BGP recursion host

Hi everyone,

 

I am trying to understand the influence of "bgp recursion host" command but I am afraid that I am missing something. As far as I understand it literally instructs the BGP process to use only /32 prefixes for BGP next-hop resolution. 

 

In SPv3 workbook Full Scale Lab 4 Task 4.7 "bgp recursion host" is set for VPNv4 address family along with "bgp additional-paths install". Why is "bgp recursion host" necessary here? When we are using VPNv4 address family we are using only /32 routes for hext-hop recursion anyway, why we should explicitly configure this only in this scenario?

 

Looking forward to getting your comments.

Comments

  • Nope the command tells CEF to not search for a longest prefix match and thus slowing down the BGP PIC convergence. So when you eanble add-paths, you disable recursion for /32 and connected interfaces in order to not slow down the convergence.

  • Nope the command tells CEF to not search for a longest prefix match and thus slowing down the BGP PIC convergence. So when you eanble add-paths, you disable recursion for /32 and connected interfaces in order to not slow down the convergence.

    add-path and "additional-paths install" are 2 completely different topics. "additional-paths install" just tells the box to install in the FIB both the best path and any additional paths that may be present in the BGP RIB (there are different knobs that allow you to custumize which additional paths you would like to select and install in the FIB. 

    add-path (the RFC) is just a mechanism used to tag routes (like an RD in VPNv4) and prevent the implicit withdraw problem. That way, routers in a network that is not fully meshed, and use a RR for example, can all receive multiple copies of the SAME route with different next hops. Once they receive multiple copies with different next hops, they can then decide to install one or more additional paths in the FIB

    bgp recursion host is NOT enabled by default - for both regular IPv4 and VPNv4. This will cause the router to waste time looking for other alternative  shorter length next-hops once a path a lost (1.1.1.0/24 instead of 1.1.1.1/32 for example).  When BGP PIC Edge, AKA 'additional-path install' is enabled on a router under VPNv4 address family (recent code), bgp recursion host is automatically enabled. Perhaps in the version of code that the workbook used the behavior was different. For IPv4, BGP recursion host is NOT enabled automatically even after enabling PIC edge. 

     

     

  • Sorry it is misleading I was using a shortname for additional-paths-install, too lazy ;)

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