3.3 Wireless
📘CCNP Encore (350-401-ENCORE-v1.1)
1. What Is Roaming in Wireless Networks?
Roaming means a wireless client device (such as a laptop, IP phone, barcode scanner, or tablet) moves from one Access Point (AP) to another while staying connected to the same wireless network.
- The device does not disconnect
- Applications continue running
- IP communication should remain active
Roaming is very important in enterprise IT environments, especially for:
- Voice over Wi-Fi (VoWiFi)
- Video conferencing
- Cloud applications
- Warehouse devices
- Hospital and campus networks
There are two main types of roaming:
- Layer 2 (L2) Roaming
- Layer 3 (L3) Roaming
2. OSI Layers Refresher (Very Simple)
| Layer | What It Handles |
|---|---|
| Layer 2 | MAC address, VLAN, switching |
| Layer 3 | IP address, routing |
- Layer 2 roaming → IP address does not change
- Layer 3 roaming → IP address changes or is preserved using special methods
3. Layer 2 Roaming (L2 Roaming)
3.1 What Is Layer 2 Roaming?
Layer 2 roaming occurs when:
- A wireless client moves between APs
- All APs are in the same VLAN
- The client stays in the same IP subnet
- The IP address does NOT change
This is the simplest and fastest type of roaming.
3.2 How Layer 2 Roaming Works (Step by Step)
- Client connects to AP-1
- Client receives an IP address from DHCP
- Client moves toward AP-2
- AP-2 is in the same VLAN
- Client re-associates with AP-2
- IP address remains the same
- Network sessions continue
Only wireless control frames are exchanged; no IP changes occur.
3.3 Key Characteristics of Layer 2 Roaming
- Same VLAN across all APs
- Same IP subnet
- No DHCP renewal required
- Faster roaming
- Minimal packet loss
- Best for latency-sensitive traffic
3.4 Advantages of Layer 2 Roaming
- Very fast handoff
- No IP address change
- Ideal for real-time applications
- Easy to deploy in small to medium networks
3.5 Limitations of Layer 2 Roaming
- VLAN must span across many switches
- Large Layer 2 domains increase:
- Broadcast traffic
- Risk of loops
- Troubleshooting complexity
- Not scalable for large enterprise campuses
3.6 Common Layer 2 Roaming Use Cases
- Single-building enterprise networks
- Warehouses using the same VLAN everywhere
- Small campus networks
- Voice-over-WiFi deployments in limited areas
4. Layer 3 Roaming (L3 Roaming)
4.1 What Is Layer 3 Roaming?
Layer 3 roaming occurs when:
- A wireless client moves between APs
- APs are in different VLANs
- APs are in different IP subnets
- Client changes its point of network attachment
Without special handling, this would normally break connectivity.
4.2 Why Layer 3 Roaming Is Needed
In large enterprise networks:
- Different buildings use different VLANs
- VLANs cannot span everywhere
- Routing is required between areas
Layer 3 roaming allows mobility without losing connectivity, even across subnets.
5. Problems Without Layer 3 Roaming Support
If Layer 3 roaming is not supported:
- Client moves to a new subnet
- Old IP address becomes invalid
- Active sessions break
- Applications disconnect
- Voice and video drop
To solve this, mobility mechanisms are used.
6. How Layer 3 Roaming Works (High Level)
When Layer 3 roaming is enabled:
- Client keeps its original IP address
- Traffic is tunneled back to its original network
- Wireless Controller manages routing and forwarding
This creates the illusion that the client never changed networks.
7. Cisco Layer 3 Roaming Concepts (Exam Relevant)
7.1 Anchor and Foreign Controllers
Cisco wireless uses:
- Anchor WLC
- Foreign WLC
| Term | Description |
|---|---|
| Anchor WLC | Original controller where client first connected |
| Foreign WLC | Controller the client roams to |
7.2 Layer 3 Roaming Process (Simplified)
- Client connects to AP under WLC-A (Anchor)
- Client gets an IP address
- Client moves to AP under WLC-B (Foreign)
- WLC-B detects client subnet mismatch
- Tunnel is built between WLC-B and WLC-A
- Client traffic is forwarded through the anchor
- IP address remains unchanged
7.3 Mobility Tunnel
- A mobility tunnel is created between controllers
- Traffic is encapsulated
- Uses CAPWAP / mobility protocols
- Enables session continuity
8. Key Characteristics of Layer 3 Roaming
- Different VLANs
- Different IP subnets
- Client IP preserved using tunneling
- Slightly higher latency than L2 roaming
- Requires controller coordination
9. Advantages of Layer 3 Roaming
- Highly scalable
- VLANs do not need to stretch everywhere
- Works across buildings and campuses
- Supports large enterprise designs
10. Disadvantages of Layer 3 Roaming
- More complex design
- Requires wireless controllers
- Slight increase in latency
- Tunnel overhead
11. Use Cases for Layer 3 Roaming
- Large campus networks
- Multi-building enterprises
- Hospitals with segmented VLANs
- University networks
- Secure networks with strict subnet separation
12. Comparison: Layer 2 vs Layer 3 Roaming
| Feature | Layer 2 Roaming | Layer 3 Roaming |
|---|---|---|
| VLAN | Same | Different |
| IP Address | Same | Preserved using tunneling |
| DHCP | No renewal | No renewal |
| Complexity | Low | High |
| Scalability | Limited | High |
| Latency | Very low | Slightly higher |
| Controller Dependency | Optional | Required |
13. Roaming and Application Impact (Exam Focus)
| Application Type | Roaming Requirement |
|---|---|
| Voice over Wi-Fi | Fast roaming (L2 preferred) |
| Video calls | Minimal packet loss |
| Cloud apps | IP continuity required |
| Database sessions | IP stability important |
Layer 3 roaming ensures session continuity across routed networks.
14. Fast Roaming Protocols (Awareness for Exam)
While not the main focus here, roaming performance is enhanced by:
- 802.11r (Fast BSS Transition)
- 802.11k (Neighbor lists)
- 802.11v (Client steering)
These improve handoff speed for both L2 and L3 roaming.
15. Exam Tips – What You MUST Remember
✔ Layer 2 roaming = same VLAN, same subnet
✔ Layer 3 roaming = different VLANs, different subnets
✔ L2 roaming is faster but less scalable
✔ L3 roaming uses anchor and foreign controllers
✔ IP address preservation is critical
✔ Roaming is essential for real-time applications
✔ Cisco uses mobility tunnels for L3 roaming
16. Summary
- Layer 2 roaming is simple, fast, and limited in size
- Layer 3 roaming is scalable and used in large enterprise designs
- Both aim to provide seamless connectivity
- CCNP ENCORE expects understanding of:
- Principles
- Differences
- Use cases
- Impact on applications
