[nsp] ¿ Is it possible to change the order "BGP best path selection algorithm" ?

[Pekka Savola]

On Mon, 19 Aug 2002 cros_m@tsm.es wrote:
> ============================================================================================
> The Question: There are many options to change the behavior of the "BGP
> best path selection algorithm".
> My doubt is if there is any option, so we can decide first by RouterID
> instead of  "eBGP is preferred over iBGP path".
> ============================================================================================
> 
>   The scenario is a very controlled one ( is for internal use in a
>   company).
> 
>   Standard Behavior
>   =================
> 
>          Prefix      MED   e/iBGP(1) Dist_NH(2)    RouterID(3)
>        ==========================================================
>       (*)prefix_a     5    (eBGP)      1            5.5.5.5    <=====
>          prefix_a     5     iBGP       5            2.2.2.2
> 
> 
>   Desired Behavior
>   =================
> 
>         Prefix      MED   e/iBGP(-) Dist_NH(-)    RouterID(1)
>        ==========================================================
>        prefix_a     5     eBGP       1            5.5.5.5
>     (*)prefix_a     5     iBGP       5           (2.2.2.2)   <======

Why wouldn't you give a better local-preference to those routes you want 
to prefer?


> More Information
> ===============
> 
> We are working in a distributed FW solution to the interconnection among
> MPLS VPN. (RFC2547)
> There can be multiple FW asociated to each VPN.
> We use an auxiliary transit VPN ("VPNfw") , so that we avoid asymmetric
> routing.
> Travelling from VPNorig to VPNdest would be as indicated below.
> 
>       VPNorig ===> FW_exit_orig ===>  VPNfw   ===> FW_enter_dest ===>
> VPNdest
> 
> 
> We have several options and in one of them we use the MED field of BGP.
> ===================================================================
>       We don't play with Route Targets (Each VPN has its RD and they
> import/export its Route Target).
>       VPNorig (VRF) have default routes pointing to their FW sites.
>       VPNfw learns the prefixes of all the final VPN trough eBGP.
>       We are "cheating" as we declare as CE BGP neighbours the VRF instance
> associated to the auxiliary VPNfw.
> 
>   |------------|                       |-------------|
>   |    * VRF1  |----- eBGP neighbour --|-* VRFfw     |
>   |PE  * VRF2  |----- eBGP neighbour -// /       PE  |
>   |    . . .   |                      /|/            |
>   |    *VRFn-1 |----- eBGP neighbour / /             |
>   |    *VRFn   |----- eBGP neighbour  /|             |
>   |------------|                       |-------------|
> 
>       The prefixes carry in the MED, the IGP metric ( set metric type
> internal).
>       We have multiple sites with "VPN/VRF fw" so that we have multiple
> paths for the same prefix, differing only in the MED value.
>       In Cisco, the BGP Router ID can not be specified per VRF, so we
> needed 2 PE instead of one.
>       We had to play with Local AS, Allow AS IN,  AS Path Prepend.
> 
>       Everything works OK but we have a small (really big) problem. The BGP
> selection criteria works well and the decision of the
>       BGP best path is always based in the MED value. The problem appears
> when the paths for a specific prefix have the same MED value.
>       We were expecting that the "Router ID" would decide, but the problem
> is that the we forgot the intermmediate step in which
>       "eBGP is preferred over iBGP path".
> 
> 
> I have omitted many details, but I didn't want to make the Email longer.
> 
> 
> ============================================================================================
> The Question: As there are many options to change the behavior of the "BGP
> best path selection algorithm".
> Is there any option, so we can decide first by RouterID instead of  "eBGP
> is preferred over iBGP path" ?
> ============================================================================================
> 
>   Standard Behavior
>   =================
> 
>          Prefix      MED   e/iBGP(1) Dist_NH(2)    RouterID(3)
>        ==========================================================
>       (*)prefix_a     5    (eBGP)      1            5.5.5.5    <=====
>          prefix_a     5     iBGP       5            2.2.2.2
> 
> 
>   Desired Behavior
>   =================
> 
>         Prefix      MED   e/iBGP(-) Dist_NH(-)    RouterID(1)
>        ==========================================================
>        prefix_a     5     eBGP       1            5.5.5.5
>     (*)prefix_a     5     iBGP       5           (2.2.2.2)   <======
>       The scenario is a very controlled one ( is for internal use in a
>   company).
> 
> 
> Thanks
> 
> 
>       Miguel
> 
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-- 
Pekka Savola                 "Tell me of difficulties surmounted,
Netcore Oy                   not those you stumble over and fall"
Systems. Networks. Security.  -- Robert Jordan: A Crown of Swords