BGP:preferred outgoing path

Unanswered Question
Nov 11th, 2008

hi gurus,

is it possible to use local preference on this kind of setup? attached here is the diagram. i want each block of ip address to have different outgoing path.


I have this problem too.
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Haris P Tue, 11/11/2008 - 22:37

I think the only way to acheive your aim will be by using route-map .



jeffersoncbriones Tue, 11/11/2008 - 22:43

Hi Haris,

route-map and then use local preference? im using ibgp ,can you provide me a sample config.

Can i influence outgoing based on source address coming from my network ?


Giuseppe Larosa Wed, 11/12/2008 - 01:10

Hello Jefferson,

>> can i influence outgoing traffic based on source which is my network address?

not using BGP you should use PBR policy based routing for this

if you want that traffic with source goes out via AS 200 and traffic with source goes out via AS 500 as showed in your picture BGP is not enough.

BGP provides a lot of tools and policies but it works on destination address.

So BGP can be helpful on trying to influence the return path from internet.

To achieve what you want the distribution multilayer switches have to use PBR with next hop the lan ip address of the correct border router (the one connecting to AS 200, and the one connecting to AS 500 in your picture)

Hope to help


jeffersoncbriones Wed, 11/12/2008 - 01:13

Hi Guiseppe,

I would create a PBR then enable it on the interface in the core network not on the border router. how bout using vrf ?

tnx a lot.

Giuseppe Larosa Wed, 11/12/2008 - 01:37

Hello Jefferson,

yes on the core router towards the border router.

>> how bout using vrf ?

if the needs are that net should access internet only and via AS 200 and the same for via AS 500 you can think to use two VRFs.

You can do this if AS 200 and AS 500 are only used for these networks.

In this way you separate the networks and you don't need PBR.

If other IP networks need to use AS 200 and AS 500 PBR is a better solution.

Hope to help


jeffersoncbriones Wed, 11/12/2008 - 01:43

Hi Giuseppe,

Yes other networks need to use this. another thing my GSR12406 dont have pbr enabled. im using 12.0(38), can i enable this one?


Giuseppe Larosa Wed, 11/12/2008 - 02:37

Hello Jefferson,

in the case of GSR what counts is the linecard if you have an engine3 4GE I think PBR should be fine.

I remember I tested PBR on these linecards.

3GE engine2 can have some limitations but it should work too.

Hope to help


Giuseppe Larosa Thu, 11/13/2008 - 07:48

Hello Jefferson,

you are right:

RT-TO-CRN-SI-E-1(config-if)#ip policy ?

% Unrecognized command

RT-TO-CRN-SI-E-1(config-if)#ip policy

I verified on one of our GSR that has a 3GE engine2 linecard

You need to verify if you can configure a GRE tunnel between access router and border router so that you can send traffic over it

Hope to help


Mohamed Sobair Thu, 11/13/2008 - 08:37


If the routers in AS100 are all IBGP peers, then yes , setting the local prefrence for a set of prefixes would have different outgoing path.

Assuming all routers in AS100 are IBGP peers or within a Confederation (Confederation Members).

By default, IBGP updates carries the following manadatory attributes:

1- As path.

2- local prefrence.

3- Origin.




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