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Unicast Flood impact

 

Hi All,

 

I was interested in knowing the impact of unicast flooding in network. In a situation where the mac table entries timeout earlier than the arp cache entries and all traffic for those hosts are unicast flooded in that vlan, will this kind of unicast flood forwarding be handled by the ASIC or will it be punted to the CPU causing the CPU util to rise ?

 

Thanks in Advance 

 

Regards

Umesh

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Posting

I believe it's hardware forwarded, so the issue isn't so much an impact to the L3 switches themselves, but to all the links/ports carrying the needless traffic.  Consider a host setting traffic at full gig rate.  This traffic would be replicated onto every link/port, basically turning your switch into a hub.

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Super Bronze

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The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

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In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

I believe it's hardware forwarded, so the issue isn't so much an impact to the L3 switches themselves, but to all the links/ports carrying the needless traffic.  Consider a host setting traffic at full gig rate.  This traffic would be replicated onto every link/port, basically turning your switch into a hub.

Community Member

 Thnx Joseph,Suppose this

 

Thnx Joseph,

Suppose this switch that is flooding the unicast is the secondary collapsed core with HSRP standby interfaces. All backup uplink trunks from the access switches are terminated to this core. When this core floods the unicast it will also flood it to these backup trunks on which the particular vlan in question is allowed.  At  the access switch this backup uplink will be either in blocking if STP is used or in Flex link backup mode,  however. This would mean the switch will discard all the packets received on this interface. Will the access switch be able to discard these packets in hardware or will it be punted to the CPU to take the decision to discard or allow.

 

Thanks in Advance

 

Regards 

Umesh

Super Bronze

DisclaimerThe Author of this

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

I believe the blocking will also be done in hardware.  One possible hardware performance issue that flooding might cause, the switch has to replicate the packet to multiple ports.  Depending on the switch architecture, this might cause some issues.

 

For example, the original 3560/3750 series has a 32 Mbps fabric.  Suppose a one gig ingress stream enters one of a 3560G's gig ports.  If the stream is replicated to all the other ports, at ingress, just that one stream might create more than 40 Gbps of egress traffic that needs to transit the fabric.

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