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BGP Design for HA across two DC's

I'm trying to do a BGP design to make our external IP address the most flexible and HA it can be.  The theory I have is that we have 4 class C's and divid two per site.  Each time we create a URL at one location we will create the same URL on the matching IP on the second block at the other location. This will give us two IP's to work with so we can manipulate DNS and global load balancing.  From a BGP perspective I want to advertise out the primary two class C's out of the DC it belongs to as the primary path and the secondary set as the back-up path for the other DC.  So another words the secondary set would have AS_PATH prepending or another form of BGP manipulation done to see it less favorable.  I don't see how this is possible without acquiring two AS numbers.  I have drawn a small ACIII diagram below to try to help.  Any ideas on how to make this better or how to do this with one AS?

DC A                      DC B


Primray          |     Primary

1.1.1.X          |      3.3.3.X

2.2.2.X          |      4.4.4.X


Secondary    |      Secondary

3.3.3.X          |      1.1.1.X

4.4.4.X          |      2.2.2.X


Laurent Aubert
Cisco Employee

Hi Robert,

Instead of provisioning your service at two different places, what do you think about virtualizing  the IP address from a load-balancing complex so the outside is not aware you have actually two sites. Your load-balancers could then balance the traffic to the servers available on both site. It assumes you have connectivity between those sites already.

Also you should be able to do what you want with a single AS so not sure why you think you need two AS but I may misunderstood something.




If you have the same service provider on both DCs, you can use communities.

It is not a bad idea to use a load balancer as it was suggested.

But all depends of your infrastructure.