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.
4
Obtaining IP Addresses on a Layer 2 Network
1.
An AP broadcasts a DHCP Discovery packet with Option 60 as Huawei AP.
2.
The two ACs receive the DHCP Discovery packet and both reply with a DHCP Offer
packet.
3.
After receiving the two DHCP Offer packets, in the Discovery phase, the AP sends
Discover Request messages to both ACs. As long as the ACs are working properly, they
will return Discover Response messages to the AP. The Discover Response messages
contain the dual-link backup flag, priorities, workload, and IP addresses of the ACs.
After receiving the Discover Response messages, the AP compares information in the
packets and selects an AC as the DHCP server, and then sends a DHCP Request packet
to the selected AC.
4.
The selected AC sends a DHCP ACK packet to the AP. In the DHCP ACK packet, the
Option 43 field contains the IP addresses of the two ACs, and the Option 51 field
contains a long IP address lease. (The 32-bit Option 51 field can specify the longest lease
of 136 years.)
5.
After receiving the DHCP ACK packet, the AP obtains its own IP address and IP
addresses of the active and standby ACs. The IP address lease is very long so that the AP
does not need to extend the lease. If no Option 43 is configured on the ACs, the AP can
broadcast a DHCP Request to discover ACs.
Obtaining IP Addresses on a Layer 3 Network
1.
An AP broadcasts a DHCP Discovery packet with Option 60 as Huawei AP.
2.
A DHCP relay agent is deployed between the ACs and APs, and the primary and
secondary DHCP servers are configured on the active and standby ACs. The DHCP relay
agent sends a DHCP Discovery packet to the active AC (in active/standby mode) or to
both the ACs (in load balancing mode).
3.
If the DHCP relay agent works in active/standby mode, the active AC receives the
DHCP Discovery packet and replies with a DHCP Offer packet. If the DHCP relay agent
works in load balancing mode, both ACs receive the DHCP Discovery packet and reply
with a DHCP Offer packet.
4.
The DHCP relay agent forwards the DHCP Offer packets to the AP.
5.
After receiving the two DHCP Offer packets, in the Discovery phase, the AP sends
Discover Request messages to both ACs. As long as the ACs are working properly, they
will return Discover Response messages to the AP. The Discover Response messages
contain the dual-link backup flag, priorities, workload, and IP addresses of the ACs.
After receiving the Discover Response messages, the AP compares information in the
packets and selects an AC as the DHCP server, and then sends a DHCP Request packet
to the selected AC.
6.
The DHCP relay agent forwards the DHCP Request packet to the selected AC.
7.
The selected AC sends a DHCP ACK packet to the AP. In the DHCP ACK packet, the
Option 43 field contains the IP addresses of the two ACs, and the Option 51 field
contains a long IP address lease. (The 32-bit Option 51 field can specify the longest lease
of 136 years.)
8.
The DHCP relay agent forwards the DHCP ACK packet to the AP.
9.
After receiving the DHCP ACK packet, the AP obtains its own IP address and IP
addresses of the active and standby ACs. The IP address lease is very long so that the AP
does not need to extend the lease.
The AP then discovers ACs and set up tunnels with the ACs.
