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CPU Congestion


Userlevel 1
Hi all,

From time to time and randomly on my network some stacked switches become unresponsive and very sluggish, my monitoring software shows that those stacks are down and if i ping them i have a RTT of about 5000ms.

When i log onto to the stack and run the cmd "debug hal show congestion" it shows congestion on all slots. to identify the type of traffic that is congestioning the CPU i run the cmd "show l2stats vlan [vlan]" several times to see which counter is incrementing and i see that the broadcast and multicast are the ones to blame.

I am wondering if anyone has experienced this problem to and how did you troubleshoot it?

3 replies

Have we ruled out a loop? Are you running ELRP?

We have seen this caused by MDNS on a customers network. Fortunately in their case they were not using the MDNS so we were able to block that and things improved.
Userlevel 1
Hi, I have to rule out loop because the switch has only one uplink, also if there was a loop I should see mac flapping, which I dont.
Userlevel 5
Hello,

If you run a "show igmp snooping cache" Do you see a lot of ports reporting on the following addresses?
224.0.0.251 (bonjour/MDNS)
239.255.255.250 (SSDP)
239.255.255.253 (SLP)
224.0.0.252 (LLMNR)

If so, we may need to block some of that traffic, if it isn't being used.

Vi block_mdns.pol

entry slp {

if {

destination-address 239.255.255.253/32;

} then {

deny;

count slp;

}

}



entry bonjour-mdns {

if {

destination-address 224.0.0.251/32;

} then {

deny;

count bonjour-mdns;

}

}

entry llmnr {

if {

destination-address 224.0.0.252/32;

} then {

count llmnr;

deny;

}

}

entry ssdp {

if {

destination-address 239.255.255.250/32;

} then {

count ssdp;

deny;

}

}


:wq to exit

apply to the edge ports on ingress to prevent the packets from even coming into the network at all.

Thanks
Brad

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