Same BGP AS across mutliple sites

Answered Question
Mar 31st, 2009

I am applying for another BGP AS for another site we are deploying and ARIN asked why we can't use our current AS? Is it possible to use the same AS across multiple sites that are connected to different service providers and have no connections between them?

I have this problem too.
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Correct Answer by Harold Ritter about 7 years 9 months ago

Robert,

It is certainly possible to do that. If you need site 1 to communicate with site 2 and vice versa, you will need to use the command "neighbor allowas-in" on the upstream provider connection, so that both sites can receive the updates from the other. I assumed that each site only has one connection to the upstream provider, correct?

Regards

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rraver Tue, 03/31/2009 - 10:06

Here is a quick image of what I am trying to do..

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|Corporate|

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AS# 12345

Subnet: 1.1.1.X

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| DC 1 |

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AS# ??NEW?? or 12345?

Subnet: 1.1.2.X

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| DC 2 |

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AS# ??NEW?? or 12345?

Subnet: 1.1.3.X

Again, none of these sites are connected with private links and have dual links to the service provider/hosting facility. The subnets are also being requested from ARIN.

Correct Answer
Harold Ritter Tue, 03/31/2009 - 10:39

Robert,

It is certainly possible to do that. If you need site 1 to communicate with site 2 and vice versa, you will need to use the command "neighbor allowas-in" on the upstream provider connection, so that both sites can receive the updates from the other. I assumed that each site only has one connection to the upstream provider, correct?

Regards

rraver Tue, 03/31/2009 - 10:43

Each site has dual connections for HA to the upstream provider(hosting facility) in the future we are looking at getting direct links from carriers but currently its from the hosting provider.

Harold Ritter Tue, 03/31/2009 - 16:53

Robert,

Another option would be not to use the allowas-in and just rely on a default that would be received from your SP in addition to the full internet routing table that you probably already receive from them.

Regards

Mohamed Sobair Tue, 03/31/2009 - 13:55

Hi,

You have One of two Options:

1) Neighbor x.x.x.x as-overide , has to be configured on the provider in order to ur own AS with it AS when BGP updates are recieved.

2) As pointed by Harold, The (Neighbor allow-as in) on ur edge router to permit updates from ur OWN AS and prevent bgp dropping the updates since ur AS in the AS path.

HTH

Mohamed

rraver Tue, 03/31/2009 - 20:20

Thanks guys, I was missing the allowas-in when thinking and it makes sense now.

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