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New BGP ACI L3Out Configuration Requirement Version 4.2(2f)

zachartl
Level 1
Level 1

Hello,

We've recently received a new BGP L3Out Requirement. We're going to need to create four eBGP connections from our two border leafs. We've already One L3Out configured for OSPF. We went to enable BGP within that L3Out only to be warned that if we enabled BGP, our OSPF routes would no longer be propagated from that L3Out. We obviously can't abide that. I've been under the impression for some time that we should only be using a single L3Out and the creation of other L3Outs may adversely affect performance of the border leafs and to some extent, the entire fabric. 

 

Will it be okay for us to create a second L3Out for BGP and will this adversely impact performance?

 

Thank you in advance,

Terry

3 Replies 3

Claudia de Luna
Spotlight
Spotlight

Hi @zachartl 

 

I'm trying to think what could have given you the impression that you could only have one L3Out or that multiple L3Outs would impact performance.   I"ve been deploying ACI for a number of years and I've actually never heard that.   Almost every one of my ACI deployments involve multiple L3Outs and I've never seen a performance issue.    I do always recommend summarizing  or just advertising default into the fabric if that is possible but not because of any performance concerns. Thats just sound practice.

 

Remember that these are line rate switches.   Unless you are trying to take in the full internet routing table I don't think I'd be worried about performance in general.   

Hello Claudia,

Perhaps I've had a misconception. In my initial correspondence I forgot to state this was a single VRF deployment. I thought I had interpreted that information from a best practices guide regarding L3Outs. Although this is an eBGP connection requirement, we're not planning for anything near a full BGP routing table. Thank you.

Hi @zachartl 

When it comes to combining BGP and OSPF in the same L3Out there are some limitations. Basically, L3Out will consider OSPF as the method of IGP for creating BGP peering IP reachability. If you need OSPF for advertising BD subnets or for transit routing, you absolutely need to configure it in different L3Outs. NOTE: if you want to still use the OSPF for BGP peering, you can do that, but you must keep the same configuration and parameters (like SVI configuration etc).

I hope I did not created more confusion, so before deploying the BGP L3Outs, I would recommend you read the L3out whitepaper: https://www.cisco.com/c/en/us/solutions/collateral/data-center-virtualization/application-centric-infrastructure/guide-c07-743150.html#L3OutBGP 

You will find a lot of details about L3Out configuration, including best practices, supported deployments, limitations and so forth.

 

Best regards,

Sergiu

 

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