This article will provide some useful and general troubleshooting for MC-LAG.

While you are configuring MC-LAG it may be a challenge to troubleshoot and find possible configuration mismatches.

Here are some troubleshooting steps learned during the last few years that may help you with MC-LAG.

  set interfaces irb unit <unit number> family inet address <IP and mask> arp <IP address of peer IRB> l2-interface <ICL LAG interface> mac <mac address of peer IRB interface> 

From the note here, it seems we can't have both VRRP and mcae-mac-synchronize configured: 

"Note: On QFX Series switches, you cannot configure both VRRP over IRB and MAC synchronization, because processing MAC addresses might not work." 

http://www.juniper.net/documentation/en_US/junos13.3/topics/concept/lag-multichassis--guidelines.html 

5. Restart ICCP? 

6. Look for any other VC-related config because MC-LAG is not supported on VC. 

7. If there is an issue with mac learning on non mc-lag vlans between the mc-lag peers, make sure 

the vlans are not trunked on the ICL. They should be trunked on their own separate LAG. 

8. Make sure the IP address of the MC-LAG peer in the statement 'set protocols iccp peer <IP address of peer> is reachable. 

9. If ICCP is not up, make sure that the ICCP vlan is not configured on the mc-ae interfaces. 

10. If STP is configured along with MC-LAG, then the Juniper device must be the root bridge: 

https://www.juniper.net/documentation/en_US/junos/topics/concept/mc-lag-feature-summary-best-practices.html 

"With RL2GP, you can configure two edge MC-LAG nodes with the same STP virtual root ID. The virtual root ID must be superior to all bridges in the downstream network". 

11. Run the following commands: 

• "set interface <interface-name> vlan-tagging/flexible-vlan-tagging" on trunk ports to increase the MTU to 1518/1522. 
• If this knob is not there then packet size of larger than 1514 will get dropped. 
• "set interface irb arp-l2-validate" to ensure there is no mistmach in arp and mac. Only valid on EX9200. Not valid on Broadcom chipsets. 
• "set forwarding-options dhcp-relay forward-snooped-clients <>" when dhcp-relay is configured. By default, dhcp packets are snooped 
• if dhcp-relay is configured and default action is dropped if it not part of the relay. 
• "set forwarding-options dhcp-relay route-suppression destination". This is to ensure that arp entry is not learnt via jdhcpd on ICL link. 
• "set protocols igmp-snooping vlan all interface <> multicast-router-interface". Always configure ICL as mrouter-interface for multicast; otherwise ICL will not forward multicast traffic. 
• "set multicast-snooping-options multichassis-lag-replicate-state" Multicast-replicate-state is required for IGMP snooping to work. 

• Check chassis configuration to make sure the device supports aggregate ethernet interfaces:

user@router>show configuration chassis                      
aggregated-devices {
    ethernet {
        device-count 200;

• Make sure the physical interface is correctly assigned to LAG:

user@router> show configuration interfaces xe-0/0/0
description test;
gigether-options {
    802.3ad ae50;
}

• Both LAG peers should have unique chassis ID.
• Both MC-LAG peers should have the same mode. The two available modes are active-active or active-standby
• Since these two MC-LAG peers looks the same device from a remote end perspective, then both MC-LAG peers should have the same LACP ID.
• Both MC-LAG peers should have the same LACP admin key.
• The switch service ID is used to synchronize applications, IGMP, ARP, and MAC learning across MC-LAG members. Therefore, we define the same service-id on both peers.

Tips:

• Do not forget to specify the child interfaces that belong to the MC-AE interfaces.
• The MC-LAG will not become active if there is no unit configuration for the AE.
• When the interface of an active -active MAC-LAG is enabled for ethernet switching, the VLAN needs to be added to the ICL as well.
• You can reuse most settings for a second MC_AE between MC-LAG peers. Only the system ID, admin key and mc-as-id need to differ on a per MC-AE basis.
• Certain MC-LAG deployments involving active routeing-protocols between the MC-LAG peers may require static ARP entries.
• If the remote end does not support LACP, use the "force-up" option to make sure that MC-AE interfaces become operational

06-16-2023:KB Created

Best Practices and Usage Notes

Advanced MC-LAG Concepts

MC-LAG Concepts

MC-LAG Examples