BGP for disaster recovery site

Scott Weeks surfer at
Mon Sep 29 15:49:22 CDT 2008


--- CHRISTINE.M.BERNS at wrote:
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.

First, unless the first two bits of the first octet are "10" it's not a "Class B".  It's a /16, which is the same size as a "Class B" but the first two bits of the first octet are not necessarily "10".

Are these two sites (main and DR) in the same ASN?

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...

I would consider another consultant if you have not used 80% of your current allocation.  ARIN will not allocate anymore space until you hit 80% utilization

...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.  

If the two sites are not in the same AS it's not a good idea unless you're experienced with such things.  See past posts in the archives about the subject.

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?

Nothing is ever guaranteed WRT routing certain prefix sizes on the internet, but you should have no problems with a /24.  Any longer (a /25) and you will not have good connectivity on the internet.


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