From: Howard C. Berkowitz (hcb@xxxxxxxxxxxx)
Date: Sun Aug 25 2002 - 13:15:39 GMT-3
At 9:00 AM -0700 8/22/02, perkinsr@WellsFargo.COM wrote:
>I need some help with BGP. When reading the labs, I never get a clear
>feeling for what the ultimate goal of configuring BGP is. When configuring
>IGPs the goal is to have connectivity from every subnet to every subnet.
>With BGP this isn't always the case.
First, you are absolutely correct that operational BGP tries to
provide the minimum but necessary amount of information to other AS.
At the level of granularity at which eBGP "thinks", you may want to
abstract the idea of a next-hop IP address to think first of a
next-hop AS, and THEN the IP address of a router in that AS.
>When I read the lab I have no problems deciding who should be neighbors
>(that is usually explicitly stated) and what kind of relationship to make
>(e/ibgp, confederation, route-reflector). Once I have that going and some
>entries in the bgp tables I start to think about synchronization and
>redistribution and then get all glossy eyed and pass out. I freeze up and
>don't know what direction I am going.
That's similar to my experience in first trying to understand it,
which I'll call the bottom-up approach. I tried understanding the
commands somewhat in isolation, and they made no sense. It became
clear to me when I began to understand what problem BGP was solving,
first focusing on BGP routing policy (different than Cisco policy
routing and not mutually exclusive), and then on Cisco commands.
Indeed, there is a Routing Policy Specification Language (RPSL),
which evolved from RIPE-181. There is a public domain tool, RtConfig,
that will translate RPSL to Cisco configuration statements, and will
let you see the relationships.
Do note that RPSL and the like deal primarily with eBGP.
As to iBGP, unfortunately, the CCIE program hits essentially obsolete
technologies such as synchronization. Your best general information
is some of the RFCs and I-Ds on reflectors, confederations, and
problems with them. I do discuss iBGP scaling in my book.
>
>After considerable thought about what my problem is I only can guess that
>the goal of BGP is to provide connectivity from every AS to every AS, and
>this doesn't mean that I can get to every subnet in every AS, just the
>subnets that are part of BGP.
>
>Does anyone have any guidance on this? I am lost and by the time I get to
>the more advanced topics in BGP I am totally out of focus and don't
>understand why I am doing what I am doing.
Several sources, at least given my preference of understanding the
problem first. Start with http://www.radb.net and get the various
RPSL tutorials. I think it will give you a pointer to RtConfig. Read
the following RFCs. starting with the tutorial:
RFC-2622: Routing Policy Specification Language
Highly Recommended Reading
The current routing language used by IRRd and by RsConfig. Certain
ambiguities in the RIPE-181 language have been dealt with, such as
the inter-as attribute (happily gone). This document should only
require a couple of read-throughs for the average user. RFC-2650 goes
with this to help provide explanations on day-to-day use of RPSL.
RPSL is much more expressive with regards to routing policy than
RIPE-181. This explains much of the complexity of the latter part of
the document.
RFC-2650: Using RPSL in Practice
Highly Recommended Reading
A definite must read for people new to RPSL. This tutorial gives many
examples of common policies in RPSL.
There are many tutorials at the NANOG meetings
(http://www.nanog.org). Offhand, Avi Friedman and I gave a pair at
the Denver meeting, Paul Ferguson has had several, and I gave some
operational aspects at the last Atlanta meeting. It is worth looking
through all the meeting agendas.
Let me now turn to books, with disclosure that I've written in this
area (OF COURSE I think you should buy it! :-))
Huitema's "Routing in the Internet" is kind of high level, but gives
good history of how the need for BGP evolved.
Halabi's book really is focused more on the Cisco configuration than
the big picture, but it's good for that. I haven't read Parkhurst's
book yet but have it on order.
The book by Smith and Greene, Cisco ISP Essentials, is an expanded
version of the Cisco briefing "IOS Essentials every ISP should know."
While it is Cisco-oriented, it's much more real-world than Halabi.
Unfortunately, real-world is not necessarily the CCIE methods. It's
a good book. I know Phil personally and he's a very good teacher.
There are several design-oriented books, John Stewart's book is a
nicely small one dealing with BGP concepts. John is also one of the
good guys.
My "Building Service Provider Networks" is about half to two-thirds
routing policy and BGP, but also gets into other ISP issues like the
transmission system, large-scale access servers, and address
management. Another book is by Nguyen, which I haven't read yet.
There's no accident that all of these are on my shelf, and I've got
drawers full of BGP RFCs and I-Ds, tutorials, and true theoretical
papers on the topic. If you want to browse anything theoretical, you
might want to search for Vern Paxson's PhD dissertation, "End to End
Packet Dynamics in the Internet" as a start.
I will suggest an I-D that I coauthored with people from Cisco,
Juniper, Nortel, and NextHop, which, among other things, tries to
clarify some confusing BGP terminology:
http://www.ietf.org/internet-drafts/draft-ietf-bmwg-conterm-03.txt
It's been up to the IESG, which asked for some clarifications before
it becomes an RFC, which I have almost finished. Hopefully, it will
be an Informational RFC in a month or two.
Unfortunately, the main BGP RFC is horribly out of date. It's worth
navigating to the IDR working group at http://www.ietf.org and
getting the more recent drafts, as well as seeing some of the other
work going on. There are some excellent I-D's (maybe one is now RFC)
on using communities by Olivier Bonadventure -- you can do a name
search for these on the I-D search engine at IETF.
--
"What Problem are you trying to solve?"
***send Cisco questions to the list, so all can benefit -- not
directly to me***
*******************************************************************************
*
Howard C. Berkowitz hcb@gettcomm.com
Chief Technology Officer, GettLab/Gett Communications http://www.gettlabs.com
Technical Director, CertificationZone.com http://www.certificationzone.com
"retired" Certified Cisco Systems Instructor (CID) #93005
books: Building Service Provider Networks (Wiley 2002), WAN
Survival Guide (Wiley 2000), Building Routing and Switching
Architectures for Enterprise Neworks (McMillan 1999), Building
Addressing Architectures for Routing and Switching (McMillan 1998).
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