The Purpose of the Forwarding Tag (FTAG) in Cisco ACI

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The Forwarding Tag (FTAG) is an important concept in the Cisco Application Centric Infrastructure (ACI) that serves a specific purpose in the fabric. FTAG is used to help identify and forward traffic in the ACI fabric.

FTAG is a label that is attached to the VXLAN traffic flowing through the fabric. This label is used to help identify the traffic and ensure that it is properly routed based on the configured policies. In particular, FTAG is used in ACI to associate traffic with the appropriate Endpoint Group (EPG) and to apply the appropriate contracts and policies to the traffic.

When traffic enters the ACI fabric, it is encapsulated in VXLAN and assigned an FTAG. The FTAG is then used to determine the correct forwarding path for the traffic through the fabric. This forwarding path is based on the policies defined for the EPG associated with the traffic. For example, if a particular EPG is allowed to communicate with another EPG, the appropriate contracts and policies are applied to the traffic to allow it to flow between those EPGs.

FTAG trees are another important component of the ACI fabric. FTAG trees are used to load balance unicast traffic and multicast traffic. These trees ensure that traffic is evenly distributed across multiple available paths in the fabric, helping to optimize the use of available resources and minimize congestion.

In summary, the purpose of the Forwarding Tag (FTAG) in Cisco ACI is to help identify and properly route traffic through the fabric based on the configured policies. FTAG is used to associate traffic with the appropriate EPG and apply the appropriate contracts and policies to the traffic. FTAG trees are used to load balance traffic across multiple available paths in the fabric, helping to optimize resource utilization and minimize congestion.