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Virtual Chassis Enhancement in JUNOS 9.3



Article ID: KB13855 KB Last Updated: 11 Apr 2009Version: 1.0
Prior to JUNOS 9.3, a VC (Virtual Chassis) split would result in both split parts electing their own master 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.
In JUNOS 9.3, a VC split would result in:
  • 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 master/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”

Which split part remains active and functional?
  • VC split triggers Master Election in each of the split parts due to the VC topology change.
  • Newly elected Master 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” master 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” master and backup of the last stable state end up in the same part/half, then this part wins.
    • If the “old” master 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 master crashing and the backup is supposed to take over the role of the master if the master crashes.

How to enable VC split-detection feature in JUNOS 9.3?
  • 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
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