BGP for disaster recovery site

We currently have a routable block (class B) of IP addresses. We are in
the process of designing a disaster recovery site. Our main site is
already dual homed to two different Internet service providers via BGP. A
consultant told us that in order to allow us to test access to the DR site
without affecting the production environment, we should get another block
of addresses from ARIN and advertise those addresses out the DR site's
Internet connection. Can we even expect to get another block from ARIN if
we already have a class B, and could we not accomplish the same thing by
advertising a subnet of our existing Class B at the DR site? I would
actually prefer to advertise a subnet of our class B, but am wondering if
there are any reasons why this is not a good idea. Also, I have seen
reference to some Internet service providers possibly not accepting /24
BGP routes and either dropping them or aggregating them to a /21 or /20
or /19. Are there recommendations as to what is the longest prefix
that we should advertise to guarantee that the prefix will be advertised
throughout the Internet?


If you have a subnet or two within your /16 that you're not using at all today, you could use those to advertise from your DR site. If you're using all of your /16 today, then you could apply to ARIN for more space, but keep in mind that just because you have a /16 today doesn't mean that ARIN will automatically hand you another /16 because you're running a DR site.

It is true that some providers might filter /24s out of 'legacy class B' space, however most providers I've seen are also loath to scribble on advertisements that they don't originate, i.e. aggregating smaller prefixes from your /16 back into that /16 if the origin AS isn't theirs.
It might also be a good idea to register route-objects with one of the routing registries (RADB, ALTDB, ARIN, etc...) since some providers do build their routing policies based on information from those sources.

There is no 100% guarantee of global reachability on any prefix you or anyone else advertises - just a reasonable expectation that things will work for the most part :slight_smile: