This article provides an explanation of the load balancing logic with an AE interface.

The ae interface does not load balance (egress) when the packets are generated by the RE.

Topology

 [MX80-a]---- [MX80-1] ==== ae11 ==== [MX80-b] ---- [MX80-2]

• xe-0/0/0  and xe-0/0/1 belongs to ae11

All Routing-Engine (RE) generated packets that egress the system by the LAG interface (e.g., protocol control traffic) will use the lowest member link in the LAG.

Below is the output of monitor interface traffic on MX80-1, when ICMP packets are sent to MX80-1 from MX80-2 using six lo0.0 addresses as source IP addresses:

Interface    Link  Input packets        (pps)     Output packets        (pps)
 lc-0/0/0      Up              0                             0
 pfh-0/0/0     Up              0                             0
 xe-0/0/0      Up            362          (1)              605          (0)<<<<<
 xe-0/0/1      Up            562          (1)                5          (0)
 xe-0/0/2      Up              0          (0)                6          (0)

Below is the output of the monitor interface traffic on MX80-1, when ICMP packets are sent to MX80-a from ​MX80-2 using six lo0.0 addresses as source IP addresses:

Interface    Link  Input packets        (pps)     Output packets        (pps)
 lc-0/0/0      Up              0                             0
 pfh-0/0/0     Up              0                             0
 xe-0/0/0      Up            509          (1)              203          (0)<<<<<
 xe-0/0/1      Up            309          (1)              404          (0)<<<<<

This is expected behavior.

• Traffic passing through the MX is load balanced.
• Traffic directed to the MX is not load balanced.

Notice: This behavior will change after the lowest number interface flap or manually disable it. When lowest number interface recovery, packet might still send out via interface with bigger number.

For example:

physical interface xe-0/0/0 and xe-0/0/1 was bundled into ae194.

After first implementation, packet will be send out through xe-0/0/0 which has the lowest number. But, after interface xe-0/0/0 flap, packet will be send out through xe-0/0/1. After xe-0/0/0 recovery, packet will still be send out through xe-0/0/1.

Add below notice:

Notice: This behavior will change after the lowest number interface flap or manually disable it. When lowest number interface recovery, packet might still send out via interface with bigger number.

For example:

physical interface xe-0/0/0 and xe-0/0/1 was bundled into ae194.

After first implementation, packet will be send out through xe-0/0/0 which has the lowest number. But, after interface xe-0/0/0 flap, packet will be send out through xe-0/0/1. After xe-0/0/0 recovery, packet will still be send out through xe-0/0/1.