CWDP CWDP Roaming and Client Mobility 2 — Questions and Answers
Question 1: What is 'roam thrashing' and how can a CWDP designer mitigate it?
- Excessive AP reboots during peak traffic; resolved by firmware updates
- Clients rapidly roaming between APs of similar signal strength; mitigated by hysteresis thresholds (Correct answer)
- Controller overload during mass roaming events; mitigated by load balancing
- Duplicate IP assignment during roam; mitigated by DHCP snooping
Correct answer: Clients rapidly roaming between APs of similar signal strength; mitigated by hysteresis thresholds
Roam thrashing occurs when clients oscillate between APs; RSSI hysteresis thresholds prevent roaming unless a meaningful improvement is detected.
Question 2: In a Layer 3 roaming scenario, what mechanism preserves a client's IP address when crossing subnet boundaries?
- DHCP relay
- Mobility tunneling (e.g., GRE tunnel back to anchor AP) (Correct answer)
- 802.11r key caching
- VLAN trunking
Correct answer: Mobility tunneling (e.g., GRE tunnel back to anchor AP)
Layer 3 roaming solutions use tunneling (GRE or proprietary) to forward client traffic back through the anchor AP, preserving the original IP.
Question 3: What is the Pairwise Master Key (PMK) caching mechanism (OKC/PKC) designed to accomplish during roaming?
- Speed up IP address assignment at the new AP
- Avoid full 802.1X re-authentication by caching session keys (Correct answer)
- Increase transmit power during the roam transition
- Enable cross-SSID roaming between different WLANs
Correct answer: Avoid full 802.1X re-authentication by caching session keys
Opportunistic Key Caching (OKC) allows clients to reuse a previously derived PMK with nearby APs, skipping the full EAP exchange.
Question 4: A hospital deploys VoWi-Fi phones. Nurse stations are 30 meters apart. What AP density consideration ensures seamless voice roaming?
- APs only at nurse stations with no overlap
- Cell coverage overlap of 15–20% at -67 dBm or better (Correct answer)
- Single AP per floor with high-gain directional antenna
- Roaming disabled to keep phones on one AP
Correct answer: Cell coverage overlap of 15–20% at -67 dBm or better
Overlapping coverage at -67 dBm ensures VoWi-Fi phones always have a strong candidate AP available before initiating a roam.
Question 5: Which metric does 802.11k neighbor reports NOT include about neighboring APs?
- BSSID
- Operating channel
- RSSI seen by the reporting AP
- Current client count on the neighbor AP (Correct answer)
Correct answer: Current client count on the neighbor AP
802.11k neighbor reports include BSSID, channel, and regulatory information but do not report the load or client count of neighbors.
Question 6: What is the key advantage of 802.11r Over-the-DS (Distribution System) fast transition versus Over-the-Air?
- It requires no client-side 802.11r support
- It allows key exchange with the target AP before the client reassociates (Correct answer)
- It increases transmit power during roam
- It works without a WLAN controller
Correct answer: It allows key exchange with the target AP before the client reassociates
Over-the-DS FT pre-stages authentication keys with the target AP via the wired network before the client completes the air-interface roam.
CWDP CWDP Roaming and Client Mobility 2
What is 'roam thrashing' and how can a CWDP designer mitigate it?