From: ccie2be (ccie2be@nyc.rr.com)
Date: Thu Mar 13 2003 - 11:35:55 GMT-3
This is one of the most basic concepts in BGP but I'll answer your question
anyway.
Consider the following:
AS1 - - - - - AS2 - - - - AS3
If traffic can flow from AS1 thru AS2 and reach AS3, then AS2 is a transit
AS.
This may or may not be good depending on whether AS2 is ISP or an "end
user".
Let's say AS2 is a big company with locations in NY and Toyko. If you ran
this company, would you want to provide free transit for the traffic of
other companies on your internal network? If your BGP configuration didn't
prevent it, it's possible that traffic could enter your network in NY,
travel over your internal network to Tokyo, and leave your Tokyo location on
it's way to it's ultimate destination making your internal network a Transit
AS. In this case, you're a Transit AS but you shouldn't be.
If, however, you're an ISP, that's the whole point. You want to provide
your customers with Internet access which means allowing traffic destined to
any destination to transit your AS.
BGP is not a trivial protocol. There's quite a bit to learn to be able to
conquer the BGP portion of the lab. However, there are some very good
sources of comprehensive information on BGP and there are 2 in particular
that I recommend: Volume II of Doyle's book, Routing TCP/IP and Internet
Routing Architectures by Halabi considered by many to be the "Bible" on BGP.
If you want to truly understand BGP, read these 2 books and practice,
practice and practice some more.
Jim
----- Original Message -----
From: "Fan Shan" <fansh@publicf.bta.net.cn>
To: "Ccielab@Groupstudy.Com" <ccielab@groupstudy.com>
Sent: Wednesday, March 12, 2003 11:13 PM
Subject: What is transit AS in BGP?
> What is transit AS in BGP?
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