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Cisco Bug: CSCvu06851 - Unknown IGMP GSQ are flooded and replicated at CPU for all ports in vlan

Last Modified

Aug 31, 2020

Products (1)

  • Cisco Catalyst 3850 Series Switches

Known Affected Releases

16.9.5

Description (partial)

Symptom:
High CPU seen from excessive replication (Inject) of IGMP GSQ. 

For every 1 GSQ thee will be N number of outbound copies made by CPU. So if there are 100 ports in the vlan we would see 1 in > 100 out replication of IGMP GSQs

Conditions:
IGMP GSQ arrives at switch that does not have the group programmed in snooping table. This causes switch to flood the packet. 

Example:
> Send in GSQ for 239.193.2.2 (NOT 2.1)

F340.26.08-3850-1#sh ip igmp snooping groups
Vlan      Group                    Type        Version     Port List
-----------------------------------------------------------------------
25        239.193.2.1              igmp        v2          Gi1/0/1 <-- only 2.1 here

F340.26.08-3850-1#sh spanning-tree vlan 25

Interface           Role Sts Cost      Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/0/1             Desg FWD 4         128.1    P2p  <-- 2.2 GSQ flood
Gi1/0/4             Desg FWD 4         128.4    P2p  <-- 2.2 GSQ flood

/// CPU pcap ///
F340.26.08-3850-1#sh mon capture 2 buffer br | i 239.193.2.2
744   0.022653    10.25.1.8 -> 239.193.2.2  IGMPv2 60 Membership Query, specific for group 239.193.2.2 <-- 1 in
745   0.022675    10.25.1.8 -> 239.193.2.2  IGMPv2 60 Membership Query, specific for group 239.193.2.2 <-- 2 out 
746   0.022696    10.25.1.8 -> 239.193.2.2  IGMPv2 60 Membership Query, specific for group 239.193.2.2
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