>Scenario:
>Let's say you have a class C subnet that you break off into two smaller
>networks:
>
>1.1.1.0/24 = 1.1.1.0/25 + 1.1.1.128/25
>
>Problem:
>You are announcing this through BGP, but your provider only allows you to
>advertise full class C addresses. The problem lies in that you have to
>announce this network as 1.1.1.0/24 but still get traffic to 1.1.1.0/25 &
>1.1.1.128/25.
>
>Solution:
>So, there are two ways to do this:
>
>1) Use an aggregate-address with a suppress map to filter this.
>[snip]
>
>
>2) Use null routes
>[snip]
>Question:
>Which way do you all feel is better?
>
>I read in Boson's test that the preferred way is to use aggregate addresses,
>but this seems to be a much more complex way to do what a simple null route
>can accomplish? Thought / comments?
First, let me deal with the problem definition. Unless you have a
more advanced multihoming scenario, why would you ever want to
advertise anything more specific than /24 to your provider? The
provider is going to use the same interface to send to both /25's,
right? Telling the provider about the more specifics doesn't, in this
example, give the provider any additional useful information.
Second, this is one of those cases where there's a CCIE Lab way and
the right way. It's my general impression that the proctors frown on
solutions that involve static routes, and that they may explicitly
say you can't use static routes. This appears to be meant to screen
out candidates that use static routes when they can't figure out how
to make dynamic routing do something it should (a Good Thing) and to
demonstrate knowledge of the full IOS command set even when there is
a neat and clean static route solution (a Bad Thing).
The blackhole null route, I would say, is the preferred technique of
most real-world ISP routing engineers. It doesn't appear to be a
technique that Cisco emphasizes in classes or tests. So Boson may be
right in its approach to test preparation. In my BGP papers at
CertificationZone, I give both methods.
>
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