our router that we manage just has a single static default route, that points to our ISP managed router.  I assume they advertise our network via BGP.

The network space is 199.196.16.0/20

Shane
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