Hi Richard,
Your description is missing the concept of RR clusters.
Each plane should be present in each cluster. And all RRs serving paths
in a given cluster need to either have equal IGP point of the network,
disabled IGP metric consideration for best path calculation or have a
way to learn what the best path of other plane is.
With this in mind your statement
"With different IGP topology, one RR may view a path as the second best
while another RR may view the same path as the third best."
will not take place as the above conditions make sure that there is path
consistency on a per cluster basis.
Note also that just like in normal IPv4/IPv6 networks where tie breaker
is IGP metric there is no need for best path consistency across clusters
without compromising loop free BGP property.
If you agree with the above I will make sure that the text in the draft
is more clear on that.
Many thx,
R.
Let's assume that three planes of RR are deployed: The first plane
distributes the best path for each route as usual, the second plane for the
second best, and the third plane for the third best. Using your example, we
take the IGP distance as the equally-good path tie-breaker. With different
IGP topology, one RR may view a path as the second best while another RR may
view the same path as the third best. For example, let's consider a route R
with paths P1, P2, P3, where P2 and P3 are equally good and thus need to be
broken by a tie breaker. On the first plane RR, P1 is viewed as the best,
and is thus distributed as we hoped. On the second plane RR, P2 is viewed as
the second best and is thus distributed. On the third plane RR, however, P2
is viewed as the third best and the P3 is viewed as the second best, and
thus P2 is distributed, but this is not something we hoped to see.
In order to make it work, all RRs on all planes should put all the paths in
the same ordering, otherwise it will not correctly distribute the nth best
path. But can you do that under all circumstances? The above example shows
you can't, unless I have missed something.
Regards,
Renwei
-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of
[email protected]
Sent: Wednesday, June 23, 2010 1:30 AM
To: [email protected]
Cc: [email protected]
Subject: [GROW] I-D Action:draft-ietf-grow-diverse-bgp-path-dist-01.txt
A New Internet-Draft is available from the on-line Internet-Drafts
directories.
This draft is a work item of the Global Routing Operations Working Group of
the IETF.
Title : Distribution of diverse BGP paths.
Author(s) : R. Raszuk, et al.
Filename : draft-ietf-grow-diverse-bgp-path-dist-01.txt
Pages : 20
Date : 2010-06-23
The BGP4 protocol specifies the selection and propagation of a single best
path for each prefix. As defined today BGP has no mechanisms to distribute
paths other then best path between it's speakers. This behaviour results in
number of disadvantages for new applications and services.
This document presents an alternative mechanism for solving the problem
based on the concept of parallel route reflector planes. It also compares
existing solutions and proposed ideas that enable distribution of more paths
than just the best path.
This proposal does not specify any changes to the BGP protocol definition.
It does not require upgrades to provider edge or core routers nor does it
need network wide upgrades. The authors believe that the GROW WG would be
the best place for this work.
A URL for this Internet-Draft is:
http://www.ietf.org/internet-drafts/draft-ietf-grow-diverse-bgp-path-dist-01
.txt
Internet-Drafts are also available by anonymous FTP at:
ftp://ftp.ietf.org/internet-drafts/
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implementation to automatically retrieve the ASCII version of the
Internet-Draft.
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