BGP: Influence Outbound

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Aug 7th, 2008
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Hi guys,

I have a question on how to influence my outbound traffic. I have 3 providers and let say i have 3 /24 networks. For the purpose of the config, i want one /24 going on my first ISP and the 2nd /24 going to the 2nd and 3rd /24 to my 3rd isp. And still when one isp goes down still the other /24 from the down isp will now go the 2nd isp and so on. Here is a sample of my diagram.

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lee.reade Thu, 08/07/2008 - 23:24
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Do you know the prefixes that you want to send to each bgp router? Ie, you say /24 network, so do you know that you want say to go to ISPa, to ISPb, to ISPc???

Or do you not know the prefixes for the remote networks?

What networks is each ISP advertising to you?



ninjakid_cisco Thu, 08/07/2008 - 23:46
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Hi LR,

Let say my networks are public and we will use private for this example:

For outbound: to go to ISPa to ISPb to ISPc

For Failover:

If ISPa goes down, my can use ISPB to go out, since its all advertised to all isp.

I think this is clear. I just want to learn this traffic engineering on the outbound and later ill ask for inbound. and im receiving full routes to all my isp =)


lee.reade Fri, 08/08/2008 - 00:28
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Since you are taking full routes to net then you wont be able to be specific what routes go to which ISP I reckon your best way forward here would be for each core router to advertise default into your core, and each internal router will use closest core exit point.

At the core exit points you can config a route-map for each isp peer, so that routes to use ISPa and routes to use ISPb on core1 and on core2 config similiar so that to routes use ISPc and routes to use ISPb.

You can do this using access-lists and route-maps by setting the weights.

Check out;



lamav Sun, 08/10/2008 - 17:08
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What are the requirements here?

From the question, it sounds like your requirement is to have traffic sourced from a certain subnet (one of the 3 you mention) to use a certain ISP connection.

The solution provided by LR does not address this. It addreses what would be a requirement for load sharing outbound traffic to the Internet, dividing outbound traffic across 3 ISPs based on the destination address.

Which is it?

ninjakid_cisco Sun, 08/10/2008 - 17:17
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Hi lamav,

Dividing the oubtound traffic across 3 ISP based on the source, which is my network (3 /24).

Another thing, my diagram shows i have a router connected to each ISP and then connected to my core routers. Is it the same configuration on local preference if i remove all routers connected to my ISP and directly connect my ISP to the core routers ?



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