Backbone OSPF area design

[quangnn87]

Hi experts,

My client has a lot of L3Out, and all are using OSPF.

I learned recently that they use a "trick" to create their backbone area:

• they configure a L3Out using area 0
• in the Logical Interface Profile of that L3Out they configure 2 vPC so each Border Leaf can receive an IP address
• physically, the ports that they configure in the vPC are connected to nothing.

As SVIs are still up even when the ports are connected to nothing, they manage to bring up a backbone without any ports connected.

I find it interesting but I cannot find any document/discussion about that design, so I'm not sure that it is considered as a valid design and if there is any hidden caveat?

Have you seen that in your network? 

Thanks.

[Wassim Aouadi]

Hello @quangnn87 ,

That has to do with the SVI Auto State feature in Cisco ACI. By default in ACI, when you configure a L3Out based on SVIs, the SVI remains up even if the member interfaces are physically down.

More details: https://www.cisco.com/c/en/us/td/docs/dcn/aci/apic/6x/l3-configuration/cisco-apic-layer-3-networking-configuration-guide-60x/node-and-interface-for-l3out-layer3-config-60x.html?bookSearch=true#d20497e2359a1635

Regarding your question of whether it is a valid design or not, it depends on what you need to achieve. If your client's ACI fabric is connected to a critical external network and the connectivity shall not go down when a single member interface goes down, then you probably should leave SVI Auto State to its default value of 'disabled'.