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live89
live89
Explorer III
February 2, 2021
Question

BGP additional-path-select

  • February 2, 2021
  • 1 reply
  • 14352 views

Hi

 

What is the difference between:

set additional-path-select<#>
under config router bgp

 

and

set adv-additional-path <#>

under config neighbor

 

attached screenshot

 

Also, I sow on the other peer this:

set additional-path receive

I disable it by:

set additional-path disable

And even after clearing and restarting BGP I can still see that HQ advertising multiple (three) paths to me

Later I noticed that my local BGP has 'ibgp-multipath' enabled

 

-- If I only enable 'set additional-path receive' under neighbor I don't see that I can learn additional paths from neighbor -- Only if I enable 'set ibgp-multipath enable' in global BGP settings I can learn additional paths from neighbor -- And if I enable 'set ibgp-multipath enable' alone without 'set additional-path receive' under neighbor I can still learn additional paths from neighbor. Why this behavior? I mean why do we need the 'set additional-path receive' if 'set ibgp-multipath enable' is doing the job alone?

What is the purpose of "set additional-path receive" if HQ still can advertise to me additional paths without it and only with 'set ibgp-multipath enable' ?

      1 reply

      emnoc
      emnoc
      New Member
      February 2, 2021

      So lets start with ibgp-multipath. That allows for mpath from iBGP only. That is my understanding of that feature. This allows for ecmp and selection of paths from ibgp.

       

      On set additional-path-select this is for additional paths and the total number of paths. 

       

      So what are your goals or desire with mpath? I believe in fortios those should be default disable and additional-path select does NOT come up as an option until you enable ibgp-mpath 

       

      I believe the additional path select was put into place to limit bgp resources from learned paths, fwiw

       

      Ken Felix

        live89
        live89Author
        Explorer III
        February 2, 2021
        Hi Ken Thanks for your answer I did some extra research for the first part of my question regarding 'set additional-path-select' and I think it is only for calculation bestpath process. And the 'set adv-additional-path ' is stating for how many bestpaths already calculated to be advertised to your neighbor.   I know what is 'ibgp-multipath' and I know its purpose is ECMP for iBGP. But my question is why do I need the 'set additional-path receive' under neighbor settings if it is not doing anything? All I need is enabling 'ibgp-multipath' under BGP global settings and I can receive multiple paths without enabling 'set additional-path receive' under neighbor section. And if I enable 'set additional-path receive' under neighbor section alone without 'ibgp-multipath' I see no results. So what is the purpose of 'set additional-path receive' ?
          emnoc
          emnoc
          New Member
          February 2, 2021

          Open a ticket with support. I personally never used it from what I can recall.

           

          Ken Felix

           

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