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BGP: Advertising one AS from multiple sites

I was wondering if there were any issues to consider in using a single AS to advertise different subnets from various sites around the globe. e.g. say you had AS 1888 with address block 220.5.4.0/24 advertised from London office, AS 1888 220.5.5.0/24 advertised from Paris, AS 1888 220.5.6.0/24 advertised from Barcelona. Each site with its own ISP. Are there any downsides or special considerations to that approach as opposed to using a different AS for each site?

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Accepted Solutions

Re: BGP: Advertising one AS from multiple sites

Hi,

You'll only face one problem, which is that for example the routes of the Paris site will travel over the internet to reach Barcelona site, but the problem is that Barcelona site will drop the routes since your AS is in the AS_PATH (since you are using the same AS at all sites), but this can be worked around simply by doing iBGP over GRE between your sites (plus of course the eBGP with the local provider), another option would be to use the bgp bestpath as-path ignore command but i don't think that it is recommended as it might introduce routing problems.

HTH,

Mohammed Mahmoud.

11 REPLIES
Cisco Employee

Re: BGP: Advertising one AS from multiple sites

I dont have much experience with that kind of topology but what I am thinking if you own the public address space with your AS number, you should be fine with it. All the three ISP's will allow you to advertise your subnets from different routes.

I cannot comment much on the downsides of it.. Let us wait if someone else can comment on it and see if our thinking is right or wrong.

-amit singh

Re: BGP: Advertising one AS from multiple sites

I don't see a problem with that.

However, do you have one site advertising everything? If you do, this site should have iBGP with all other sites in a separate or private link, in case those sites with longer prefix advertisement upstream ISP went down.

About the downside, lets wait for others who have implemented the same or similar to comment.

Community Member

Re: BGP: Advertising one AS from multiple sites

Each site would only advertise its /24 block. Not planning any iBGP between the sites. This is part of creating a mesh DMVPN topology amongst sites and BGP is for ISP redundancy at each site. Each site will actually have two ISPs - hence the requirement for BGP. We'll be receiving partial routes and a default and setting the larger pipe at each site to higher localpref. Thanks.

Bronze

Re: BGP: Advertising one AS from multiple sites

Hi

Using same AS number at different sites is absolutly fine. Let me give you an example, like an Class A ISP is using its same AS number for Internet service in different countries :)

and as you will be advertising the routes as different subnets from different sites and these sites will not be connected to each other, so just check any of site's subnet should not overlap otherwise in case of failure at one site other site with smaller prefix will receieve the traffic ..

and I hope company will remain same at all the sites as not very much sure about the administrative constarints/guidelines from RIPE

rgds

Re: BGP: Advertising one AS from multiple sites

Hi,

You'll only face one problem, which is that for example the routes of the Paris site will travel over the internet to reach Barcelona site, but the problem is that Barcelona site will drop the routes since your AS is in the AS_PATH (since you are using the same AS at all sites), but this can be worked around simply by doing iBGP over GRE between your sites (plus of course the eBGP with the local provider), another option would be to use the bgp bestpath as-path ignore command but i don't think that it is recommended as it might introduce routing problems.

HTH,

Mohammed Mahmoud.

Cisco Employee

Re: BGP: Advertising one AS from multiple sites

Mohammed,

The bgp bestpath as-path ignore will not help in this scenario. You probably meant the "allowas-in" option. This would certainly allow updates advertised by one site to be accepted by other sites with the same AS.

Additionally, a default route received from the upstream service provider could also be a way to provide inter site connectivity in this context.

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
Cisco Employee

Re: BGP: Advertising one AS from multiple sites

Harold,

This was good one. Rated !!!

-amit singh

Cisco Employee

Re: BGP: Advertising one AS from multiple sites

I would appreciate, if you someone could identify himself, who has rated my last post.

May I know the reason, why it was rated..

I rated the post given by Harold, as I simply liked his idea and post.

Remember, I didnt post anything wrong or mentioned a wrong solution.

We all guys are very busy in our daily schedule and try our best to solve the problems of the others. Using a wrong rating system, will simply " discourage" the efforts putting in here.

-amit singh

Re: BGP: Advertising one AS from multiple sites

hi Harold,

Totally agree with you, sorry for my confusion yes i did meant to say "allowas-in" in order to ignore the local AS when received inside the AS_PATH of a route from a remote site using the same AS (another branch), while on the other hand " bgp bestpath as-path ignore" is used to bypath the AS_PATH length check in the BGP path selection (to be compliant with RFC 1771 which does not include this step) which has nothing to do here.

I've one comment though, wouldn't the default-route work around be not recommended in cases of dual homed.

Harold, i am really grateful for having you around commenting on our posts, this is really professional.

Amit, sorry about what happened, this happened to me many times, but you know what never mind, we are here to help people, and hundreds of people rate us well and more than that they are very happy and satisfied by our help, then for those few who use the system wrong ... we don't care about them or what they think or do.

Both posts rated indeed :)

BR,

Mohammed Mahmoud.

Cisco Employee

Re: BGP: Advertising one AS from multiple sites

Mohammed,

Taking the default route on top of a full routing table doesn't really affect anything in a dual homed scenario. This would simply provide connectivity to the other sites within the same AS.

The "allowas-in" knob would allow the reception of the specific prefixes from the other sites and would therefore be more granular than the default route approach in a dual homed scenario.

You are very welcome. It is always a pleasure for me to jump in and comment whenever I think it can help.

Regards,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
Hall of Fame Super Blue

Re: BGP: Advertising one AS from multiple sites

Amit

I think for all of us regular posters we sometimes get marked inappropriately. Some people just do not appreciate the effort many of us make to help them.

You do a great job in these forums and i hope it won't discourage you from continuing. I know i've learnt a lot from you.

Posting rated.

Jon

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