This article describes the issue of preemption not being supported in multichassis link aggregation (MC-LAG) when running in Active-Standby mode (prior to 12.3 and later releases).
Consider an MC-LAG scenario in which one link is active and the other is in standby mode (also known as Active-Standby mode). For more information, refer to the following links:
Note: When the active link fails, it switches to the standby link. When the active link is reestablished, it is not automatically reverted.
Initial state - on the active node:
root@MX240-16-re0# run show interfaces mc-ae Member Link : ae0 Current State Machine's State: mcae active state Local Status : active < Local State : up Peer Status : standby Peer State : up Logical Interface : ae0.0 Topology Type : NULL Local State : up Peer State : Unknown Peer Ip/MCP/State : Unknown
Initial state - on the standby node:
root@MX960-138-RE1# run show interfaces mc-ae Member Link : ae0 Current State Machine's State: mcae standby state Local Status : standby < Local State : up Peer Status : active Peer State : up Logical Interface : ae0.0 Topology Type : NULL Local State : up Peer State : Unknown Peer Ip/MCP/State : Unknown
Active link fails (here, the failure of the active link is simulated by disabling AE on the active node):
[edit] root@MX240-16-re0# set interfaces ae0 disable [edit] root@MX240-16-re0# commit commit complete
On the initial active node:
root@MX240-16-re0# run show interfaces mc-ae Member Link : ae0 Current State Machine's State: mcae standby state Local Status : standby < Local State : down Peer Status : active Peer State : up Logical Interface : ae0.0 Topology Type : NULL Local State : up Peer State : Unknown Peer Ip/MCP/State : Unknown root@MX960-138-RE1# run show interfaces mc-ae Member Link : ae0 Current State Machine's State: mcae active state Local Status : active < Local State : up Peer Status : standby Peer State : down Logical Interface : ae0.0 Topology Type : NULL Local State : up Peer State : Unknown Peer Ip/MCP/State : Unknown
Now the active link is reestablished:
[edit] root@MX240-16-re0# delete interfaces ae0 disable [edit] root@MX240-16-re0# commit commit complete
root@MX240-16-re0# run show interfaces mc-ae Member Link : ae0 Current State Machine's State: mcae standby state Local Status : standby < still in standby. not reverted. Local State : up Peer Status : active Peer State : up Logical Interface : ae0.0 Topology Type : NULL Local State : up Peer State : Unknown Peer Ip/MCP/State : Unknown
On the standby node:
root@MX960-138-RE1# run show interfaces mc-ae Member Link : ae0 Current State Machine's State: mcae active state Local Status : active < still standby. not reverted. Local State : up Peer Status : standby Peer State : up Logical Interface : ae0.0 Topology Type : NULL Local State : up Peer State : Unknown Peer Ip/MCP/State : Unknown
This behavior, as seen in the above example, is expected behavior as preemption is not supported with the MC-LAG feature on Junos OS releases prior to 12.3.
Configuration support for manual and automatic link switchover mechanisms on the multichassis link aggregation interface is mentioned in the 12.3 Release Notes, New Features, Layer 2 section:
http://www.juniper.net/techpubs/en_US/junos12.3/information-products/topic-collections/release-notes/12.3/index.html?topic-69609.html#jd0e9311
In the 13.3 docs and later, configuring revertive or nonrevertive mode is discussed here:
http://www.juniper.net/techpubs/en_US/junos13.3/topics/task/configuration/interfaces-configuring-mc-lag-link-switchover.html