Understanding Eth-Trunk Load Balancing of S Series Switches

This document describes Eth-Trunk forwarding fundamentals, load balancing mode, and how to configure Eth-Trunk load balancing.

This document uses S5720HI as an example. There may be differences in the implementation of different switch models and versions. For details, see the corresponding product documentation.

Because an Eth-Trunk between two devices consists of multiple physical links bundled together, an Eth-Trunk may transmit data frames of the same data flow over different physical links. A potential problem arises in that the second data frame may arrive at the remote device earlier than the first data frame, resulting in out-of-order packets.

To prevent out-of-order packets, Eth-Trunk uses flow-based load balancing. This mechanism uses the hash algorithm to calculate the address in a data frame and generate a hash key based on which the system searches for the outbound interface in the Eth-Trunk forwarding table. Each MAC or IP address corresponds to a hash key, so the system uses different outbound interfaces to forward data. This mechanism ensures that frames of the same data flow are forwarded on the same physical link and implements load balancing of data flows. Flow-based load balancing ensures the correct sequence of data transmission, but cannot ensure efficient bandwidth usage.

In Figure 1, an Eth-Trunk is located at the data link layer, between the MAC address and LLC sub-layers.

Figure 1-1 Eth-Trunk in the Ethernet protocol stack

The Eth-Trunk module maintains a forwarding table that consists of the following entries:

• HASH-KEY value

  The HASH-KEY value is calculated through the hash algorithm based on the MAC address or IP address in a data packet.

• Interface number

  Eth-Trunk forwarding entries are related to the number of member interfaces in an Eth-Trunk. Different HASH-KEY values map to different outbound interfaces.

  For example, an Eth-Trunk supports a maximum of eight member interfaces. If physical interfaces 1, 2, 3, and 4 are bundled into an Eth-Trunk, the Eth-Trunk forwarding table contains four entries, as shown in Figure 2. In the Eth-Trunk forwarding table, hash keys are 0, 1, 2, 3, 4, 5, 6, and 7, and the corresponding outbound interface numbers are 1, 2, 3, 4, 1, 2, 3, and 4. Therefore, optimal load balancing can be implemented only when two, four, or eight interfaces are added to an Eth-Trunk.

  Figure 1-2 Example of an Eth-Trunk forwarding table
  

The Eth-Trunk module forwards a packet according to the Eth-Trunk forwarding table:

1. The Eth-Trunk module receives a packet from the MAC sub-layer, and then extracts its source MAC address/IP address or destination MAC address/IP address.
2. The Eth-Trunk module calculates the HASH-KEY value using the hash algorithm.
3. Using the HASH-KEY value, the Eth-Trunk module searches the Eth-Trunk forwarding table for the interface number, and then sends the packet from the corresponding interface.

You can set the load balancing mode based on traffic models. When a field of traffic changes frequently, you can set the load balancing mode based on this field to ensure that the traffic is load balanced evenly. For example, if IP addresses in packets change frequently, use the load balancing mode based on the destination IP address, source IP address, or both source and destination IP addresses. If MAC addresses in packets change frequently and IP addresses are fixed, use the load balancing mode based on the destination MAC address, source MAC address, or both source and destination MAC addresses.

According to your network requirements, you can configure load balancing based on the following information:

• Source MAC address
• Destination MAC address
• Source and destination MAC addresses
• Source IP address
• Destination IP address
• Source and destination IP addresses
• VLAN IDs and source physical port numbers for Layer 2, IPv4, IPv6, and MPLS packets in enhanced load balancing mode

For example, DeviceA has two TCP packet flows. In one TCP packet flow, the source IP address is 192.168.1.1 (MAC address: a-a-a; source port number: 50), and the destination IP address is 172.16.1.1 (MAC address: b-b-b; destination port number: 2000). In another TCP packet flow, the source IP address is 192.168.1.1 (MAC address: a-a-a; source port number: 60), and the destination IP address is 10.1.1.1 (MAC address: c-c-c; destination port number: 2000). If load balancing based on the source MAC address is configured on DeviceA, only one outbound interface is available for sending packets out. On the other hand, if load balancing based on the destination IP address is configured on DeviceA, packets destined for different destination IP addresses are sent out through different outbound interfaces.

When configuring a load balancing mode, pay attention to the following points:

• The load balancing mode takes effect only on traffic leaving outbound interfaces. If traffic arriving at inbound interfaces is uneven, change the load balancing mode of the uplink outbound interface.
• If possible, ensure data flows are load balanced among all active links. If data flows are transmitted over one link, traffic congestion may occur, affecting services.

  For example, when data packets have only one destination MAC address and one destination IP address, perform load balancing based on the source MAC address and source IP address. If load balancing based on the destination MAC address and destination IP address is used, traffic is transmitted over one link, causing congestion.

If traffic is unevenly load balanced after the configuration is complete, rectify the fault according to How Do I Adjust Eth-Trunk Configurations on S Series Switches When Eth-Trunk Load Balancing Is Uneven.

You can configure a common or enhanced load balancing mode. Configure either of them as required.

<HUAWEI> system-view
[HUAWEI] interface eth-trunk 1
[HUAWEI-Eth-Trunk1] load-balance src-dst-mac
[HUAWEI-Eth-Trunk1] quit
[HUAWEI] display eth-trunk 1 load-balance
Eth-Trunk1's load-balance information:
 Load-balance Configuration: SA-XOR-DA
 Load-balance options used per-protocol:
  L2  : Source XOR Destination MAC address, Vlan ID, Ethertype, Ingress-port
  IPv4: Source XOR Destination MAC address, Vlan ID, Ethertype, Ingress-port
  IPv6: Source XOR Destination MAC address, Vlan ID, Ethertype, Ingress-port
  MPLS: Source XOR Destination MAC address, Vlan ID, Ethertype, Ingress-port

<HUAWEI> system-view
[HUAWEI] load-balance-profile a
[HUAWEI-load-balance-profile-a] ipv6 field sip protocol
[HUAWEI-load-balance-profile-a] quit
[HUAWEI] interface eth-trunk 1
[HUAWEI-Eth-Trunk1] load-balance enhanced profile a
[HUAWEI-Eth-Trunk1] quit
[HUAWEI] display eth-trunk 1 load-balance
Eth-Trunk1's load-balance information:
 Load-balance Configuration: ENHANCED
 Load-balance enhanced profile: a
 Load-balance options used per-protocol:
  L2  : Source XOR Destination MAC address
  IPv4: Source XOR Destination IP address
  IPv6: Source IP address, IP protocol
  MPLS: Top XOR Second label

S1720, S2700, S5700, S6720 V200R011C10 Configuration Guide - Ethernet Switching