3.3 Wireless
📘CCNP Encore (350-401-ENCORE-v1.1)
1. Overview of AP Discovery and Join Process
In a Cisco wireless network, Access Points (APs) do not work alone.
They must discover, connect to, and register with a Wireless LAN Controller (WLC).
The full process has two main stages:
- AP Discovery – How the AP finds one or more WLCs
- AP Join – How the AP selects a WLC and registers itself
This process is automatic once basic IP connectivity exists.
2. Prerequisites for AP Discovery
Before an AP can discover a WLC, these conditions must be met:
- AP has power (PoE or external)
- AP has IP address
- Either statically configured
- Or received from DHCP
- AP can reach the network (Layer 2 or Layer 3 connectivity)
Once the AP boots and gets an IP address, it starts the discovery phase.
3. AP Discovery Methods (Discovery Algorithms)
Cisco APs use multiple discovery methods, tried in a specific order.
This order is very important for the exam.
Discovery Order (High Priority to Low Priority)
- Previously Joined WLC (Primary)
- DHCP Option 43
- DNS (cisco-capwap-controller)
- Layer 2 Broadcast
- Layer 3 Broadcast (Limited)
3.1 Previously Joined WLC (Highest Priority)
If the AP has joined a WLC before, it remembers:
- WLC IP address
- Controller name
When the AP reboots:
- It first tries to contact the last joined WLC
- Uses CAPWAP protocol (UDP ports 5246 and 5247)
Why this exists
- Ensures AP reconnects quickly after reboot
- Common in enterprise networks
📌 Exam tip:
This method always has highest priority.
3.2 DHCP Option 43
If no previous WLC is reachable, the AP checks DHCP Option 43.
What is DHCP Option 43?
- A special DHCP option
- Provides WLC IP address(es) to the AP
How it works
- AP sends DHCP request
- DHCP server replies with:
- IP address
- Gateway
- Option 43 → WLC IP(s)
Why this is important
- Used in large Layer 3 networks
- Common when APs and WLCs are in different subnets
📌 Exam tip
- Option 43 is vendor-specific
- Used mainly for centralized controllers
3.3 DNS Discovery
If Option 43 is not configured, the AP tries DNS discovery.
DNS entry used
cisco-capwap-controller
How it works
- AP queries DNS
- DNS returns IP address of WLC
- AP tries to connect using CAPWAP
Important points
- DNS name must exist
- DNS server must be reachable
- Works across Layer 3 networks
📌 Exam tip
- DNS name is fixed
- Case-insensitive
- Very commonly tested
3.4 Layer 2 Broadcast Discovery
If DNS fails, AP tries Layer 2 broadcast.
How it works
- AP sends CAPWAP discovery broadcast
- Only works if:
- AP and WLC are in same VLAN
- Broadcast traffic is allowed
Limitations
- Does not work across routers
- Rare in large enterprise designs
📌 Exam tip
- Layer 2 only
- Same subnet required
3.5 Layer 3 Broadcast (Rare / Limited)
Some older deployments allow limited Layer 3 broadcast using helper configurations, but:
- Not common
- Not scalable
- Not recommended
📌 Exam tip
- Mentioned rarely
- Not preferred in modern designs
4. CAPWAP Discovery Process
Once a discovery method finds a WLC, this happens:
- AP sends CAPWAP Discovery Request
- WLC replies with CAPWAP Discovery Response
- Response includes:
- WLC name
- WLC IP
- Number of APs currently joined
- Controller priority
- Controller capacity
The AP may receive multiple responses from different WLCs.
5. WLC Selection Process
After receiving responses, the AP must choose one WLC.
This selection is based on specific criteria.
5.1 WLC Selection Criteria (Order Matters)
- Primary / Secondary / Tertiary Controller
- Controller Priority
- AP Capacity
- Lowest Load
- Tie-Breaker (Lowest IP)
5.2 Primary, Secondary, and Tertiary WLC
Each AP can be configured with:
- Primary WLC
- Secondary WLC
- Tertiary WLC
Behavior
- AP always tries Primary first
- If Primary is unavailable → Secondary
- If Secondary unavailable → Tertiary
📌 Exam tip
- These are AP-specific settings
- Used for redundancy and failover
5.3 Controller Priority
Each WLC has a priority value:
- Higher value = more preferred
- Range depends on platform
Used when:
- AP receives responses from multiple WLCs
- No Primary/Secondary is defined
📌 Exam tip
- Priority influences AP load distribution
5.4 AP Capacity and Load
The AP checks:
- How many APs are already joined
- Maximum supported APs per controller
The AP prefers:
- WLC with more available capacity
- Lower load
📌 Exam tip
- Helps prevent controller overload
5.5 Tie-Breaker (Lowest IP Address)
If everything is equal:
- AP chooses WLC with lowest IP address
📌 Exam tip
- Simple but commonly tested fact
6. AP Join Process (Registration)
Once a WLC is selected, the join phase begins.
6.1 Join Request
- AP sends CAPWAP Join Request
- Includes:
- AP model
- MAC address
- Software version
6.2 Authentication and Authorization
WLC checks:
- Is AP allowed to join?
- Is AP in the AP authorization list?
- Is AP model supported?
6.3 Image Download (If Needed)
If AP software version does not match WLC:
- WLC pushes new image to AP
- AP reboots
- Discovery and join process restarts
📌 Exam tip
- Software mismatch causes automatic upgrade
6.4 Final Registration
Once software matches:
- AP joins successfully
- AP status becomes Registered
- WLC pushes:
- Configuration
- SSIDs
- RF settings
- Security policies
AP is now operational.
7. CAPWAP Ports (Exam Critical)
CAPWAP uses UDP, not TCP.
| Purpose | Port |
|---|---|
| Control | UDP 5246 |
| Data | UDP 5247 |
📌 Exam tip
- These ports must be allowed through firewalls
8. Summary for Exam Preparation
You MUST remember:
- Discovery order:
- Previously joined WLC
- DHCP Option 43
- DNS (cisco-capwap-controller)
- Layer 2 broadcast
- CAPWAP protocol and ports
- WLC selection logic
- Primary/Secondary/Tertiary controllers
- Software image download behavior
9. Why This Topic Is Important in Real IT Environments
- Ensures scalable wireless deployments
- Enables centralized management
- Allows automatic AP provisioning
- Supports high availability and redundancy
