Inter-Device Link Aggregation and Local Preferential Forwarding
Inter-Device Link Aggregation
A stack supports inter-device link aggregation through Eth-Trunks. Physical Ethernet interfaces on different member switches can bundle into an Eth-Trunk, which connects the stack to an upstream or downstream device. If a member link of the Eth-Trunk or a member switch fails, traffic on this link or switch can be distributed to other member links through the stack cables connecting member switches. Inter-device link aggregation implements link and device backup and ensures reliable data traffic transmission.
In Figure 3-23, traffic sent to the core device on a network is evenly distributed to member links of an Eth-Trunk set up between stack member switches. If a member link fails, traffic on this link is distributed to the other link through the stack cables. This link backup mechanism improves network reliability.
In Figure 3-24, traffic sent to the core device on a network is evenly distributed to member links of an Eth-Trunk set up between stack member switches. If a member switch fails, traffic toward this switch is distributed to the other switch. This device backup mechanism improves network reliability.
Local Preferential Forwarding
Inter-device link aggregation implements reliable data traffic transmission and backup between stack member switches. Because stack cables provide limited bandwidth, inter-device forwarding increases the load on stack cables and reduces forwarding efficiency. To improve the forwarding efficiency and reduce traffic on stack cables, a switch provides the local preferential forwarding function. The function allows traffic reaching the local switch to be preferentially forwarded through a local interface. If the local device has no outbound interface or all outbound interfaces fail, traffic is forwarded through an interface on the other member switch.
In Figure 3-25, SwitchA and SwitchB form a stack, and their uplink and downlink interfaces bundle into Eth-Trunks. If local preferential forwarding is not configured, traffic reaching SwitchA is load balanced between the Eth-Trunk member links. Some traffic is forwarded from SwitchA to SwitchB through the stack cables and is sent out from a physical interface on SwitchB. If local preferential forwarding is configured, traffic reaching SwitchA is forwarded through a local physical interface and does not pass through the stack cables. By default, the local preferential forwarding function is enabled on the device.
