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Cisco Bug: CSCvs91165 - Cat4500 STP not stable when receive multicast traffic more than 12G

Last Modified

Jun 18, 2020

Products (1)

  • Cisco Catalyst 4000 Series Switches

Known Affected Releases


Description (partial)

Cat4500 series chassis with SUP8-E and WS-X4712-SFP+E linecard installed
There are multiple 10G links terminated on this linecard
If there are multiple multicast sources on these 10G links connected on WS-X4712-SFP+E linecard sending at high rate [over 12Gbps] STP topology might reconverge frequently due to loss of STP BPDUs

Cat4k switch with WS-X4712-SFP+E linecard
This linecard is 2,5:1 oversubscribed.
Each 3 front panel SFP+ ports are belonging to the same port-group.
48Gbps backplane bandwidth is split evenly into each of the 4 port-groups [12Gbps per port-group]
This means that 3 SFP+ front panel interfaces [3x10Gbps] have 12Gbps connection to the backplane, hence 2,5:1 oversubscription.
If there is more than 12Gbps in total arriving on a SFP+ ports belonging to the same port-group then due to oversubscription traffic loss will occur and if STP BPDUs are lost it will result in STP instability and frequent reconvergence
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