BGP Multihomed / Dual ASN's

[geburns]

Hello All,

Has anyone ever dealt with a BGP configuration where they configured Multi-homing along with two unique ASN's?  

In this example Company "A" acquired company "B".  Company "B" also owns a public IP address space and ASN number.  Rather than close Data Center "B" Company "A" decides to utilize company "B" Data Center as a hot site.  So the question is, is there anything wrong with  continuing to use two different ASN's within the new company and taking advantage of Multi-homing?

Example:

• The two Data Centers "A" and "B" are connected via a Private "Data Center Interconnect".  
• Each data center is connected to the Internet using it's own unique ASN number. 

                                     Data Center "A" ASN=1000  <====== eBGP Connection  ====>  ISP "A"  ASN =1

                                                           []

                                                           []

                    eBGP Connection  ----> []     (Data Center Interconnect) 

                                                           []

                                                           []

                                     Data Center "B" ASN=2000  <======= eBGP Connection  ====>  ISP "B"  ASN =2

I can provide a better landscape if necessary.  I admit this is a little cryptic!

Thanks,

[Reza Sharifi]

Hi,

I don't see anything wrong with this design. You basically have 2 sites each with their own public IP segment and ASN. So, you can simply use one site as your active site and the other one as stand-by site, or you can use them both as active/active.  In active/active case, you maybe dealing with some Asymmetric routing where the traffic goes out one data center and come back the other one and I know some applications don't work well with that. 

HTH

[geburns]

Hello Reza,

Thanks for responding.  Regarding the Asymmetric routing, I am glad you brought that up.  One thing I did not mention was the IP blocks and how they would be announced from each site.

First off, lets start with an IP block, say 10.90.240.0/20.  I am using a private IP block for the sake of explanation.

Break 10.90.240.0/20 block in half using a /21

             10.255.240.0/21  Used for Site A

             10.255.248.0/21  Used for Site B

Site A:

Announce both blocks from site A, but prepend the Site B (10.255.248.0/21) announcements (ASN = 1,1,1,1)

Site B:

Announce both blocks from site B, but prepend the Site A (10.255.248.0/21) announcements (ASN = 2,2,2,2)

This should resolve the Asymmetric routing issue.  Anything I have missed?

Thanks,

geb

[Reza Sharifi]

Hi Geb,

Prepending the subnet should work but please remember you would need to talk to each provider about it. I have seen cases where you prepened say 10.255.248.0/21 multiple times on site A, but you still see traffic coming through site A, and that is because how your provider peers with other providers and who they have better business relationship with.

HTH