BGP Multihoming

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Mar 9th, 2007
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We have a t1 and a t3 for internet connectivity. Both connectons come into the same router. We are using BGP on the router. Rather than tinker with bgp parameters to try and make the t3 used as a first priority, should I somehow just put the t1 interface in a backup mode. I am interested in hearing opinions on how you guys would handle this. I am using weight commands to make the t3 the preference for outbound traffic. I am going to do some work with set as-path prepend statements this weekend to try to manipulate inbound traffic to use the t3. I am just now starting to think that maybe I just want to make the t1 used only if the t3 is completely down.

any ideas or know hows on this one?

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devang_etcom Fri, 03/09/2007 - 15:09
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i think you are on the right track for infulancing the inbound and the outbound traffic path selection... for the outbound traffic you can have static routes with the different metrics... even you can have HSRP also...

sundar.palaniappan Fri, 03/09/2007 - 15:23
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Hmm, I'm not sure I would do that. Why use static routes when BGP is an option?

I would recommend going with a BGP solution and that would be more reliable. Moreover, your setup doesn't require a complex solution.

The problem with making T1 int a backup interface is it would work only when the T3 interface goes down. If the T3 interface remains up but doesn't pass traffic then this solution wouldn't work.

Given the fact, you have a single router BGP peering with 2 providers the primary/redundant setup can be done without any complex configuration. BGP gives you a range of options to influence inbound/outbound traffic but you can use something like this to achieve the desired outcome.

For outbound traffic:

Set the weight higher for prefixes learnt via T3

For inbound traffic:

Prepend the AS path via T1 link to make it less desirable. However, check with the ISP if this is OK or have them recommend options for you.



bfledderjohn Fri, 03/09/2007 - 20:25
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prepending the as path didn't help me at all.

do you have any other ideas?


devang_etcom Fri, 03/09/2007 - 21:14
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why AS Path Prepending didnt help you... i mean will you please give us issue?

you can have then your service provider should advertise the MED value with the update send to you... normally Update with lowest MED is prefered... so tell your ISP to configure the router which is connected with T3 link to advertise update with the lowest MED... and advertise route to link T1 with the higer value MED than T3... but for that your ISP should be agree with you to configure their router...



libanm Fri, 03/09/2007 - 21:51
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You are on the right track, but let me ask you this, are you connected to the same ISP? because if you are connected to two different ISP, then i would be careful to route traffic that belongs to T-1 ISP to T3 ISP, where the path could be longer :)

bfledderjohn Sat, 03/10/2007 - 06:39
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I use PRTG and I still more traffic coming in via the t1 instead of the t3.

when i use to test our speed, it shows t1 speed. when i shut down the serial interface for the t1 and test again, I get t3 speeds.

I have one router, 2 different ISP's. I am already using weight on the t3 interface for outgoing traffic.


Danilo Dy Fri, 03/09/2007 - 21:58
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For outgoing you can use Weight since you only have one router. Weight is local to the router, however, if you have more than one router, use local-preference which is local to the AS domain. However, if you use Weight on T3, you are not utilizing the outgoing of your T1. To load share your T1 and T3 outgoing bandwidth, use as-path route map, see this link for sample

For incoming, Prepend is not the best option. You can try MED.

bfledderjohn Sat, 03/10/2007 - 06:42
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I have one router, 2 different ISP's. I am already using weight on the t3 interface for outgoing traffic.

PRTG shows more incoming traffic to the t1 then the t3 and speedtests show t1 speeds unless I shut down the t1 serial interface and retest.

When users download files, our internet gets bogged beacuse incoming traffic is flodding the t1. I can see that their requests go out the t3 but returning traffic uses the t1.

sundar.palaniappan Sat, 03/10/2007 - 12:12
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The original poster clearly indicated in his first posting that he has one router terminating both ISP connections. Hence, weight is an ideal option to influence how the outbound traffic is routed as weight is on the top of the list in the BGP path selection mechanism. As far as the inbound traffic is concerned, I am not sure why prepending AS paths to two providers doesn't work and we need more information to determine the reason. As I had indicated earliler there are many ways to influence inbound traffic but that's something that has to be worked out with the provider(s).

Here's an option for you. You can configure bgp conditional route injection feature to advertise your network to the provider connected via T1 connection only when the T3 network/link is down. The good thing about this feature is you control the whole thing and the provider's assistance isn't required.

This link has some cool configuration examples and take a look at it before configuring it on your router.

Good Luck!!




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