inconsistenet-as issue

Answered Question
May 27th, 2007

Hi Everyone,

I need your opinion, we have a customer who needs to be multihomed (via our ISP and another ISP) to achieve redundancy, he wants to run BGP via private-as (he won't have the justification to own his own PI addresses and ASN).

The solution i reached (actually i found many ISPs using it) is using 2 class Cs, one from us and the other from the other ISP, and the customer should advertise both to both of us via BGP, and both ISP would have to remove the private-as and both ISP will advertise both classes to the internet (each will be primary for its class and backup for the other ISP class - using the perpending).

And of course we will need to have 2 route objects for both classes at RIPE (to attach the 2 classes to the 2 ASN of the 2 ISPs in order to be accepted on the internet routers through our upstream providers).

The issue here is, since this types of routes is seen as inconsistent-as in the internet routing table, would this have any drawbacks or impact, is there any problem going through with this solution, has any one experienced this solution, if so please give me your feedback.

Thanks in advance for your replies.

BR,

Mohammed Mahmoud.

I have this problem too.
0 votes
Correct Answer by Paolo Bevilacqua about 9 years 6 months ago

Hi,

I was looking at this better. There are currently about 800 prefixes announced with inconsistent as, although most people is liberal in accepting routes, I don't know what the general consensus on using them.

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Paolo Bevilacqua Sun, 05/27/2007 - 12:38

Hi Mohammad,

Since you are willing to make a extensive configuration for this customer, and the objective is to have mutual backup for the respective prefixes, why don't you look also at the conditional BGP feature:

http://cisco.com/en/US/tech/tk365/technologies_configuration_example09186a0080094309.shtml

That should avoid the inconsistent AS in the normal situation, still allowing each of the ISPs to begin advertising both prefixes on circuit failure.

Good luck!

mohammedmahmoud Sun, 05/27/2007 - 23:24

Hi Paolo,

First of all, thank you very much for your feedback.

I thought about conditional advertisement, but i had some doubts, would the design go that i match the other provider's prefix that i receive from the internet using the "non-exist-map" and then if the other provider stops sending them (the circuit of the customer fails and the provider stops relaying the routes to the internet) i should now send them via the "advertise-map", would the " neighbor advertise-map command" support matching on the same prefix in both the "non-exist-map" and the "advertise-map".

One last question, in your experience what are the drawbacks of having the route as inconsistent-as.

BR,

Mohammed Mahmoud.

Paolo Bevilacqua Mon, 05/28/2007 - 07:01

Hi,

yes you have described how the mechanism would work.

Wrt the drawbacks of inconsistent as, lot of time has passed since I had operational BGP experience, hence I can't tell. Chances are nothing will happen, still I think ISP's must avoid it.

Correct Answer
Paolo Bevilacqua Mon, 05/28/2007 - 11:01

Hi,

I was looking at this better. There are currently about 800 prefixes announced with inconsistent as, although most people is liberal in accepting routes, I don't know what the general consensus on using them.

mohammedmahmoud Mon, 05/28/2007 - 11:15

Hi Paolo,

Thank you for your professional interaction and support, i'll go with the conditional advertisement and i'll let you updated with the results, and i'll also let you know if i found something new about inconsistent-as.

BR,

Mohammed Mahmoud.

fortis123 Thu, 04/17/2008 - 08:49

Hi All,

Can you shoot me a link to an config scenario to use Conditional advertisement.

2 nternet routers, Dual ISP (rtr1-ISPA, Rtr2 -ISPB ), having our own AS# and class C N/w being advertised and EBGP with both carriers.

Wanted to run Primary/backup autofailover.

Carriers not supporting Communities.

Thank you in advance

MS

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