Inter-AS Option AB

Have any of you tested out Inter-AS Option AB? Its a hybrid between option A and option B. I have been reading about it and implemented it for the first time in the lab for testing purposes. I must say that I really like this brings the best of both worlds between A and B. 

For those of you that have not heard about it, here is the doc:


I also like that it can easily be extended to support CSC. Worth taking a look at!




  • I have it on my list for tonight, I'll report back on how awesome it surely is [:D]

  • Right I just finished doing 'regular' option AB.  It's pretty straightforward, few odd extra commands in the VRF and under BGP, no rockets or science.

    For the life of me I can't figure out why anyone would feel the need to deploy this in the real world, except for maybe migrating from A to B, which I guess is part of the point.  It's seems utterly pointless for a finished article, you either go A or you go B but not a mishmash. You could see someone deploying this for a migration and then never bothering to finish the job off. Like everyone says, you never, or rarely, see options B or C in a production network because of the complexity of managing coodinated systems with a compeditor, it's hard enough to get your own internal systems in order. Why did Cisco develop this? Surely some large ISP decided they wanted it so Cisco said yeah we want your money, let;s make this for you. And as for AB+, yikes.

    On to CSC now. I hope it's more exciting :D

  • I guess the main advantage of this design is that it only requires 1 session between the ASBRs (eBGP VPNv4). Option A would require separate sessions (eBGP or whatever IGP) per VRF, adding extra overhead comparing it to being able to just peer once and exchange routes for all VRFs through 1 session. 


  • Inter-AS deployments are hard to find. The only one I have seen so far is when we merged 2 networks so that we can then get rid-off the old one. Then we used Option-10A which was easy to deploy.

    CSC are like unicorns.

    Inter-AS AB gives you best of Option-10A and 10B, but is it just another unicorn?



  • I guess the thing I don't see the point with is if you are exchanging VPNv4 with the oether carrier why would you go through the burden of creating a subinterface per VRF, potentially scalaing to thousands.


    @AmitB snap on the merging networks.

  • CSC are like unicorns. 


    True.  I believe primary goal of CSC in the SP track  is to give a complete understanding of using label stacks and tunneling through multiple networks without the need for a total transit route exchange.


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