Subject: my examples

From: Richard Draves <[email-masked]>
Date: Thu, 11 Nov 1999 14:24:19 -0800

It sounded like the ISP preference examples were confusing.

Here are the slides with pictures:
ftp://ftp.research.microsoft.com/users/richdr/slides.pdf
ftp://ftp.research.microsoft.com/users/richdr/slides.ppt

The setup is that ISP 1 is using prefixes 2004::/16 and 2006::/16. ISP 2 is
using the prefix 2005::/16. Site A is connected to ISP 1 and is using prefix
2004:A::/48. Site B is connected to both ISP 1 and ISP 2 and is using
prefixes 2005:B::/48 and 2006:B::/48.

When Site A initiates communication with Site B, it has to choose a
destination address. Site B nodes will have two addresses in the DNS,
because Site B is using two prefixes.

By default the algorithm will always pick the "wrong" address. This is
because a source address in 2004:A::/48 has a longer match against the
2005:B::/48 address compared to a 2006:B::/48 address. So a node in Site A
will pick the 2005:B::/48 destination address, and traffic will get routed
through ISP 2 instead of staying within ISP 1.

Adding an entry in Site A's policy table for 2006::/16, giving that
destination prefix higher precedence, fixes the problem. For completeness I
also showed an entry for 2004::/16, Site A's ISP's other prefix. These
entries don't do anything special with Label & MatchSrcLabel.

The next example is when Site B initiates communication with Site A. It has
to choose a source address. Again, the default longest-matching-prefix rules
pick the "wrong" source address.

If Site B cares about this, it can add policies to fix the problem. As in
the first example, there are policies to give higher precedence to Site B's
ISPs' prefixes. But now the policies also control source address selection
using Label and MatchSrcLabel - communication with sites attached to ISP 1
will use a 2006::/16 (or 2004::/16 but that's moot) source address, so
return traffic comes back via ISP 1. In this example, the policy for ::/0
says that communication with other sites will use a 2005::/16 source
address, so ISP 2 is the default ISP for return traffic.

Hope this makes it clearer...

Thanks,
Rich
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