Multipod L3Out Preference

[dm2020]

Hi All,

I'm currently deploying ACI Multipod with two pods.

I've configured a single VRF and single BGP L3Out that connects the leaf switches within each pod to our core network. This L3Out is then assigned to each BD which results in the the associated BD subnet being advertised out of both pods. This is all working as expected.

We have a requirement to start advertising BD subnets out of only one pod and I'm trying to determine the best way to achieve this. For example, I need to advertise BD subnet 10.1.1.0/24 out of only pod1. So far I have tested the following:

1) Used route profiles on the single L3Out to match prefix 10.1.1.0/24 and either drop or apply AS path prepending when advertised out to the BGP neighbors in pod2

2) Configured two L3Outs, one for each pod, and assign the BD with subnet 10.1.1.0/.24 to the L3Out associated to pod1.

Both work ok however I dont know which is the suggested/recommended approach. Are there any issues/gotchas with either of these or is there a better way to achieve this?

[Remi-Astruc]

Hi @dm2020 ,

Using AS path prepend is the best way to go, and gives you redundancy.

Making 2 L3outs would give you headache when it comes to External EPG design.

[dm2020]

Hi @Remi-Astruc 

Thanks for taking the time to reply.

Taking this a step further, is it possible to influence what Pod/L3out is to used for egress path selection on a per BD basis? For example, BD1 with subnet 10.1.1.0/24 uses the L3Out in Pod1 for ingress and egress routing, and BD2 with subnet 10.1.2.0/24 uses the L3Out in Pod2 for ingress and egress routing?

I'm aware that I can use using import route control and manipulating BGP attributes to select outbound path selection via a specific L3Out, however this would impact all BDs associated to the VRF. Is it possible to do this on a per BD basis? Would PBR achieve this?

Thanks

[Remi-Astruc]

Hi @dm2020 ,

There is a trick for doing that with PBR but I would not recommend that unless very specific need.

Maybe re-think your real requirements. For example, if you need the behavior difference you describe, what about splitting BDs in 2 different VRFs, then you can tweak the outgoing path for each.

Hope this helps