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

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

Aug 19, 2002, 7:42 AM

Post #1 of 3 (1756 views)

============================================================================================
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

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

pekkas at netcore

Aug 19, 2002, 8:17 AM

Post #2 of 3 (1702 views)

Permalink

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
>
> _______________________________________________
> cisco-nsp mailing list real_name)s@puck.nether.net
> http://puck.nether.net/mailman/listinfo/cisco-nsp
> archive at http://puck.nether.net/pipermail/cisco-nsp/
>

--
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

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

hank at att

Aug 19, 2002, 11:54 AM

Post #3 of 3 (1719 views)

Permalink

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
>
> _______________________________________________
> cisco-nsp mailing listreal_name)s@puck.nether.net
> http://puck.nether.net/mailman/listinfo/cisco-nsp
> archive at http://puck.nether.net/pipermail/cisco-nsp/
>

Hank Nussbacher