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Cisco Bug: CSCuq21372 - 4500-X VSS Drops DHCP Offer through VNET Trunk on L3 Etherchannel

Last Modified

Nov 27, 2020

Products (150)

  • Cisco 2600 Series Multiservice Platforms
  • Cisco Catalyst 2960X-24PS-L Switch
  • Cisco Catalyst 2960S-F48FPS-L Switch
  • Cisco Embedded Service 2020 24TC NCP B Switch
  • Cisco Catalyst 2960C-12PC-L Switch
  • Cisco IE 2000-16TC-G Industrial Ethernet Switch
  • Cisco Catalyst 3560X-48U-S Switch
  • Cisco Catalyst 2960X-24PD-L Switch
  • Cisco Catalyst 2960X-48LPS-L Switch
  • Cisco Catalyst 3560CG-8TC-S Compact Switch
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Known Affected Releases

15.2(1)E 15.2(2)E

Description (partial)

DHCP Negotiation fails when passing through a 4500-X VSS Pair utilizing IP Helper, Easy Virtual Network (EVN), DHCP Snooping, and ARP Inspection functions in combination. DHCP Offers from the server will be dropped.

The DHCP Server offers must arrive on the EVN VNET trunk interface. If they arrive on the standby 4500-X switch in the VSS, they will be dropped. If they arrive on the Active 4500-X switch, they will be forwarded and DHCP will work. Depending on ether-channel load-balancing, this behaviour will flip. 

VNET Trunks must be configured, as well as IP Helper (DHCP Relay), DHCP Snooping and ARP Inspection on the VLAN where the clients DHCP Discover packet initially arrives.
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