Roaming / Mobility Basics

Mobility, or roaming, is a wireless LAN client’s ability to maintain its association seamlessly from one access point to another securely and with as little latency as possible. In This post I will try to elaborate how mobility works when controllers are included in a wireless network.


In wireless networiking , roaming means to the ability to move from one AP coverage area to another without interruption in service or loss in connectivity. This is the key component in wireless network deployment.

Mobility Groups:

A mobility group is a set of Wireless LAN Controllers, by the same mobility group name, that defines of seamless roaming for wireless clients. By creating a mobility group, we can enable multiple WLCs in a network to dynamically share information and forward data traffic when inter-controller or inter-subnet roaming occurs. Controllers in the same mobility group can share the context and state of client devices as well as their list of access points so that they do not consider each other’s access points as rogue devices. With this information, the network can support inter-controller wireless LAN roaming and controller redundancy.

Roaming is the action for a Wireless client to move from one AP to another AP while actively transmitting data without any interruption.

For Voice WLAN: It very common to have roaming.

For Data WLAN: For data deployment, it can be nomadic; user can get slight interruption while moving from one coverage area to another without impacting the user experience.

When a wireless client connects and authenticates to an AP, the AP´s controller (where AP is connected) put an entry for that client in its database, which contains the information of client like: Mac address, IP address, WLAN associate with and the AP where client is connected…etc.

If we have the controller based deployments then Roaming can be of three types.

  1. Intra(All AP on same controller)
  2. Inter or layer 2(Different controller with same network means same subnet)
  3. Layer 3 (Different controller with totally different subnets)


 Intra Controller Roaming:

Intra Controller Roaming

  • Intra controller roaming is the roaming between LAP’s managed by the same WLC, obviously in the same IP subnet.
  • It is necessarily about the subnets that the clients are using that are serviced by the APs.  So, let’s say we have 2 APs, both of which are servicing a specific IP subnet.  When client roam from one AP to the other, which is layer 2 roaming.  The client still maintains its IP address.
  • Wireless Clients move from one AP to another AP in same controller.
  • When client moves its association from one access point to another, the controller simply updates the client database with the newly associated access point.
  • This entry includes the client’s MAC and IP addresses, security context and associations, quality of service (QoS) contexts, the WLAN, and the associated AP.
  • Sometimes it does also establish the new security context means if during roaming a clients session timeout or key change occurs then this information should pass to WLC. If we have open authentication then WLC don’t need to establish or update security Context.

AP: Encryption and Decryption

WLC: Mobility, QoS and Security Management

  • This Roaming process take less then 10ms (It almost seamless).

Lab result and logs will follow soon……………………………………………..

Layer 2 – Inter Controller Roaming:

 L2 - Inter Controller Roaming

Now when we do a layer to roam and multiple controllers are involved, then both controllers need to service the same subnet.  This would mean that we have a dynamic interface in subnet x configured on WLC1 and another interface that is also configured on subnet x on WLC2.  This would be Inter controller roaming (Layer 2).

  •  The wireless user moves from one AP to another AP connected to another controller in the same subnet (as the first controller).
  • It means that the client is roaming between two different controllers, but, these controllers can be part of the same Mobility Group and the same subnet.
  • When the client tries to join the new AP, both controllers exchange the client details (database entry and credentials).
  • The new WLC exchange mobility message with the original WLC and the client entry is moved to new WLC.  This entry includes the client’s MAC and IP addresses, security context and associations, quality of service (QoS) contexts, the WLAN, and the associated AP.
  • Client database entry is updated for the new access point. This process takes less then 20ms and remains transparent to the user.

Lab result and logs will follow soon……………………………………………

Layer 3 – Inter Controller Roaming:

 L3 - Inter Controller Roaming

The wireless user moves from one AP to another AP connected to another controller in a different subnet or if the clients roam between APs registered to different controllers and the client WLAN on the two controllers is on different subnet, then it is called inter-controller L3 roam.

  • It’s similar to inter subnet roaming.
  • Controllers exchange mobility messages on the client roam. However, instead of moving the client database entry to the new controller, the original controller marks the client with an “Anchor” entry in its own client database.
  • The database entry is copied to the new controller client database and marked with a “Foreign” entry in the new controller.
  • The main advantage is clients maintain its original IP address even after changing the controller.
  • The process takes less than 30 ms.
  • The roam remains transparent to the wireless client. 

Lab result and logs will follow soon……………………………………………


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