From: Marko Milivojevic (markom@markom.info)
Date: Fri Dec 05 2008 - 18:20:30 ARST
First of all, is this a lab or real-life scenario?
If it's a lab, you are doing something wrong, as this should work.
However, if you are dealing with real, live, wild, Internet, things
are not that simple out there. While AS_PATH prepending is perfectly
valid "traffic engineering" technique, it doesn't always work. Large
number of carriers will always prefer "customer" routes over "peer"
and "transit" (upstream) ones. If you are a customer of relatively
large carrier, no amount of prepending will help you, as they will
always prefer the direct route. That route will then be announced to
their customers, peers and transit carriers. Depending where in the
"food chain" your other ISP is, you may get very little or no traffic
over the other link, even though prepending would suggest otherwise.
Now, this clearly appears to be a problem. Luckily there are
solutions. To understand them, I will spend a minute to explain how
BGP in large networks usually works. Dealing with as-path filters,
prefix-lists, etc. is fun and interesting thing to do when you have
2-3 peers. There are networks that have thousands of BGP sessions and
peers. Some of them peer in multiple locations and in some of these
locations, different policies are applied. Can you imagine doing that
with access-lists... of _any_ sort? No, neither can I. This is why we
have BGP communities.
Prefixes are tagged with comprehensive set of communities at the
"routing ingress" (i.e. when received from a customer). All outbound
policies in the entire network act only on these communities. AS_PATH
and other things are not considered in the decision which prefixes to
announce to other networks. Customers get all, or subset, peers get
customer routes, etc. Now, when routes are tagged with communities,
they are usually assigned LOCAL_PREF, as well. This attribute is
assigned based on what sort of session you have. You get the idea.
Now, the remedy for this situation is to ask your carriers to provide
you with the information about the communities they will accept from
you. Vast majority of serious SP's provide customers some control over
their routing, ranging from simple prepending (where your community
instructs them to prepend elsewhere), to region-based prepending (i.e.
prepend my routes to your peers in Europe) and remote-triggered
blackholing. Some of these communities control what sort of preference
your routes get in the carrier's network. This is what you need :-).
-- Marko CCIE #18427 (SP) My network blog: http://cisco.markom.info/Blogs and organic groups at http://www.ccie.net
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