Understanding "Invalid FTE" Error with 802.11r Roaming

 When investigating Fast Transition (FT) roaming issues in enterprise wireless networks, packet captures often hold the key to diagnosis. Let's analyze this specific roaming failure case where a client fails to roam from one access point to another.


Problem Statement: Random Devices are facing FT roaming issue and requires reauthentication. 

While the problem statement sounds easy, capturing roaming failures could be challenging when the problem is random/intermittent. It is also important to know where to capture; Remember CWAP Guidelines? For roaming we need to capture near multiple AP's.  In this case we used RUCKUS AP Remote PCAP feature to stream captures from multiple AP's to the Wireshark and Voila! We were lucky to capture the failure  use case. 

Now lets see what does it look like:


The packet captures shows us an "Invalid FTE (0x0037)" error. FTE stands for Fast Transition Element, a critical component of the 802.11r protocol that facilitates seamless client roaming between Access Points within the same mobility domain. But, why "Invalid FTE"? The answer to that is in the Authentication Frame for this transaction. So lets look at the Authentication Frame:



The PMK-R0 Key Holder Identifier (R0KH-ID) shows the Hex equivalent of the AP's MAC which holds the PMK-R0 for the respective Station. However, upon looking at the PMK Table of that AP, I noticed no matching PMK for that station. One of the common reason for an "Invalid FTE" error, is a missing/ expired or deleted PMK-R0 (Pairwise Master Key R0) for the station on the first AP where the client initially associated. This key is essential for the 802.11r Fast BSS Transition process.

To understand why this causes a failure, we need to examine the 802.11r key hierarchy. 

  • PMK-R0: The root key generated when a client first associates with an AP in the mobility domain. This key is bound to the:
    • Client's MAC address
    • R0KH-ID (PMK-R0 Key Holder Identifier) of the initial AP
    • MDID (Mobility Domain Identifier)
  • PMK-R1: Derived from PMK-R0 and distributed to other APs in the mobility domain for FT roaming.

    • When the PMK-R0 is missing on the initial AP, the target AP cannot validate the FT Authentication elements presented by the client during roaming. 

    Potential Resolutions:

    To fix this type of FT roaming issue:

    1. Ensure the initial AP hasn't purged the PMK-R0 due to PMK cache timeout.
    2. Verify all APs in the roaming path share the same mobility domain ID and have consistent R0KH/R1KH configurations.
    3. Ensure the Initial AP is properly distributing the PMK-R1 keys to potential target APs.
    Note: You would need a Support Expert to validate the PMK distribution and records as this information is not exposed to the basic command line interface. 

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