Huawei AC6605 Manuale utente

AC6605 Access Controller

V200R001C00

Dual-Link Backup White Paper

Issue

Date

2012-05-30

HUAWEI TECHNOLOGIES CO., LTD.

Issue 01 (2012-05-30)

Huawei Proprietary and Confidential

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AC6605 Access Controller

Dual-Link Backup White Paper

About This Document

Issue 01 (2012-05-30)

Huawei Proprietary and Confidential

About This Document

Purpose

This document describes the dual-link backup technology of the AC6605. The dual-link

backup technology provides high reliability between important network nodes to ensure

service availability.

This document covers the mechanism and networking modes of dual-link backup supported

by the AC6605, and provides configuration examples for typical dual-link backup

applications.

Intended Audience

This document is intended for:

Data configuration engineers

Commissioning engineers

Network monitoring engineers

System maintenance engineers

Symbol Conventions

The symbols that may be found in this document are defined as follows.

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Dual-Link Backup White Paper

About This Document

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iii

Change History

Changes between document issues are cumulative. The latest document issue contains all the

changes made in earlier issues.

Issue 01 (2012-05-30)

This is the first formal issue.

AC6605 Access Controller

Dual-Link Backup White Paper

Contents

Issue 01 (2012-05-30)

Huawei Proprietary and Confidential

Contents

About This Document ............................................................................................................... ii

1 Dual-Link Backup.................................................................................................................... 1

1.1 Introduction to Dual-Link Backup.............................................................................................................. 1

1.2 Availability................................................................................................................................................ 2

1.3 Principles .................................................................................................................................................. 2

1.3.1 Dual-Link Backup Mechanism.......................................................................................................... 2

1.3.2 AP Login Process.............................................................................................................................. 3

1.3.3 STA Login Process............................................................................................................................ 6

1.3.4 Active/Standby Switchover and Revertive Switchover ....................................................................... 6

1.3.5 Loop Prevention in VLANs............................................................................................................... 7

1.3.6 Dual-Link Backup in Layer 2 Networking and Configuration Notes .................................................10

1.3.7 Dual-Link Backup in Layer 3 Networking and Configuration Notes .................................................18

1.4 Applications .............................................................................................................................................29

1.4.1 Direct Forwarding in Layer 3 Chain Networking ..............................................................................29

1.4.2 Tunnel Forwarding in Layer 2 Branched Networking........................................................................33

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1 Dual-Link Backup

About This Chapter

1.1 Introduction to

1.2 Availability

1.3 Principles

1.4 Applications

1.1 Introduction to Dual-Link Backup

Definition

Dual-link backup is implemented by deploying a standby AC at the same layer as the active

AC. The standby AC has the same configuration as the active AC. When the active AC fails,

the backup AC starts to manage services quickly.

Purpose

An AC usually controls thousands of APs and tens of thousands of STAs; therefore, the AC

must be highly reliable. Configuring dual-link backup can ensure stable service operating on a

WLAN network.

Benefits

The dual-link backup technology provides high reliability between important network nodes

to ensure service availability.

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1.2 Availability

Version Support

Product

Product Version

AC6605

V200R001C00

Constraints

Dual-link backup has the following constraints:

The active and standby ACs must have the same WLAN service configuration.

APs must be able to communicate with both the active and standby ACs. When APs

communicate with the ACs through a Layer 2 network, APs must belong to the same

VLAN as the ACs, and AP IP addresses must be on the same network segment as AC IP

addresses.

If the active AC functions as a DHCP server to allocate IP addresses to APs, configure

static IP addresses for APs or configure an IP address pool only on the active AC. If you

need to configure IP address pools on both ACs, ensure that the IP address pools on the

two ACs do not overlap. Otherwise, IP address conflicts may occur.

The ACs cannot allocate IP addresses to STAs or work as gateways for STAs.

1.3 Principles

1.3.1 Dual-Link Backup Mechanism

Dual-link backup is enabled on active and standby ACs so that APs can establish Control and

Provisioning of Wireless Access Points (CAPWAP) tunnels with both ACs. The status of an

AP is displayed as normal on the active AC and displayed as standby on the standby AC.

When both the active and standby ACs are working properly, only the active AC manages

services of APs and delivers configurations to the APs.

An AP sends Echo packets to monitor the status of the links connected to the ACs. When the

active AC is unavailable because of an AC fault or network failure, an AP detects that the link

connected to the active AC is Down, and the AP status on the standby AC changes from

standby to normal. The standby AC then delivers configurations to the AP and manages

services of the AP.

When the AP detects that the link connected to the active AC recovers, it switches service

traffic back to the active AC within 500 seconds if revertive switching is enabled. After the

switchover, the AP status is displayed as normal on the active AC and displayed as standby on

the standby AC. If revertive switching is disabled, the AP still sends service traffic to the

standby AC.

As shown in Figure 1-1, the APs set up CAPWAP tunnels with the active AC (AC1) and

standby AC (AC2) simultaneously when they go online. The APs and ACs exchange

handshake packets to monitor the link status. When the APs detect that the links connected to

AC1 fail, they switch service traffic to AC2. As they have established CAPWAP tunnels with

AC2, service switching time is reduced. However, users need to go online again after the

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switchover. Users that were authenticated using open system or WEP authentication do not

need to go online again.

Figure 1-1 Dual-link backup

AC1 AC2

Aggregation

switch

Access switch

AP AP

STA

CAPWAP primary link

CAPWAP backup link

Impact of an active/standby switchover on services is as follows:

During an active/standby switchover, services are interrupted for a period because an AP

needs to detect the failure of primary link using CAPWAP heartbeat packets. The service

interruption time is determined by the heartbeat interval (3s to 300s) and number of

detection attempts (2 to 120). Users cannot use Internet services, but they are still online

and do not need to be reauthenticated after the switchover is complete. The AP status on

the standby AC changes from standby to normal after the AP detects a failure on the

primary link.

When traffic is switched back from the standby AC to the active AC, services are not

affected because both the primary link and backup link are working properly. Users are

still online and do not need to be reauthenticated after the switchover is complete.

1.3.2 AP Login Process

After an AP is powered on, it uses DHCP to obtain its own IP address and IP addresses of the

active and standby ACs. The AP then discovers the ACs and establishes primary and backup

links with the ACs to implement dual-link backup.

If neither of the active and standby ACs is used as a DHCP server, you only need to set

Option 43 to IP addresses of the active and standby ACs on the DHCP server and do not need

to consider the IP address lease. The following describes the AP login process when an AC

functions as a DHCP server.

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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.

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Setting Up the First Tunnel

1. After the AP obtains its own IP address and IP addresses of the two ACs, it sends a

Discover Request packet to both ACs to discover ACs. If the AP obtains only its own IP

address, it broadcast a Discover Request packet to discover ACs.

2. If the ACs are working properly, they send a Discover Response packet with the

dual-link backup flag, workload, and priority to the AP.

3. After receiving the Discover Response packets, the AP compares the priorities and IP

addresses of the ACs to select an AC and establishes a CAPWAP tunnel with the

selected AC. The AC with a smaller priority value becomes the active AC. If the ACs

have the same priority value, the AC with a lower workload becomes the active AC. If

their workloads are also the same, the AC with smaller IP address becomes the active

AC.

If an AC fails to return a Discover Response packet, the AP selects the other AC to set up the CAPWAP

tunnel. The tunnel set up first may not be the primary tunnel. The AP will determine the primary and

backup tunnels after it successfully establishes tunnels with both the two ACs.

4. The subsequent negotiation and configuration delivery processes are the same as those

for setting up a single CAPWAP tunnel.

5. After the tunnel is set up, the AC delivers configurations to the AP. When the AP works,

STAs can go online and use network services.

Setting Up the Second Tunnel with the Other AC

1. The AP determines to set up a CAPWAP tunnel with the other AC only when the

Discover Response packet sent by the first AC contains the dual-link backup flag. The

AP starts to set up the second tunnel after the first AC has delivered configurations to the

AP. This avoids repeated configuration delivery.

2. If the AP has obtained the other AC's IP address, it sends a unicast Discover Request

packet to this AC, and then sends a broadcast Discover Request packet to discover ACs.

If the AP has not obtained the other AC's IP address, it sends a broadcast Discover

Request to discover ACs. The AC that has set up a CAPWAP tunnel with the AP ignores

the Discover Request packet.

3. If the AC is working properly, it returns a Discover Response packet containing the

dual-link backup flag and priority to the AP.

4. The AP knows that the dual-link backup function is enabled after receiving the Discover

Response packet, and saves the priority of the AC.

The second AC will not become the active AC even if it has a higher priority than the first one. The AP

switches traffic to this AC only after it sets up a CAPWAP tunnel with the AC.

5. The AP sends a Join Request packet, notifying the AC that the configurations have been

delivered. The AC will not deliver configurations to the AP after receiving the Join

Request packet.

6. After the tunnel is set up, the AP compares the priorities and IP addresses of the two ACs

and determines the active and standby ACs. The AP performs a revertive switchover if

the second AC becomes the active AC.

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