CCNA Wireless Summary Notes: Understanding Roaming

Roaming with Autonomous APs

  • Moving from one autonomous AP  to another autonomous AP

Intracontroller Roaming:

  • Moving from one lightweight AP that is connected to a WLC to another AP that is connected to the same WLC.
  • Controller updates its client association table
  • Takes less than 10 ms
  • Client has no knowledge of what is happening.
  • Processes that occur:
    • Client reassociation
    • May get new DHCP lease or renew his
    • Client authentication (this may slow the process down)
  • Fast roaming techniques
    • Cisco Centralized Key Management (CCKM) – WLC has the databse of the clients and the keys on behalf of the APs and can provide to other WLCs and APs when clients move. Cisco Compatibility Extensions CCX support is required from the clients
    • Proactive key caching (PKC) or sticky pairwise master key ID caching (SKC) – Clients have a list of the keys they used before. The key for the destination AP should be there. Max of 8 AP-key entries.
    • 802.11r – client can cache a portion of the authentication server’s key and present that to future APs as it roams

Intercontroller Roaming

Moving from one lightweight AP that is connected to a WLC to another AP that is connected to a different WLC.

  • Layer 2 Roaming (Local to Local)
    • Client roams but stays in same VLAN and subnet
    • Client can keep its IP address
    • Fast roaming (<20ms)
  • Layer 3 Roaming (Local to Foreign)
    • Client changes subnet
    • Avoid DHCP to save time.
    • WLCs compare the clients VLAN IDs, if same, then L2 roaming will occur, otherwise L3 roaming.
    • Tunnel is built between original WLC (anchor) and foreign WLC (foreign). Traditionally it was Ethernet over IP (EoIP) tunnels. For the new codes, they create CAPWAP tunnels . Tunnel connects the client to its original controller no matter the location.
    • Anchor and foreign controllers are determined automatically. For like the guest WLAN, you can have 1 WLC as a static anchor so all the rest of the WLCs send the traffic to it via l3 tunnels.

Mobility Groups

If WLCs are in the same static groups, clients can roam between them. If in different mobility groups, they can still roam but inefficiently.

Controllers have a list of the MAC addresses of its own and others in same group. Each controller also has a mobility group name. List can have max of 72 controllers with 24 in each group.

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