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Cisco Bug: CSCtc40859 - IPSG - Incorrect TCAM programming when 2 hosts swap their DHCP addresses

Last Modified

Mar 01, 2018

Products (1)

  • Cisco Catalyst 6000 Series Switches

Known Affected Releases


Description (partial)


Machine1 has IP X.
Power down machine1, power up machine 2 and assign the same IP X.
Power down machine2, power up machine 1 and assign the same IP X.
Power up machine2 and assign IP Y.

Before machine 2 is powered up again, IP X is associated with both machine1 and machine2. When machine2 eventually gets assigned IP address Y as its address, it is seen by switch as machine2 moving from address X to address Y. So TCAM entry of X is removed and programmed with Y. 

But at this point we need to also check if address X is associated with some other host (in this case machine1) before we purge it. This is the bug - machine1 has no entry in the TCAM and no traffic passes until further DHCP traffic is seen.


IPSG enabled which adds IP addresses learned by DHCP into the TCAM.
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