Hi,
I haven't the BSCN book, but using standard BGP terminology is easy to
answer your question.
Suppose you are multihomed to two providers' ASs, e.g. AS1 and AS2
> 1) Default Routes from All providers
This means both AS1 and AS2 will send you a default route (prefix
0.0.0.0/0), and you will typically use one of the links as primary and the
other as a backup; you won't have any information to help you choose
which path is "best" for your outgoing traffic, so you'll need to tweak
local_pref to prefer the primary, and you will need to influence traffic
coming into your AS by AS_PATH-prepending on your backup link or by using
communities, if supported by your provider.
> 2) Customer & Default Routes from AS providers
This is almost the same as 1), but the provider which you intend to use as
backup (e.g. AS2) will send you not only a default, but also the routes
for its customers. This way, you can make better use of your backup link,
by using it not only as a backup but also as a way of reaching quickly all
AS2's customers, without going via AS1 and who-knows-what
interconnections.
As to the outbound traffic, the local_pref and AS_path setup may stay very
similar to 1), since all AS2 customers will be reached by the backup links
since there will be a specific route as opposed to a default; the inbound
traffic configuration is slightly more tricky, and to solve it correctly
you might need to use communities, or accept a small percentage of
"primary" traffic coming in via the backup link.
> 3) Full Routes from All providers
You get full routing tables from both providers - this way you are free to
use both links in a way that minimizes AS_PATH length for all
destinations. You usually do this if you are NOT using a primary/backup
setup, but you "simply" want to interconnect to different provider, chose
the best path to any destination and make sure that traffic can reach your
network in the best way. You will also need bog boxes (not 2511)!!!
Please check Halabi's BGP book for examples and case studies!
Hope this helps,
Saverio
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