What is Eth-Trunk for S series switches

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Eth-Trunk, also called link aggregation, is a technique that bundles multiple physical links to a logical link to increase link bandwidth. For relevant protocols and standards, see IEEE 802.3ad.
As the network scale expands increasingly, users pose increasingly higher requirements on the bandwidth and reliability of backbone links. Traditional technologies often use high-speed cards or devices supporting high-speed interface cards to increase the bandwidth. This method, however, is costly and inflexible.
Link aggregation bundles multiple physical interfaces into a logical interface to increase the bandwidth without upgrading the hardware. In addition, link aggregation adopts the link backup mechanism to improve reliability of the links between devices.
Link aggregation has the following advantages:
1. Increase bandwidth. The maximum bandwidth of a link aggregation group is the sum of bandwidth of the member interfaces.
2. Improve reliability. When a link fails, traffic is automatically switched to other member links. This ensures reliability of the link aggregation group.
3. Achieve load balancing. A link aggregation group can balance load among active member links.

Other related questions:
The difference between E-trunk and Eth-Trunk on S series switches
For S series switches (except the S1700): Eth-Trunk, also called link aggregation, bundles multiple physical links to form a logical link to increase link bandwidth. The bundled links can enhance link reliability through mutual backup. E-Trunk (Enhanced Trunk), developed from LACP (a link aggregation protocol for a single device), is a mechanism that implements link aggregation among multiple devices. E-Trunk achieves device-level link reliability, instead of card-level link reliability previously achieved by LACP.

How load balancing is implemented on S series switches when link aggregation is configured
For S series switches (except the S1700), there are two load balancing modes: per-packet load balancing and per-flow load balancing. 1. Per-packet load balancing mode When there are multiple physical links between the two devices of the Eth-Trunk, the first data frame of a data flow is transmitted on one physical link, and the second data frame is transmitted on another physical link. In this case, the second data frame may arrive at the peer device earlier than the first data frame. As a result, packet mis-sequencing occurs. 2. Per-flow load balancing mode This mechanism uses the hash algorithm to calculate the address in a data frame and generates a hash key value. Then the system searches for the outbound interface in the Eth-Trunk forwarding table based on the generated hash key value. Each MAC or IP address corresponds to a hash key value, 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 flow-based load balancing. Per-flow load balancing ensures the correct sequence of data transmission, but cannot ensure the bandwidth usage. Notes: Currently, S series switches support only per-flow load balancing mode, including the following: 1. Load balancing based on the source MAC address of packets; 2. Load balancing based on the destination MAC address of packets; 3. Load balancing based on the source IP address of packets; 4. Load balancing based on the destination IP address of packets; 5. Load balancing based on the source and destination MAC addresses of packets; 6. Load balancing based on the source and destination IP addresses of packets; 7. Enhanced load balancing for L2, IPv4, IPv6, and MPLS packets based on the VLAN ID and source physical interface number. When you configure load balancing modes, follow these guidelines: The load balancing mode only takes effect on the outbound interface of traffic. If load is unevenly distributed on the inbound interfaces, change the load balancing mode on the uplink outbound interfaces. Configure load balancing to ensure data flow is transmitted on all active links instead of only one link, preventing traffic congestion and ensuring normal service operation. For example, if data packets have only one destination MAC address and IP address, you are advised to configure load balancing based on the source MAC address and IP address. If you implement load balancing based on the destination MAC address and IP address, the data flow may be transmitted on only one link, causing traffic congestion.

Link aggregation support on S series fixed switches
S series fixed switches (except the S1700) support the link aggregation function. For details about link aggregation support on the fixed switches, see the "Ethernet Switching - Link Aggregation" in the S1720&S2700&S3700&S5700&S6700 Product Use Precautions.

The type of Eth-Trunk member interfaces on S series switches
Observe the following guidelines when adding member interface to an Eth-Trunk on S series switches (except the S1700): 1. The member interface cannot be configured with some services or static MAC address entries. For example, when an interface is added to an Eth-Trunk, the interface must use the default link type. 2. An Eth-Trunk cannot be added to another Eth-Trunk. Member interfaces of an Eth-Trunk must use the same Ethernet type and rate. Interfaces that use different Ethernet types and rates cannot join the same Eth-Trunk. For instance, GE interfaces and FE interfaces cannot join the same Eth-Trunk, but GE electrical interfaces and GE optical interfaces can join the same Eth-Trunk. 3. Both devices of the Eth-Trunk must use the same number of physical interfaces, interface rate, duplex mode, and flow control mode. 4. If an interface of the local device is added to an Eth-Trunk, an interface of the remote device directly connected to the interface of the local device must also be added to an Eth-Trunk; otherwise, the two ends cannot communicate.

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