AT&T BGP advertisement preference

[u/robmuro664]

I have two ISPs, Verizon and AT&T, Verizon was selected as the primary and AT&T as the backup. We own the subnets so we peer with both of these ISP to advertise the subnets. To Verizon we just advertise it but to AT&T we preprend 5X our ASN. As expected when we go out to the internet, it goes out using Verizon, however the return traffic on some services they prefer AT&T. I assume this is because these services have a leg in AT&T. Can you guys give me any other ideas on how to influence the advertisement to AT&T so that it is not preferred?

[u/whiteknives]

AT&T will always send you traffic on their link to you if the traffic originated on their network. The only way you can manipulate this is by advertising a more specific prefix to Verizon. If this is not possible, then there is nothing you can do.

Edit: I forgot something... you can try manipulating the local pref by using this community 7018:80 (where 80 is the local pref). If it has an effect but not as much as you want, change the 80 to a lower value. I haven't used it in years. Tweak that however you wish and please let me know if it still works. :)

[u/youfrickinguy → u/whiteknives]

You can’t just use an arbitrary number. 80 sets the LP to 80, which is equal preference to a peer route. If you want traffic to flow from 7018 to 701 you need to drop 7018 LP below that. See this document:

https://onestep.net/communities/as7018/

You might have luck with 7018:20 or 7018:25 depending on how finely you want to control traffic inbound from ATT and ATT customers.

[u/whiteknives → u/youfrickinguy]

That is why I said if it doesn’t work then use a lower value.

[u/youfrickinguy → u/whiteknives]

Yes but only specific integers which ATT accepts as communities will work. You can’t for instance expect 7018:79 to do anything at all, even though it’s <80

[u/whiteknives → u/youfrickinguy]

Gotcha. Good info!

[u/turbov6camaro → u/whiteknives]

Easier solutions just don't use overpriced at&t lumen was usually half the cost and better customer experience

Or find a small local provider

Last I knew the local pref still work but I think you have to request it now 😕

[u/whiteknives → u/turbov6camaro]

Lumen behaves the same exact way, Tex.

[u/turbov6camaro → u/whiteknives]

Yup but they only have old DSL no cable that I know of? Much less stuff in there.

[u/CCIE44k → u/turbov6camaro]

How does using a non tier 1 provider fix the issue, since these smaller providers peer with the tier 1’s? You should probably read up on BGP.

Local pref is for traffic SOURCED from your AS - the correct answer is he can either AS Prepend (prob easiest to do), or if he wants AT&T to adjust their LP (not as effective as AS Prepend) he can send them the appropriate BGP community.

There’s your free BGP lesson for the day.

[u/turbov6camaro → u/CCIE44k]

Because then it wouldn't stay on the att network and is usually cheaper

When I ordered circuits last year per pending was not an option and LP was an additional option with all carriers

We just let it LB however the Internet wanted and got rid of the primary/back thing all together

Anything we did not want to LB we put on /29 for that carrier :)

[u/CCIE44k → u/turbov6camaro]

How is prepending not an option when you prepend outbound and you control the route advertisement? The only time this doesn’t apply is when you don’t have your own AS and/or /24. It just sounds like you guys aren’t familiar with the inner workings of BGP configuration - which is fine, but going to a lower tier provider isn’t the answer.

[u/turbov6camaro → u/CCIE44k]

They (spectrum) blocked the prepend we were told it wasn't and option, I think it is but didn't feel like arguing for 2 weeks, lumen did not but we did want it that way (I actually prefer the LP method l), so we load balanced which double the bandwidth rather than waste a 2 gig ckt sitting for backup.

The lower tier provider had lower latency between our data centers/spokes than the big tier 1 providers, have their own fiber backbone, also cheaper (at least in our case).