Table of Contents
Description ... 3
Benefits... 4
Theory of Operation... 4
Interaction with Other Features... 7
Configuration ... 8
Application Examples ... 10
Description
The Xirrus Wi-Fi Array supports Gigabit Ethernet interfaces for its wired connection to the network. There are 2 GigE interfaces on the 8-radio XS8 and 16-radio XS16 models, and 1 GigE interface on the 4-radio XS4 model. This Application Note applies to the operational modes of the 2 GigE ports on the XS8 and XS16 Array models.
The advent of the new high performance 802.11n Wi-Fi standard is driving the requirement for greater system performance in both wireless and wired network infrastructure. With 802.11n, Gigabit Ethernet uplinks from the Wi-Fi radio device to the wired network are essentially mandatory. The Xirrus Array architecture allows for flexibility in the deployment of the Array in the wired network as well as means to troubleshoot connections, including aggregation, daisy chaining, and redundancy.
The Wi-Fi Array Gigabit Ethernet ports support the following GigE port modes:
• 802.3ad Link Aggregation • Load Balancing
• Broadcast • Link Backup • Bridged • Mirrored
IEEE 803.2ad link aggregation allows the Wi-Fi Array’s Gig1 and Gig2 ports to be aggregated as one, providing the external wired switch supports this protocol. 802.3ad is a standard for bonding two wired ports together and is commonly supported by Ethernet switches. If the switch connected to the Wi-Fi Array does not support 802.3ad, the Wi-Fi Array supports a simpler mode identified in this application note as Load Balancing. This mode allows both ports to be connected to the same switch and carry traffic, with a simple algorithm used to divide traffic flows between the two ports.
Broadcast and Link Backup modes are similar in that they both have been implemented for fault tolerance. Broadcast mode will transmit traffic on both ports simultaneously. If one GigE port fails, the other will continue to service. Link Backup allows the individual Gigabit ports to be connected to the same switch or to different switches on the same Layer 2 subnet, with one of the GigE ports set to standby. If one GigE port fails, the other will take over. If connected to the same switch, this achieves GigE fail-over if a port fails on the Wi-Fi Array or switch, or the cable connection is cut. When the GigE ports are connected to different switches, this achieves fail-over in case one of the switches fail. Bridge mode provides IT administrators the flexibility to daisy-chain Wi-Fi Arrays. This allows for a wired extension of the Wi-Fi network without the added cost of an additional cable run to the switch closet and corresponding switch port. For example, if a certain area needs a new Array deployed for Wi-Fi coverage but the location is not currently cabled, the closest Wi-Fi Array to that location can be used in Bridge mode and an RJ-45 cable run from it to the new Array.
Benefits
The Xirrus Wi-Fi Array supports multiple connectivity options for its dual Gigabit Ethernet uplinks, each containing a set of associated applications and benefits.
• 802.3ad and Load Balancing modes increase the resiliency of the wired connection by providing
dual active connections. If one link fails, the other will continue to service traffic.
• 802.3ad and Load Balancing modes achieve greater uplink performance by providing a total 2
Gbps throughput connection from the Array.
• Bridge mode extends the reach of a wireless network by allowing Arrays to be wired directly to
each other in a daisy chain fashion, eliminating cable runs and wired switch ports in the wiring closet.
• Link Backup provides for fail-over resiliency of the wired connection if a GigE port fails. • Broadcast mode provides fault tolerance by transmitting all traffic on both GigE ports. • Mirror mode simplifies network troubleshooting of your network. If Gig1 is connected to your
corporate network, Mirror mode can be configured on that Wi-Fi Array with a wired packet capture connected to Gig2 to monitor inbound and outbound Gig1 traffic.
Theory of Operation
802.3ad
802.3ad Link Aggregation has been implemented in the Wi-Fi Array to overcome bandwidth issues for 802.11n. It is an economical way to set up a high-speed link that transfers much more data than any one single gigabit port can deliver. If one port fails, the other port will continue to transmit traffic. In this mode, the GigE ports act together as a single logical interface using the standard Link Aggregation Control Protocol (LACP) to establish and maintain the connection. A load balancing algorithm is used to balance traffic across the two ports. The destination IP address of the packet is used to determine the outgoing adapter. For non-IP traffic (such as ARP), the last byte of the
destination MAC address is used to do the calculation. The network switch must also support 802.3ad.
Load Balance
Load Balance mode is similar to 802.3ad mode with the key difference being the load balancing algorithm used to determine the outgoing adapter. Load Balance mode uses a “balanced XOR” mechanism – the same algorithm used to distribute traffic across multiple radios in a WDS (Wireless Distribution System) link. The exclusive OR (XOR) function is applied to the least significant bits of the source and destination MAC addresses of the packet to select the interface on which to transmit traffic. This approach has the effect of ensuring that all traffic from any given client will always go over the same interface and also ensures that the traffic is always ordered correctly.
Figure 2: Load Balance Mode
Bridge
In Bridge Mode, the Wi-Fi Array will pass traffic received on Gig1 out Gig2 and vice versa via a MAC address lookup table. This allows the Wi-Fi Array to act as a wired bridge and enables a wired daisy-chain connection of multiple Arrays together. It is recommended that a maximum of 3 Wi-Fi Arrays be linked together when using Bridge mode to avoid performance degradation of the wired link.
Broadcast
In Broadcast mode, all traffic is transmitted out both GigE interfaces. Traffic directed to the Array is accepted on both interfaces. This mode provides fault tolerance.
Figure 4: Broadcast Mode
Link Backup
In Link Backup mode, only one GigE port at a time is active. If Gig1 is active and the link between Gig1 and the switch fails, Gig2 will become active. The bond's MAC address is externally visible on only one port to avoid confusion in the switch connected the Wi-Fi Array.
Mirror
In Mirror mode, all traffic received on the Wi-Fi Array is transmitted out both Gig1 and Gig2. This allows a network analyzer (such as Wireshark) to be connected to one port to capture traffic for
troubleshooting purposes, while the other port continues to provide network connectivity for data traffic.
Figure 6: Mirror Mode
Interaction with Other Features
The Wi-Fi Array Gigabit port modes interact with VLANs in the following ways: 1. 802.3ad mode does not currently support VLANs.
2. Load Balancing mode does not currently support VLANs. 3. Broadcast mode supports VLANs.
4. Link Backup mode supports VLANs.
5. Bridge mode supports VLANs when Arrays are daisy chained. This mode works similar to a WDS VLAN scenario. If a switch is connected to both the Gig1 and Gig2 ports of the Array, both ports need to be configured as VLAN trunk ports.
Configuration
Gigabit port mode configuration is a simple process no matter the method used (WMI or CLI) and only requires a few steps.
GigE Port Mode Configuration Using the Web Management Interface (WMI)
1. First configure the Gigabit Ethernet switch(es) to which the Arrays will be connected. If 802.3ad is being used, the switch ports selected for the trunk need to be set up.
2. On the Array, login to the WMI. 3. Click on Network.
4. Click on Interfaces.
5. Under the Gigabit Ethernet 1 Settings heading: a. Click on the drop down field for Port Mode. b. Select the appropriate Port Mode from the list.
GigE Port Mode Configuration Using the Command Line Interface (CLI)
1. First configure the Gigabit Ethernet switch(es) to which the Arrays will be connected. If 802.3ad is being used, the switch ports selected for the trunk will need to be set up.
5. Depending on the mode you would like to deploy, type: a. “port-mode 802.3ad” b. “port-mode load-balance” c. “port-mode broadcast” d. “port-mode link-backup” e. “port-mode bridge” f. “port-mode mirror”
Xirrus Wi-Fi Array
ArrayOS Version 3.4-0632
Copyright (c) 2005-2008 Xirrus, Inc. http://www.xirrus.com
XS16-Array# configure
XS16-Array(config)# interface gig1 XS16-Array(config-gig1)# port-mode ?
802.3ad Aggregate traffic from gig1 & gig2 using 802.3ad bridge Bridge traffic between gig1 & gig2
broadcast Transmit traffic on both gig1 & gig2
link-backup Active backup (gig1/2 fail over to each other) load-balance Load balance traffic between gig1 & gig2
Application Examples
To further understand these features, the following diagrams illustrate different application scenarios for the various GigE port modes.
802.3ad and Load Balance Modes
Broadcast and Link Backup Modes
Bridge Mode
Mirror Mode
Mirror mode is a simple way to mirror traffic on the Array’s GigE network connection. For example if a wireless station cannot associate to an external RADIUS server, instead of searching for extra gear to enable a packet capture tap on the GigE connection, Mirror mode can be used to monitor packets on the Gig1 port by connecting directly to the Gig2 port.
Tips and Recommendations
1. A maximum of 3 Arrays should be daisy chained together when using Bridge mode.
2. When possible, use GigE instead of 10/100 Ethernet for Array backhaul connections for best performance and to future proof for 802.11n.
