Link Aggregation in CSS Scenarios

Concepts

• CSS

  A CSS is a logical device formed by connecting multiple devices through CSS cables. In Figure 3-11, DeviceB and DeviceC are connected in a CSS.

• Inter-chassis Eth-Trunk

  Different physical device interfaces in a CSS aggregate to form a logical Eth-Trunk interface. When a device in the CSS or a physical device interface in the Eth-Trunk fails, traffic can be transmitted between devices through CSS cables. The inter-chassis Eth-Trunk ensures reliable transmission and implements device backup.

• Preferential forwarding of local traffic

  As shown in Figure 3-11 (b), traffic from DeviceB or DeviceC is only forwarded through local member interfaces. In Figure 3-11 (a), traffic is forwarded across devices through CSS cables.

  Figure 3-11  Inter-chassis Eth-Trunk
  
  

Preferential Forwarding of Local Traffic by an Inter-Chassis Eth-Trunk

In a CSS, an Eth-Trunk is configured to be the outbound interface of traffic to ensure reliable transmission. Member interfaces of the Eth-Trunk are located on different devices. When the CSS forwards traffic, the Eth-Trunk may select an inter-chassis member interface based on the hash algorithm. This forwarding mode occupies bandwidth resources between devices and reduces traffic forwarding efficiency.

In Figure 3-11, DeviceB and DeviceC form a CSS, and the CSS connects to DeviceA through an Eth-Trunk. After the Eth-Trunk in the CSS is configured to preferentially forward local traffic, the following features are realized:

• Forwarding received traffic by the local device

  If DeviceB has member interfaces of the Eth-Trunk and these interfaces are properly functioning, the Eth-Trunk forwarding table of DeviceB contains only local member interfaces. Therefore, the hash algorithm selects a local member interface, and traffic is only forwarded through DeviceB.

• Forwarding received traffic by another device

  If DeviceB does not have any member interfaces of the Eth-Trunk or all member interfaces are faulty, the Eth-Trunk forwarding table of DeviceB contains all available member interfaces. Therefore, the hash algorithm selects a member interface on DeviceC, and traffic is forwarded through DeviceC.

• This function is only valid for known unicast packets, and does not work with unknown unicast packets, broadcast packets, and multicast packets.
• Before configuring an Eth-Trunk to preferentially forward local traffic, ensure that member interfaces of the local Eth-Trunk have sufficient bandwidth to forward local traffic; otherwise, traffic may be discarded.