Do we need a new ASN for branch office BGP?

Answered Question
Jul 1st, 2010

We already have BGP implemented (ISP redundancy) at the main office building. It's got its ARIN approved ASN and a /24 prefix announced to the upstream ISPs.

My question is that if we were going to implement BGP at a branch office, do we need to apply a new AS number? Will it be a brand new setup or we can somehow use the same ASN? The branch office has its own /24 prefix and Internet connection. It talks to the main office through a L2L VPN tunnel.

If we can use the same ASN, what are the special configurations we need to be aware of? Thanks in advance.

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Correct Answer by Mohamed Sobair about 6 years 5 months ago

Hi,

Use the same AS number and run IBGP between your branch and main office. There is no special configuration, but you will be able to have loadsharing and redundancy between both ISPs.

HTH

Mohamed

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Chetan Kumar Ress Thu, 07/01/2010 - 09:08

Hi

It is not requried or mandatory .

You can use same AS number for all locaiton ie. IBGP over SP network.

You can use same ANS number but for that you need to have a word with your SP , SP provider need to configure for that one.

SP need to configure an extra command for Branch locations under Address-Family.

i.e neighbor x.x.x.x as-override

http://fengnet.com/book/MPLS%20Configuration%20on%20Cisco%20IOS%20Software/ch06lev1sec1.html

Regards

Chetan Kumar

Correct Answer
Mohamed Sobair Thu, 07/01/2010 - 09:31

Hi,

Use the same AS number and run IBGP between your branch and main office. There is no special configuration, but you will be able to have loadsharing and redundancy between both ISPs.

HTH

Mohamed

jackawang Thu, 07/01/2010 - 09:48

Assuming iBGP peers need to be fully meshed and they need to be able to reach each other through either IGP or EGP network. We can implement Route-reflector or Confederation but how do we do that across public IP space WAN?

Mohamed Sobair Thu, 07/01/2010 - 10:30

Hi,

How many routers do you require to run IBGP on a single site?

As a side note, BGP route-reflector and confederations needs only IP reachability between routers, as long as you have IP reachability you can configure either one.

BGP router-reflector and Confederations reduces the number of TCP sessions required by fully meshed IBGP and disables the Split Horizon rule in BGP.

HTH

Mohamed

jackawang Thu, 07/01/2010 - 11:17

I have two routers running BGP on each side. I am sure we can do route-reflector or confederation to make iBGP working. However I am concerned about peering iBGP across multiple hops over the Internet. The hello packet is sent every 60 seconds by default. I just wanted to make sure this is the best practice and commonly deployed in the field.

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