MC-LAG Modes

This chapter has touched on the two different MC-LAG modes: active-standby and active-active. When MC-LAG was first released, the only available mode was active-standby, which works on both DPC and MPC line cards. Because it was the first mode to be released and because of the simplicity of its design, the active-standby mode is generally more common. With the introduction of Trio and MPC line cards, MC-LAG was upgraded to support an active-active mode. This new mode will only work using Trio-based line cards such as the MPC.


The active-standby mode works by selecting a PE router to be the active node while the other PE router is the standby node. Only one of the PE routers can be active at any given time. When a PE router is active, it will signal via LACP to the CE router its child link is available for forwarding.

MC-LAG Mode Active-Standby.

Figure 8-9. MC-LAG Mode Active-Standby.

Figure 8-9 illustrates MC-LAG in the active-standby mode. In this example, router PE1 is active and PE2 is the standby node. This mode forces all traffic through the active node PE1. For example, a frame destined to VLAN 100 would be forwarded to PE1 and then directly to H1. A frame destined to VLAN 200 would also be forwarded to PE1, then to PE2, and finally to H2.

Let’s take a look at the LACP information from the vantage point of CE1:

dhanks@CE1-RE0>show lacp interfaces Aggregated interface: ae1 ...

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