How is Layer 3 roaming accomplished in a unified wireless deployment?

Layer 3 roaming occurs when a wireless client moves between Access Points (APs) managed by different Wireless LAN Controllers (WLCs) that reside on different IP subnets. To maintain the client's original IP address and active sessions, the controllers establish a specific relationship. The client's original controller becomes the anchor controller, and the new controller it roams to becomes the foreign controller. The client's database entry on the original WLC is updated to reflect its anchor role, while the new WLC creates a foreign entry for the client. This anchor-foreign relationship facilitates the tunneling of the client's traffic from the foreign controller back to the anchor controller, allowing the client to egress onto its original subnet.

A. An EoIP (or CAPWAP data) tunnel is created between the foreign controller and the anchor controller, not between the client and the anchor controller. The client is unaware of this underlying tunnel.

B. The client entry is not simply "passed." The original controller maintains the client's entry, marking it as an anchor, to continue servicing the client's traffic from its original subnet.

C. Assigning a new IP address from the new subnet would break the client's existing sessions. The primary purpose of Layer 3 roaming is to prevent this by preserving the original IP address.

1. Cisco

Enterprise Mobility 8.5 Design Guide

"Mobility Fundamentals" chapter

"Layer 3 Roaming" section.

"When the client associates to an AP joined to a new controller

the new controller exchanges mobility messages with the original controller

and the client database entry is moved to the new controller. The new controller now sets up the client with a “foreign” role. The original controller is marked as the “anchor” for this client." This directly supports the anchor/foreign entry description in the correct answer.

2. Cisco

Cisco Wireless LAN Controller Configuration Guide

Release 8.10

"Configuring Mobility Groups" chapter

"Information About Inter-Controller Roaming" section.

"In a Layer 3 roam

the client keeps its original IP address. The client's data is tunneled from the foreign WLC to the anchor WLC." This confirms that the goal is to keep the IP address (making option C incorrect) and that the mechanism involves the two distinct controller roles.

3. Cisco

Unified Wireless Network (UWN) Solution Design Guide

"Mobility" chapter

"Inter-Controller Mobility" section.

"The WLC to which the client is currently associated is known as the foreign WLC. The original WLC is known as the anchor WLC... The anchor WLC maintains the original client entry in its client database." This reference explicitly defines the anchor and foreign roles and confirms that the original controller maintains the client entry

invalidating option B.