07-26-2007 08:58 AM - edited 03-03-2019 06:02 PM
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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07-27-2007 04:17 AM
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.
07-26-2007 08:36 PM
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
07-26-2007 08:46 PM
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.
07-26-2007 08:54 PM
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.
07-26-2007 09:55 PM
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
07-27-2007 04:17 AM
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.
07-28-2007 06:31 PM
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,
07-28-2007 09:23 PM
Harold,
This was good one. Rated !!!
-amit singh
07-28-2007 10:03 PM
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
07-28-2007 10:52 PM
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.
07-29-2007 01:24 PM
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,
07-29-2007 01:17 AM
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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