Inter-Device Link Aggregation and Local Preferential Forwarding in a Stack

Inter-Device Link Aggregation

A stack supports inter-device link aggregation (Eth-Trunk). That is, physical Ethernet ports on different member switches can bundle into one Eth-Trunk. The Eth-Trunk link works even when a member switch or a member link in the Eth-Trunk fails, ensuring reliable data transmission. Inter-device link aggregation prevents single-point failures in a stack and greatly improves network reliability.

In Figure 1-27, traffic sent to the core device on the network is equally distributed to member links of an Eth-Trunk between the stack member switches. If an Eth-Trunk member link fails, traffic on this link is distributed to the other link. This link backup mechanism improves network reliability.

Figure 1-27 Link backup through inter-device link aggregation

In Figure 1-28, if a member switch in the stack fails, traffic is switched to the Eth-Trunk member link on the other member switch. This device backup mechanism improves network reliability.

Figure 1-28 Device backup through inter-device link aggregation

Local Preferential Forwarding

When an inter-device Eth-Trunk is configured in a stack, the stack uses the hash algorithm to select outbound interfaces in the Eth-Trunk. Therefore, traffic received on a member switch may be forwarded through the other member switch. Inter-device forwarding consumes bandwidth on stack links. As bandwidth provided by a stack cable is limited, this forwarding mode increases loads on stack cables and reduces forwarding efficiency. Local preferential forwarding solves this problem. This function ensures that traffic reaching the local switch is preferentially forwarded through a local interface. If the local outbound interface fails, traffic is forwarded through an interface on the other member switch.

In Figure 1-29, SwitchA and SwitchB set up a stack, and their uplink and downlink interfaces are bundled to Eth-Trunk interfaces. Without the local preferential forwarding feature, traffic reaching SwitchA is load balanced between the Eth-Trunk member links. Some of traffic is forwarded through the stack cables and sent out from a physical interface on SwitchB. If local preferential forwarding is enabled, traffic reaching SwitchA is forwarded through a local physical interface.

This function is valid only for known unicast packets but not for unknown unicast packets, broadcast packets, and multicast packets.

Figure 1-29 Local preferential forwarding