[nsp] Re: [nsp]
¿ Is it possible to change the order "BGP best path
selection algorithm" ?
cros_m@tsm.es
cros_m@tsm.es
Tue, 20 Aug 2002 09:16:53 +0200
Hello:
* bgp bestpath as-path ignore (In the proposed scenario the as_path is the
same for all the prefixes, so this is not necessary).
* bgp bestpath compare-routerid.( This is really needed, and it is active
by default in some IOS versions).
About the URL you have given ( http://www.elemental.net/~lf/undoc/#d0e1=
295
) . This was the kind of information that I was hoping would give an
undocumented command , but the entries about BGP don't help in this case.
Thanks
Miguel.
19/08/2002 20:54
Hank Nussbacher <hank@att.net.il>
Destinatarios: cros_m@tsm.es
CC: cisco-nsp@puck.nether.net
Asunto: Re: [nsp] ¿ Is it possible to change the order "BGP best path
selection algorithm" ?
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 "B=
GP
> best path selection algorithm".
> My doubt is if there is any option, so we can decide first by RouterI=
D
> 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 am=
ong
> MPLS VPN. (RFC2547)
> There can be multiple FW asociated to each VPN.
> We use an auxiliary transit VPN ("VPNfw") , so that we avoid asymmetr=
ic
> 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 BG=
P.
> ========================
============================================
> 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 inst=
ance
> 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 appea=
rs
> when the paths for a specific prefix have the same MED value.
> We were expecting that the "Router ID" would decide, but the prob=
lem
> 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 long=
er.
>
>
>
============================================================================================
> 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"eB=
GP
> 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