Prior to JUNOS 9.3, a VC (Virtual Chassis) split would result in both split parts electing their own primary and backup; thus start functioning as two individual VCs with identical configuration. This causes potential disastrous consequences for the entire network.   JUNOS 9.3 introduced an enhanced feature for VC, called 'split-detection', to prevent this potential network-wide outage.

• No more than one split part will remain active and operational running the original VC configuration.
• The other split part(s) will be “de-activated” and their switch operations ceased
  • All the VC members will act in “Linecard” role, no primary/backup
  • Can only be brought back “online” when:
    • Re-merged back to the “active” half, i.e. faults repaired
    • Manually “load factory-default”
  • Cannot function as a standalone switch or rejoin another VC until being “load factory-default”

• VC split triggers Primary Election in each of the split parts due to the VC topology change.
• Newly elected Primary in each split part makes the decision on whether the given split part should remain active or inactive.
• The decision is made based on where the “old” primary and backup switches of the last “stable state” end up after the split.
  • “Stable state” is defined as one in which all members are still connected together, i.e. “original” VC.
• The complete VC “break-up” rules and decision tree are as follows (for the first split):
  • If the “old” primary and backup of the last stable state end up in the same part/half, then this part wins.
  • If the “old” primary and backup of the last stable state end up in different parts, then the part with the most members wins.
  • In case the number of members is equal, then the part having the “stable state” old backup wins.
    • backup cannot distinguish a VCP link failure from the event of the primary crashing and the backup is supposed to take over the role of the primary if the primary crashes.

• Split detection feature is enabled by default in JUNOS 9.3.
• Disabling split-detection would cause VC split results, the same results as that found in versions prior to JUNOS 9.3.
  • Disable command:
    • juniper@vc# set virtual-chassis no-split-detection