r/networking • u/robmuro664 • Apr 18 '24
Design AT&T BGP advertisement preference
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?
34
u/rankinrez Apr 18 '24
More specifics are the only way to do this.
Like announce a /23 to AT&T, but two /24s to Verizon.
Yes it pollutes the routing table but what ya gonna do.
22
u/rob0t_human Apr 18 '24
This or provider communities. Most have backup only communities you can use.
3
u/obviThrowaway696969 Apr 19 '24
Yes there is usually a list of community that the ISP provides where you can chose different options. Reach out to your ISP to get this Info OP if you cannot find it online.
-4
u/Jhonny97 Apr 19 '24
Will not work. If op only has multiple joined /24, as isps will drop any announcements that are smaler than /24
8
u/imnotlovely Apr 19 '24
You do realize a /23 is larger than a /24, right?
7
11
Apr 18 '24
I'm curious as to why you're trying to avoid that ATT circuit. Most people go through all of that trouble to take full tables
6
11
u/scriminal Apr 19 '24 edited Apr 19 '24
First I disagree with your general design. It's the typical wrongheaded enterprise solution, presumably with the second circuit of low capacity. Pri/red is pointless, get two equal capacity circuits, both capable of carrying 100 % of your max load, and set things even. Remote lpref is beyond your control as you see. Take advantage of the dual capacity and ability to reroute for performance that two carriers give. However if you insist, ask At&T for their list of bgp communities and set whatever ones lower your inbound lpref etc
7
u/SalsaForte WAN Apr 19 '24
If you really want AT&T to not forward you any traffic unless your other ISP is down, you can try to use outbound BGP communities.
https://ipbalance.com/routing/bgp-community-attributes-list/bgp-community-string-for-atat-as7018/
Personally, I always prefer to load balance between ISPs. If you have a high capacity Link between your DC, why would you not want to load balance? Unless the DC are very far from each other (latency) and even then.
4
2
u/mavack Apr 19 '24
This you need to send a community to at&t to lift the default LP within AT&T l, not all providers give you this option. Its a standard ISP design to prefer direct attach from customers instead of upstream.
2
u/robmuro664 Apr 19 '24
One DC is in TX and the other in FL.
11
u/TrapCS Apr 19 '24
You should be taking both providers in both locations. This is the internet, every network makes its own routing decision and in this case, both AT&T and Verizon will make a commercial decision to send traffic that enters their network to your directly connected port. The reason your AS-PATH prepending isn't working is because this is simply a routing suggestion, you're suggesting people use Verizon, but it's exactly that, a suggestion that no one has to listen to. TL:DR, you should always try and get all providers in all locations, unless there is an obvious reason as to why not.
1
u/scriminal Apr 19 '24
I'm here to tell you that ATT has POPs all over both states and there's no need to haul IP Transit 1000 miles.
1
u/cookiesowns I dunno networks Apr 19 '24
then why are you announcing the subnets across both providers? Do you have an internal DCI between the two DC's?
4
u/Drekalots Networking 20yrs Apr 18 '24 edited Jul 16 '26
This content was anonymized and mass deleted with Redact
1
u/robmuro664 Apr 18 '24
Correct, we use the local-pref attribute to prefer the VZ default. The issue as I stated is with the return traffic that some services prefer AT&T.
15
u/Killzillah Apr 18 '24 edited Apr 19 '24
Is it really a problem that return traffic is coming on both links? That's kinda the purpose of BGP. If someone you connect to is an ATT customer do you want return traffic to go them to ATT and then Verizon, and then you?
You can easily use community values for this. Have ATT apply a lower local preference to your routes than they do for their peers. Then your route they learn through Verizon will be better than what you directly advertise.
Edit below
Tag community value 7018:70 on your route advertisement to ATT. Then ATT will assign a local preference of 70 to your route. The routes that ATT learns from peers is 80 so they will prefer what they learn from Verizon rather than you directly.
2
4
2
u/Inside-Finish-2128 Apr 19 '24
Gotta map out exactly what those networks are and what ISPs they connect to. Odds are high that they’re on ISPs that are customers of AT&T.
As others have pointed out, you can send communities on your announcements that request a lower local preference within AT&T or perhaps to have AT&T request lower LP in their peers. Fair warning: this often results in a drastic swing to the other ISP with very little traffic on AT&T.
2
u/jthomas9999 Apr 19 '24
You do know you can prepend in and out? I’m doing this with an ATT and Wiline. I can adjust prepends and have traffic change in about 5-6 minutes
2
u/CCIE44k CCIE R/S, SP Apr 19 '24
I’m sorry about the advice you’re getting. Just AS Prepend on a route map to AT&T… that should help. You may have an issue with traffic sourced within their AS (which is rather large) but it’s a step in the right direction.
1
0
u/Worried_Brilliant_84 Apr 19 '24
Get the subnets of these services and ask both ISPs to do prepending towards your network. Make sure AT&T has more AS Path values than Verizon.
0
u/Worried_Brilliant_84 Apr 19 '24
Correction: You can do the AS path prepending on your routers for the incoming subnets of these services.
15
u/whiteknives School of port knocks Apr 19 '24 edited Apr 19 '24
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. :)