Auto anchor, also known as Mobility Anchor, is a feature specifically designed to ensure that a client's traffic is tunneled to a pre-designated anchor Wireless LAN Controller (WLC). When a client roams to a new WLC (the foreign controller), even one in a different mobility group, the auto-anchor configuration on the WLAN forces the client's session to be anchored back to the designated WLC. This ensures the client maintains its original IP address and that all its traffic egresses from a consistent point on the network, which is essential for seamless Layer 3 roaming across disparate network segments or mobility domains.

A. interface groups: This feature is used for VLAN pooling and load balancing of clients across multiple interfaces on a single WLC, not for managing inter-controller roaming.

B. RF groups: This feature is used for Radio Resource Management (RRM) to dynamically manage channel and power settings among WLCs; it is unrelated to client data tunneling or IP address retention during a roam.

C. AAA override: This allows a RADIUS server to dynamically assign attributes like VLANs or ACLs to a client upon authentication but does not provide the tunneling mechanism needed for an inter-controller Layer 3 roam.

1. Cisco Enterprise Mobility 8.5 Design Guide:

Section: Mobility Architecture > Inter-Controller Mobility > Mobility Anchor

Content: "Mobility anchor (or Auto-anchor) is a feature where a WLAN is configured to tunnel client traffic to a specific controller or set of controllers... This feature is used to provide a single point of presence for a client on the network." This guide explains that auto-anchoring is the mechanism for this type of forced tunneling

which is a common requirement for guest access and roaming between different subnets or administrative domains.

2. Cisco Wireless LAN Controller Configuration Guide

Release 8.5:

Chapter: Configuring Mobility Groups > Mobility Anchor

Content: "You can use the mobility anchor feature to restrict a WLAN to a single subnet

regardless of the entry point of a client into the network... When a client associates to a WLAN with a mobility anchor

the client is sent to the anchor controller

and the client traffic is tunneled from the foreign controller to the anchor controller." This explicitly states the feature's purpose is to maintain the client on a single subnet via tunneling

which is the core requirement of the question.