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.
Active-Standby
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.

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
:
{master:0}
dhanks@CE1-RE0>show lacp interfaces
Aggregated interface: ae1 ...
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