Everyone  mostly know what MC-Lag does, for the benefit its a variant of LAG where the Down stream Devices share LAG interface on two Physical devices instead of One, I know its confusing let see a  sample topology



Vqfx1 will see the upstream as 1 ae1 instead of two different VMX devices, this has its own advantage and the entire discussion is something out of scope of this blog post.


Blog Post Goal – Demonstrate MC-Lag on VMX and Quickly highlight the options Common and different in MC-LAG, a ready reference for someone going for an exam or a implementation.



-> ICCP is the protocol between the nodes


  • Uses TCP/IP to replicates control plane and forwarding traffic between peers
  • One stand-by and one active (active responsible status control)
  • ICCP messages exchange configuration information to ensure both peers use correct LACP Parameters
  • ICL-PL (interface between Mx1 and Mx2 ex) supports Multiple MC-Lag Between the peers so its recommended to be a AE.




  • A Keep-alive message is exchanged between MC-LAG peers which is recommended to be the management connection
  • If ICL-PL fails, keep alive is still through the management connection and in that scenario, the stand-by brings down its local member link to avoid split-brain scenarios


-> MC-LAG Modes


  • Active/Stand-by & Active-Active (QFX Only supports Active/Active MC-LAG Mode)
  • A/A supports traffic distribution and if one link fails, the traffic will be through ICL-PL connection as ICL-PL will have all the VLANS allowed
  • All Mac-Addresses are shared among MC-LAG Peers using ICCP
  • Mcast Flooding happens on all the links if MC-LAG peers are members of VLANS associated with traffic
  • IRB routing is possible


Phase-1 - Setting up ICCP (ICL Peers)

Two things here


-> If you are operating at VMX level, if you want this to work

     – Configure Static ARP for IRB else ICCP will not establish

     – Configure LACP Session-ID else LACP will not form

     – I have seen LACP session getting initiated with different ID’s at different flaps which resulted in AE0 Flaps.



When Observed from vQFX, since its Active-standby, one interface will be attached, while the other is in distributing 


Values common on both the Peers

Mc-ae-id <1>

Mode active-active

lacp active

lacp admin-key <1>

lacp system-id <01:01:01:01:01:01>


Different on Both the peers

Chassis-id 0

Status-control active