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

[cros_m@tsm.es]

============================================================================================
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)   <======






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