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Cisco Bug: CSCur72866 - Stale mroutes lead to blackholing due to OIF not populated in hardware

Last Modified

Sep 20, 2019

Products (1)

  • Cisco Nexus 7000 Series Switches

Known Affected Releases

6.2(16) 7.1(0)D1(0.320) 7.1(0)N1(0.401)

Description (partial)

Traffic blackholing due to OIF not being programmed in egress module .

module-9# show system internal forwarding vrf <> multicast route source x.x.x.x group x.x.x.x detail instance 0
Hardware Multicast FIB Entries:
Flags Legend:
  * - s_star_priority
  S - sg_entry
  D - Non-RPF Drop
  B - Bi-dir route  W - Wildcard route
(x.x.x.x/32, 239.x.x.x/32), Flags: *S
  Dev: 0, HWIndex: 0xb9 Priority: 0x3d59, VPN: 10
  RPF Interface: Ethernet<>, LIF: 0x112
  MD Adj Idx: 0x152, MDT Idx: 0xca, MTU Idx: 0x4, Dest Idx: 0x2911
  PD oiflist Idx: 0xb6, EB MET Ptr: 0xa7
>>>>>>> NO OIF here

module-9# show system internal forwarding multicast met inst 0 met-index 0xa7
0xa7        0x0       0x0     0x15d 
0x15d       0x35      0xffff  0x0   >>>>> INCORRECT

- Race condition during many removal and re-adds of the same mroutes
- practical example of trigger resulting in the issue is moving L3 interfaces across VRFs (while mcast traffic still flows). Again this is effectively removing an mroute and re-adding it within few seconds, though this is just one practical example leading to the same symptom. 
- susceptibility to this issue increases with increased scale
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