Hot!BGP additional-path-select

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live89
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2021/02/02 01:12:41 (permalink)
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BGP additional-path-select

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' ?
post edited by live89 - 2021/02/02 07:04:29

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#1

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    emnoc
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    Re: BGP additional-path-select 2021/02/02 07:33:34 (permalink)
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    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

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    #2
    live89
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    Re: BGP additional-path-select 2021/02/02 09:19:38 (permalink)
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    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' ?
    post edited by ac89live - 2021/02/02 11:31:56

    Thanks
    #3
    emnoc
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    Re: BGP additional-path-select 2021/02/02 11:02:03 (permalink)
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    Open a ticket with support. I personally never used it from what I can recall.
     
    Ken Felix
     

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    Toshi Esumi
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    Re: BGP additional-path-select 2021/02/02 11:11:39 (permalink)
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    Although I haven't used multipath, I see it's disabled at a neighbor by default after enabling under BGP globally (6.4.4). It could be a bug depending on the version. I would open a ticket.
    #5
    live89
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    Re: BGP additional-path-select 2021/02/02 13:44:33 (permalink)
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    Thank you both
    I investigated the issue with support, and it was a misunderstanding of this command
    As i ran two IPSEC VPNs towards the HQ additional to the main IPVPN line, I was always receiving three routes if ibgp-multipath is enabled, no matter if 'set additional-path receive' is set or not under neighbor. Because those routes are already learned by the main line and the other two IPSECs, and they're not additional routes.
     
    'set additional-path receive' was taking position only with ADVPN shortcuts in my scenario. So if I was trying to speak to my other office and shortcut created and set additional-path receive is enabled I will see in my routing table two additional routes to the destination. but with 'set additional-path disable' no additional routes was added to the routing table.

    Thanks
    #6
    emnoc
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    Re: BGP additional-path-select 2021/02/02 18:44:36 (permalink)
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    Thanks for the update, duly noted
     
    Ken Felix

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