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Technical Overview—Link Aggregation

 

Link aggregation technology has been a mainstay in data center and campus wired networks for many years. Link aggregation and protocols such as Link Aggregation Control Protocol (LACP) are well understood and deployed frequently for both capacity and resiliency reasons.

While link aggregation has been common on the wired network, it is only with the introduction of 802.11ac that the wireless network has exceeded the capacity of a single Gigabit Ethernet link. Other Ethernet technologies exist (or are being drafted) such as 2.5- and 5-Gigabit Ethernet and 10-Gigabit Ethernet, where a single uplink could meet the 802.11ac demands. However, these technologies do not support a standard vehicle for 10 Gigabit Ethernet PoE, suffer from cabling distance limitations, or are not standardized. These reasons make it more practical and cost effective to group multiple Gigabit Ethernet connections into one logical connection for the AP.

For the vast majority of enterprise WLANs, a single gigabit backhaul for APs is more than sufficient. 802.11ac theoretical limits are much higher than a single gigabit connection, but the practical maximum only slightly exceeds the 1-gigabit limit. Before deploying link aggregation, you should consider wireless client capabilities and data demands, as well as the cost of running two Ethernet cables and dedicating two wiring closet switch ports to each AP.

APs exceeding the capacity of a single link will only occur in extreme cases—for example, concurrent dual-band operation over the widest possible channels and highest possible MCS rates in each band, with all traffic flowing in the same direction. In such cases, a single Gigabit Ethernet backhaul will saturate and limit the AP capacity to less than 1 Gbps. To alleviate this backhaul limitation, you can use link aggregation to bond multiple Gigabit Ethernet links into a single, high-capacity logical link.

Link Aggregation on EX Series Switches

Link aggregation is a standard feature in Junos® OS. This rich aggregation feature set is supported between Juniper Networks devices, servers, and third-party network equipment, such as Ruckus Wireless equipment.

EX Series switches support a number of link aggregation models that can be used effectively with the Ruckus Wireless solution. When connecting an AP to a single EX Series switch, you can use a standard dual-homed configuration. If you are using Juniper Networks Virtual Chassis technology in the wiring closet, you can distribute the link aggregation group across multiple switches for increased resiliency. Examples of the possible connection models are shown in Figure 1.

Figure 1: Connecting EX Series Switches to Ruckus Wireless APs
 Connecting EX Series Switches
to Ruckus Wireless APs

For more information about link aggregation on EX Series switches, see Understanding Aggregated Ethernet Interfaces and LACP.

For more information about Virtual Chassis, see EX Series Virtual Chassis Overview.

Link Aggregation on Ruckus Wireless APs

The ZoneFlex R710 AP supports link aggregation and the LACP protocol (Link Aggregation Control Protocol) as defined in the 802.1ax (formerly 802.3ad) standard, allowing the bonding of its two Gigabit Ethernet ports to form a single 2 Gbps link.

In addition to allowing link bonding, the ZoneFlex R710 AP link aggregation feature includes the following options:

  • LACP rate option—Defines the rate at which the AP asks its link partner (for example, an EX Series switch) to transmit LACP control packets (LACPDUs). A slow rate and a fast rate are supported:

    • Slow rate—Requests the link partner to transmit LACPDUs every 30 seconds. This rate is adequate for the vast majority of enterprise WLAN cases.

    • Fast rate—Requests the link partner to transmit LACPDUs every 1 second. A faster rate allows the link endpoints to respond quicker to any changes on the physical interface—for example, to fail over more quickly in case one of the ports is disconnected—at the expense of more overhead.

  • Transmit hash option—Defines how the AP chooses to distribute packets between the two physical Ethernet links that comprise the aggregated link. You should consider network topology and expected traffic flows when choosing which transmit hash option to use, with the goal of spreading traffic as evenly as possible between the two physical links.

    The supported transmit hash options are:

    • Layer 2 hashing—Uses the source and destination MAC addresses in the packet to determine which physical link the packet is sent over. This is a fully 802.1ax-compliant option and is the default option.

      Layer 2 hashing is most appropriate for environments with predominately east-west traffic patterns, as shown in Figure 2. In these environments, wireless clients are communicating directly with other wireless or wired clients on the same VLAN. Layer 2 hashing on the AP load-balances the traffic based on the MAC addresses of the devices and distributes traffic across the LAG members.

      Figure 2: Example of Environment with Predominately East-West Traffic
      Example of Environment
with Predominately East-West Traffic
    • Layer 3 and Layer 4 hashing—Uses source and destination IP addresses as well as source and destination ports. This hashing mode uses upper layer protocol information, when available, to generate the hash. Using this information allows packets destined for a particular network peer to be distributed across both physical links on a per-flow basis. For fragmented packets, Layer 4 information is omitted. This hashing mode is not fully 802.1ax-compliant.

      Layer 3 and Layer 4 hashing allows the AP to evenly distribute flows across the LAG members in the case where the Layer 2 next-hop address might be common. This hashing mode results in the most optimal balancing of traffic across links in environments where traffic is primarily north-south, as shown in Figure 3. In these environments, wireless clients access network resources through a default gateway rather than having direct communication with peers.

      Figure 3: Example of Environment with Predominately North-South Traffic
      Example of Environment
with Predominately North-South Traffic
    • Layer 2 and Layer 3 hashing—Uses both Layer 2 source and destination MAC addresses and Layer 3 source and destination IP addresses. This hashing mode places all traffic to a particular network peer on the same physical link. For traffic other than IP traffic, Layer 2 hashing is used. This policy is intended to provide a more balanced distribution of traffic than Layer 2 alone, especially in environments where a Layer 3 gateway device is required to reach most destinations. This algorithm is 802.1ax-compliant.

      Leveraging Layer 2 and Layer 3 information for the LAG hashing provides a balance between east-west traffic and north-south traffic flows. This load balancing scheme is more flexible and is completely standards compliant, but traffic might not be as evenly distributed across LAG members in all cases. Figure 4 provides an example of an environment where traffic is both local and across a gateway device.

      Figure 4: Example of Environment with Mixed East-West and North-South Traffic
      Example of Environment with
Mixed East-West and North-South Traffic