[nsp] bgp - aggregates and specific routes

[Marko Milivojevic]

> If the /24 link goes down, the aggregate will still be sent out BUT
> other BGP peers - say in Timbuktu - will use the more specific /24 if
> said downstream customer has other eBGP peers up...
> Ok, basically your post is what Timothy.Hall said.  However Tim's post
> was a lot easier to follow...

    Now that we have the understanding, let's state that this is "in theory"
scenario. In practice, it may not work as you expect.

    First of all, not all ISP's in the world will accept /24 advertisement.
Those will only be getting your /19 aggregate (for that reason, whatever you
do with your upstream, make sure to send aggregate with more-specifics, or
make a deal with the upstream to send it for you - unlikely, but you can
try). So, let's make some drawing (and hope it doesn't get too messed up):

  +--INTERNET--+
  |            |
[NSP-1]-----[NSP-2]
   |           |
   |           |
[ISP-1]     [ISP-2]
   |           |
   |           |
 [YOU]-----[CUSTOMER]

    So, CUSTOMER is advertising /24 to YOU and ISP-2. YOU are advertising
/19 and /24 to ISP-1. Both ISP-1 and ISP-2 have no filters and they
advertise all they receive to NSP-1 and NSP-2. Both NSP-1 and NSP-2 apply,
say, /20 filters. Now, what do you have in NSP-2? You have only /19 path
through NSP-1 and YOU towards CUSTOMER's /24. If your connection to CUSTOMER
goes down, it is likely you will be blackholing some traffic. This is
well-known effect of route aggregation (like any decent routing book will
tell you). The solution for this problem would be direct connection between
YOU and ISP-2.

    The second problem you are facing is the fact that ISP-2 might be
reluctant to route such a small portion of your address space, which is most
probably PA. In any case, you need to make sure that ISP-2 *will* do this.
There is only political solution to this problem :-).


Marko.