Large WLAN networks (for example, those found on large campuses) may require IP session roaming at layer 3 to enable application and session persistence while a mobile client roams across multiple VLANs. For example, when a user on a VoIP call roams between APs on different VLANs without layer 3 roaming, the user's session will be interrupted as the external server must re-establish communication with the client's new IP address. During this time, a VoIP call will noticeably drop for several seconds, providing a degraded user experience. In smaller networks, it may be possible to configure a flat network by placing all APs on the same VLAN.
However, on large networks filled with thousands of devices, configuring a flat architecture with a single native VLAN may be an undesirable network topology from a best practices perspective; it may also be challenging to configure legacy setups to conform to this architecture. A turnkey solution designed to enable seamless roaming across VLANs is therefore highly desirable when configuring a complex campus topology. Using Meraki's secure auto-tunneling technology, layer 3 roaming can be enabled using a mobility concentrator, allowing for bridging across multiple VLANs in a seamless and scalable fashion.
I am not a Cisco employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
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