BGP Inbound route influence other than Prepend

Unanswered Question
May 10th, 2007

I have 3 ISP's connected to a 7206 VXR router via BGP. 2 T1's and a T3. I am trying to get all my inbound traffic to take the T3 route most of the time unless the T3 goes down. I have set up prepended to add 10 of my AS numbers to the T1 connections and I have verified the prepends are propigating properly across the internet. However, I still am getting a large amount of trafic inbound on my T1's at any given time. Is there anyway other than prepending to influence inbound BGP routes other than prepending?

I have this problem too.
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Loading. Thu, 05/10/2007 - 15:32

One more option is using MED. Configure lower value MED for your inbound traffic. But as path prepending has to be removed before applying MED as AS PATH has higher preference in route selection.

Harold Ritter Thu, 05/10/2007 - 17:45


Setting the MED will only help if the 3 connections are to the same provider (same AS).

Hope this helps,

Harold Ritter Thu, 05/10/2007 - 17:51

BGP Conditional Advertisement would allow you to advertise certain prefixes via the T1 links only if the T3 link is down (or not receiving certain prefixes from its peer).

Please refer to the following URL for more information on BGP Conditional Advertisement:

Hope this helps,

NPT_2 Thu, 05/10/2007 - 19:01

I thought about doing Conditional advertising so the prefixes were only advertised over the T1's when the T3 was down, but wouldn't the convergence time be really long? The way I understood it, the time for a new BGP network advertisement to propagate worldwide could be 24 hours or more. What do you think?

Harold Ritter Thu, 05/10/2007 - 19:19


I agree the convergence time will be a bit longer but it will be in order of minutes rather than hours. Moreover, this is the safest way to achieve what you are looking for.

Hope this helps,

ruwhite Fri, 05/11/2007 - 03:58

To answer one question on the thread.... I think the next hop tracking work in BGP made conditional advertisement event driven:

Another option, besides MEDs and AS Path Prepend (neither of which work very well) is to send RFC1998 communities. Most providers support these now, and by using overlapping advertisements and adjusting the provider's local preference through RFC1998 communities, you can get pretty good control on inbound traffic flow (within limitations--the provider still makes the final path decision towards you).



jbrunner007 Sat, 05/12/2007 - 13:25

Wait, time out...

No one *should* have the prepended routes, unless those prepends are not working properly!

He said he has 3 different ISP's. All traffic should go in ONLY on the T-3. There is an inconsistent view of your as. here is how you can fix it...

Make sure the T-1 isp's BOTH allow the prepends

Make sure the T-1 isp's DONT have any static routes redistributed into their IGP, you could be getting customer ingress traffic of those ISP's coming into your T-1's. If your T-1 ISP is like MCI/Qwest and has many customers attached to their IGP, you could be missing alot of ingress traffic on the T-3

It may be a problem where your T-1 providers are choosing your prepended path as the best path, when they should choose the T-3 path off their network (its still fewer AS hops :)

Guess what?

They may be doing some cold potato routing via communities, etc i.e. set local pref to 5000 for peers whose next_hop address = our ip space...

I have worked with some ISP's as a consultant who did a similar policy. The goal was to save $$$ and latency at the IX's where they were charged by using "preferred links"... kind of cold potato routing for economics as apposed to performance...

get your T-1 providers to relax their routing policies for your IP blocks you advertise in BGP... They may do this for you to avoid them using a connected as, despite as path length as I suspect they are.


Danilo Dy Sat, 05/12/2007 - 21:32


BGP give us "total" control for outbound traffic but not for inbound traffic. Think about this, if it give us "total" control for both, the internet will be chaotic.

Though you use prepending you still see large amount of incoming traffic in your 2 x T1 link, they might be;

1. Traffic coming from your upstream T1 ISPs local domain.

2. Traffic coming from other ISP transiting to your upstream T1 ISPs who doesn't have a path to your T3 ISP (direct or indirect).

If your T1 ISPs have a path to your T3 ISP (direct or indirect), talk to them to choose T3 ISP as primary for their outbound traffic to your AS domain;

1. All outbound, includes traffic transiting to them and traffic coming from their local domain

2. Only traffic transiting to them.

They may however deny the request for many reason.

1. It's against their religion (or policy) :)

2. They don't like T3 ISP :)

3. They don't know how to do it and may find it risky :)

4. They don't like additional work :)

I have tried this approach many times, not to my direct neighbor though, but to an ISP in the other side of the globe - in the end, both of us benefited in the change.



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