Cisco ACI - Forwarding L3 Inside and Outside connection

First post of the several ones to follow where I will attempt to decipher the inner workings of the packet forwarding in Application Centric Infrastructure (ACI).

To the uninitiated, of the other tables that a leaf switch maintains, we will be focussing on the "Global Station table"

If a leaf receives a packet with a destination IP, the host route (/32) for which already exists in its global station table, it would imply that the leaf switch is aware of that IP's existence in the ACI fabric and the it knows where to forward the packet.

If the leaf receives a packet with a destination IP, the host route (/32) for which doesn't exist in its global station table, it would check if the IP belongs to the IP address range of the tenant.

If the IP address range exists in the ACI fabric but the leaf doesn't know how to reach the destination IP (obviously since it doesn't have the entry in its global station table), it would encapsulate the packet with the VXLAN header - the spine proxy IP as the destination IP of the outer VXLAN header. The spine proxy checks the inner destination IP and checks its proxy database to find the destination leaf and then forwards it.

If the destination IP is outside the fabric it would check the external routing table to match the routes. The external routing table provides the VTEP address of the border leaf where the packet is ultimately forwarded.

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Tejas Jain

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