>Hello,
>If a cust. is running bgp with 2 ISP's, what is the recommend way of
>controlling inbound and outbound traffic?
It depends. Let's put it this way...I had to write lots and lots of
case studies for my new book on service provider networks. All of
these things have various applications, but their usage can depend on
many things, including business relationships, QoS, etc.
I've written up the order of attributes being selected, and it's in
the archives. The new BGP specification being written clarifies some
of this, and also notes that all vendor implementations are not the
same. Contrary to urban legend, AS path length is NOT a decision
factor in the RFC1771 standard, but it will be in the new document.
Let's review the scope of the various methods of changing route
preference on routes advertised TO you, which you use to SEND traffic.
Weight affects only the router on which it is configured
Local preference affects all routers in the local AS
MED affects the session between the local AS and a directly connected
(i.e., adjacent) AS. Its traditional use is to set preferred entry
points for specific traffic when there are more than one BGP
session
between a pair of AS. There is an option to let it be compared
among all the directly connected AS, which is typically used at an
exchange point. If you don't know what an exchange point is, you
don't need to
You can receive MED (i.e., to know how someone wants to send to them)
or send MED (i.e., how you want an adjacent AS to send to you).
You can also send or receive AS paths with prepending. It is the
only supported mechanism that will affect the choice of AS not
directly connected to you, unless all AS involved agree to specific
communities that can indicate preference. It's quite common to use
an ISP-defined community to suggest to them what local preference
they
should use for the route. AS path prepending, depending on the path
topologies and the policies of each AS, may or may not cause the
choice
you want in a distant AS.
>When would one use Local Pref. over MED over AS-Path. These attribs. seem to
>be for inbound traffic. True??
>If a cust. has 2 routers on his end, then would local pref. be the preferred
>one?
>How about for outbound? Is weight usually the one favored? This one seems to
>be the case if the cust. has only one router on his end.
>Any ideas on how a cust. could do this?
>Thank you.
>Regards.
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