AC6605 Access Controller
Dual-Link Backup White Paper
1
Dual-Link Backup
Issue 01 (2012-05-30)
Huawei Proprietary and Confidential
Copyright © Huawei Technologies Co., Ltd.
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1.3.3 STA Login Process
Association Authentication
STA association authentication is performed by the active AC, and the standby AC does not
save STA information. After an active/standby switchover, the new active AC synchronizes
STA information from APs through CAPWAP tunnels.
IP Address Allocation
When STA addresses are allocated by an AC, STAs face two issues: address conflicts and
address lease extension. In addition, STAs cannot be allocated static IP addresses because they
connect to wireless networks randomly.
Two solutions are available to address the two issues:
(Recommended) Deploy an independent DHCP server to allocate IP addresses to STAs.
Deploy DHCP servers on the ACs and configure non-overlapping IP address pools on the
ACs. Set the lease of IP addresses to the longest time STAs may stay online, for example,
seven days. The non-overlapping IP address pools on the ACs prevent IP address
conflicts. Because most STAs go offline before their IP addresses expire, STAs do not
need to extend their IP address leases. There is a very low probability that an STA stays
online for a long time and extends its IP address lease during an active/standby
switchover. This low probability is acceptable.
Additionally, the ACs cannot function as gateways for STAs. If an AC functions as the
gateway for STAs, the gateway IP address changes after a switchover. However, STAs cannot
change the gateway IP address, causing service interruption.
Data Forwarding
Data of STAs is forwarded in either of the following modes:
Direct forwarding: Data of STAs is forwarded by APs locally.
Tunnel forwarding: Data of STAs is forwarded by the active AC. After an active/standby
switchover, data of STAs is sent to the new active AC for forwarding.
1.3.4 Active/Standby Switchover and Revertive Switchover
Active/Standby Switchover
After an AP sets up tunnels with the active and standby ACs, it sends Echo packets to monitor
the tunnel status. The Echo packets contain the primary/secondary status of tunnels. When an
AP detects that the primary tunnel fails, the AP sends an Echo packet to the standby AC to
trigger an active/standby switchover. The failed tunnel then becomes the secondary tunnel,
and the original secondary tunnel becomes the primary tunnel. The Echo packet sent from the
AP to the standby AC contains the primary link flag. After receiving this Echo message, the
standby AC changes to active state and sets the AP status to normal. The AP then send service
data to the new active AC. The AP periodically sends Discovery Request packets to check
whether the failed link recovers.
