With Layer 3 roaming, the user is roaming from an AP on subnet A to an AP on subnet B. As a result, the Layer 3 network address must change to maintain Layer 3 connectivity on subnet B. Aruba Layer 3 Mobility allows the highly mobile device (HMD) client to maintain the same IP address even though it is roaming to a different subnet.
When a client first connects to an Instant network, a message is sent to all configured Virtual Controller IP addresses to see if this is an L3 roamed client. On receiving an acknowledgement from any of the configured Virtual Controller IP addresses, the client is identified as an L3 roamed client. If the AP has no GRE tunnel to this home network, a new tunnel is formed to an AP (home AP) from the client's home network.
Each foreign AP has only one home AP per Instant network to avoid duplication of broadcast traffic.
Separate GRE tunnels are created for each foreign AP / home AP pair. If a peer AP is a foreign AP for one client and a home AP for another, two separate GRE tunnels are used to handle L3 roaming traffic between these APs.
If client subnet discovery fails on association due to some reason, the foreign AP identifies its subnet when it sends out the first L3 packet. If the subnet is not a local subnet and belongs to another Instant network, the client is treated as an L3 roamed client and all its traffic is forwarded to the home network via a GRE tunnel.