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

[Hank Nussbacher]

On Mon, 19 Aug 2002 cros_m@tsm.es wrote:

Not going to fully check this but you might want to try 'bgp bestpath
as-path ignore' or 'bgp bestpath compare-routerid'.  Both of these
(and others on that page) change the "BGP best path selection
algorithm" but perhaps not as much as you would like.

See:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios121/121cgcr/ip_r/iprprt2/1rdbgp.htm#xtocid4
http://www.cisco.com/univercd/cc/td/doc/product/software/ios121/121cgcr/ip_r/iprprt2/1rdbgp.htm#xtocid5

Let me know if this helps.

-Hank

> 
> 
> ============================================================================================
> 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
> 
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Hank Nussbacher