Multihoming BGP with Private AS number

[qazisalman]

Hi,

I have a situation that i need to set up Multihomed BGP with private AS nunmber. We have purchased already public Ip range that we will use on our servers that need access to internet. But we are not willing to buy AS number at this moment.

Subject:  One router and two different ISPs want to use Private AS number

1- I have done some research on this and found that a public AS number needs to be bought for this type of situation. Can someone please advise what other options we have available or if i try to set up EBGP using Private AS number, would that work or what kind of issues i could face later.

2- I need to advertise a whole server ip range , how i could set up preference for ISP1 and make ISP2 as backup internet circuit.

Please advise some solutions(configs) and drawbacks of this kind of setup.

Thanks.

Regards,

[Bilal Nawaz · Accepted solution]

Hello,

1) There is no problem at all using private AS numbers - you treat it the same way as if you were using public AS numbers. You will have EBGP peerings with your ISPs. As long as the AS numbers are different Private AS numbers, then you can create EBGP relations.

Problem may happen if you want to move to a Public AS number in the future. Since your router can only belong in one AS, it means you will have to tear down all neighborships, take all the BGP config out, and re-do with the public AS number.

2) Outbound traffic can be manipulated on the router that connects to the ISPs by using Weight attribute. The routes from a particular neighbor with highest weight value will take preference over other routes.

For inbound, you could do AS Path attribute which will do prepending to the AS path. This will make it seem like it takes longer route to get to your prefixes. This can be set for the secondary ISP, so it looks like the shorter route to get to you is via ISP 1.

If you want to do a particular range, then might be a bit tricky. Maybe you have to do PBR for outbound, and AS path prepend for the specific Server prefix back in towards your network.

Hope this helps.

Please rate useful posts & remember to mark any solved questions as answered. Thank you.

[paul driver · Accepted solution]

Hello

You can use the bgp command

Neighbour xxx local-as no prepend


This can hide your private as from your isp while transitioning from private as to a public one

http://www.cisco.com/en/US/docs/ios/12_2s/feature/guide/fsbgphla.html

Res
Paul


Sent from Cisco Technical Support Android App

[qazisalman]

Thanks much guys for your help.

Thanks again.