The Layer 2 Roaming Process

As previously discussed, Layer 2 roaming happens when a user moves to another AP but stays on the same VLAN and the same IP subnet. As far as the user is concerned, nothing special has happened. The client isn't notified that he is roaming. He also keeps his IP address, and all active transmissions stay active. This process is handled within a single controller. This process is called intracontroller roaming and takes less than 10 ms. Behind the scenes, the client, when roaming to a new AP, sends a query to request authentication. The query is sent from the AP to the controller, where the controller realizes that the client is already authenticated, just via another AP. The client is then registered as roaming in the controller, although you do not see this in the controller or in the WCS, and life goes on. Figure 12-7 depicts this scenario.

Guest_Access

Figure 12-7 Intracontroller Roaming

Guest_Access

Figure 12-7 Intracontroller Roaming

Now take that same scenario and add another controller, as shown in Figure 12-8. Here, the client associated with Controllerl is on VLAN10. Upon roaming to AP3, which is managed by Controller2, the connection stays active. What happened? In this situation, intercontroller roaming happened. This occurs when a user roams from one controller to another but remains on the same VLAN and does not have to perform a DHCP process again, which would force the session to break. The two controllers are configured with the same mobility group. The two controllers then exchange mobility messages. Using mobility messages, the client database entry on Controllerl is moved to Controller2. This happens in less than 20 ms. Again, the process is transparent to the user. He roams, data keeps flowing, sessions stay active, and life is good.

Key Topic

VLAN_10 "Internal"

"Internal" Roam:— Client

Controlled

"Internal" Roam:— Client

Figure 12-8 Intercontroller Roaming

Both intracontroller roaming and intercontroller roaming allow the user to roam and remain on the same IP subnet. This is Layer 2 roaming. Now let's explore Layer 3 roaming.

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