AP Join Process

IP Address Allocation for an AP

An AP can obtain an IP address in static, DHCP, or stateless address autoconfiguration (SLAAC) mode.

• Log in to the AP and configure a static IP address for it.
• Configure a DHCP server so that the AP serves as a DHCP client and requests an IP address from the DHCP server. DHCP is the most common method for APs to obtain IP addresses.
  Figure 19-12 Message exchange between the AP and DHCP server for IP address allocation
  

  The following figure shows an example of the exchanged messages.

  1. The AP broadcasts a DHCP Discover message carrying its own MAC address, requested parameters, and broadcast flag bit.
  2. A DHCP server selects an address pool on the same network segment as the IP address of the interface receiving the DHCP Discover message, and from the address pool selects an idle IP address. Then the DHCP server sends a DHCP Offer message carrying the allocated IP address to the AP.
  3. The AP broadcasts a DHCP Request message to notify all the DHCP servers that it has selected the IP address offered by a DHCP server. Then the other servers can allocate IP addresses to other clients.
  4. After receiving the DHCP Request packet, the DHCP server replies with a DHCP ACK message, indicating that the IP address carried in the DHCP Request message is allocated to the AP.
• SLAAC mode: The AP obtains an IP address in SLAAC mode, which supports only IPv6.

  In SLAAC mode, the prefix of a network address is obtained from a Router Advertisement (RA) message, and then an interface ID is automatically generated. The prefix and the generated interface ID form an IPv6 address.

  The following figure shows an example of the exchanged messages.

The following table lists common problems that may occur when an AP attempts to obtain an IP address:

AC Discovery

Figure 19-13 shows message exchange in the AC discovery phase.

Figure 19-13 Message exchange in the AC discovery phase

The following figures show examples of the exchanged messages:

Discovery (An AP discovers an AC.)

DTLS connect (The AP establishes a DTLS connection with the AC.)

The AC discovery mechanism allows an AP to discover available ACs and selects an optimal one to set up a CAPWAP link.

1. After obtaining an IP address, the AP sends a Discovery Request message carrying its own version and mode (Fit or Fat) to discover available ACs on the network. After the AP initiates the AC discovery process, the CAPWAP state of the AP changes from Idle to Discovery.
2. After receiving the Discovery Request message, the AC determines whether to allow the AP to access the AC based on the configured IP version, AP blacklist and whitelist, AP authentication mode (MAC address authentication, SN authentication, or no authentication), and license resource restrictions, and records the determination result. If AP access is permitted, the AC unicasts a Discovery Response message carrying the AC name, AC version, CAPWAP source address, and DTLS status to the AP. If AP access is denied, the AC does not respond with a Discovery Response message.
3. If the AP receives Discovery Response messages from multiple ACs, it selects an AC based on AC priorities and loads (number of APs connected to the AC).

The AP then needs to obtain the AC's IP address in static or dynamic mode.

On a Layer 2 network, the AP can discover the AC in broadcast mode, without the need to manually specify the AC's IP address. On a Layer 3 network, you must specify the AC's IP address; otherwise, the AP cannot discover the AC in broadcast mode.

If no AC's IP address is specified, the AP broadcasts a Discovery Request message to discover an AC. If the AC's IP address is specified, the AP unicasts a Discovery Request message to the specified AC.

• Static mode: The IP addresses of ACs are specified on the AP.
• Dynamic mode: The AP can dynamically obtain the AC's IP address in DHCP or DNS mode. When the AP obtains an IP address from the DHCP server, the DHCP server embeds an option carrying the AC's IP address (DHCP) or domain name (DNS) in the DHCP Response message destined for the AP. The options are described as follows:
  • Option 43: carries the list of ACs' IPv4 addresses.
  • Option 52: carries the list of ACs' IPv6 addresses.
  • Option 15: carries the ACs' IPv4 domain names.
  • Option 24: carries the ACs' IPv6 domain names.

  The following figure shows an example of a DHCP Response message carrying Option 43.

If DTLS is enabled on the AC, the AC informs the AP of the DTLS status through the Discovery Response message. The AP then starts DTLS negotiation to establish a DTLS connection with the AC. After the DTLS connection is set up, packets transmitted between the AP and AC will be encrypted. CAPWAP data packets and control packets transmitted over CAPWAP tunnels can be DTLS-encrypted separately.

The following table lists common problems that may occur in this phase.

AP Access Control

Figure 19-14 shows message exchange in the AP access control phase.

Figure 19-14 Message exchange in the AP access control phase

The following figure shows an example of the exchanged messages.

1. After a DTLS connection is set up between the AP and AC, the AP sends a Join Request message to the AC selected in the previous phase to apply for joining the AC. The AP then enters the Join state.
2. The AC determines whether to allow the AP access. If this operation has been performed in the Discovery phase, the AC directly uses the determination result buffered in that phase, without the need for repeated determination. If no result is buffered, the AC goes through the process shown in Figure 19-15 to determine whether to allow the AP access. Based on the determination result, the AC sends a Join Response message containing the expected AP version to the AP.
   Figure 19-15 AP access control process
   
3. After receiving the Join Response message, the AP checks whether the current system software version is the same as that expected by the AC. If not, the AP enters the Image data state and starts to download the upgrade file and upgrade its software version in AC, FTP, or SFTP mode. After the upgrade is complete, the AP restarts and repeats the preceding phases. If the upgrade fails, the AP restarts and repeats the previous phases. Therefore, if the AP upgrade configuration is incorrect, the AP may continuously repeat the preceding phases until the AP software version is upgraded correctly.

The following table lists common problems that may occur in this phase.

AC Configuration Delivery

When the AP version is consistent with the AC version, the AC starts to deliver configurations to the AP.

Figure 19-16 shows message exchange in the AC configuration delivery phase.

Figure 19-16 Message exchange process in the AC configuration delivery phase

The following figure shows an example of the exchanged messages.

Configure

Data check

1. After receiving a Join Response message from the AC, the AP checks whether the AC allows its access and whether the running software version is the same as the expected one. If so, the AP sends a Configuration Status Request message containing multiple Radio Administrative State message elements to the AC to report its current configuration and then enters the Configure state.

2. After receiving the Configuration Status Request message, the AC sends a Configuration Status Response message to the AP and delivers the initialization configuration to the AP. In this phase, the AC does not deliver service configurations. Instead, it delivers service configurations only after the CAPWAP link is established.
3. After receiving the Configuration Status Response message, the AP enters the data check state and performs the initialization configuration based on the message content.
4. After the initialization configuration is complete, the AP sends a Change State Event Request message carrying the radio status and configuration execution result to the AC.

5. After receiving the Change State Event Request message, the AC sends a Change State Event Response message to the AP and updates AP information as required.

The following table lists common problems that may occur in this phase.

CAPWAP Tunnel Maintenance

Figure 19-16 shows message exchange in the CAPWAP tunnel maintenance phase.

Figure 19-17 Message exchange in the CAPWAP tunnel maintenance phase

The following figure shows an example of the exchanged messages.

After the preceding phases, the AP has gone online on the AC. Then we need to maintain the CAPWAP tunnel between the AC and AP.

The AP and AC detect the connectivity of the CAPWAP data tunnel by exchanging Keepalive messages and detect the connectivity of the CAPWAP control tunnel by exchanging Echo messages.

The AP starts a timer to send Keepalive and Echo messages and starts a tunnel detection timeout timer. If the AP receives Keepalive and Echo messages within a specified period, it resets the timeout timer; otherwise, it determines a message timeout.

Configuration Update

Figure 19-16 shows message exchange in the configuration update phase.

Figure 19-18 Message exchange process in the configuration update phase

1. After the AP goes online on the AC, the AC sends a Configuration Update Request message to the AP to deliver configurations.
2. After receiving the Configuration Update Request message, the AP changes from the Run state to the Config state to complete the configuration delivery.
3. After all configurations delivered by the AC are received, the AP sends a Configuration Update Response message to the AC, notifying the AC of the configuration delivery result.

The following table lists common problems that may occur in this phase.