Today I came across a customers 3750 switch stack which had a flapping stackwise link. The stackwise link was transitioning up/down around 3 times a second and causing massive issues with connectivity and EIGRP routing for the site.
Previously I believed that I would need to physically remove the Stackwise cable in order to restore service by shutting the flapping link. It seems it is possible to shut the Stackwise port from the CLI although it is done from enable mode rather than Configure terminal.
The command is..
Switch#switch 1 stack port 1 ?
disable Disable stack port
enable Enable stack port
The first number 1 would indicate the switch number in the stack and the second number 1 after the port is the Stackwise port number you want to shut. Make a note of which switch and port you shut as it will not show up in the config or the show outputs which could prove tricky when you want to reenable it..
You can determine the status of the ports using the command below but not how is just shows the port as down. There is no indication that it has been manually shut down.
Switch#show switch stack-ports
Switch # Port 1 Port 2
-------- ------ ------
1 Ok Ok
2 Down Ok
3 Ok Down
On thing to note about Stackwise is that it utilises Bidirectional flow. "Packets are allocated between two logical counter-rotating paths. Each counter-rotating path supports 16 Gbps in both directions, yielding a traffic total of 32 Gbps bidirectionally. The egress queues calculate path usage to help ensure that the traffic load is equally partitioned. When a break is detected in a cable, the traffic is immediately wrapped back across the single remaining 16-Gbps path to continue forwarding."
Effectively Stackwise cables are always in use (they do not leave one link in standby) if you remove one of the cables in the loop you reduce the total throughput to 16Gbps.
You can determine if the stack is running in Full or Half mode by using the following command.
Switch#show switch stack-ring speed
Stack Ring Speed : 16G
Stack Ring Configuration: Half
Stack Ring Protocol : StackWise
Previously I believed that I would need to physically remove the Stackwise cable in order to restore service by shutting the flapping link. It seems it is possible to shut the Stackwise port from the CLI although it is done from enable mode rather than Configure terminal.
The command is..
Switch#switch 1 stack port 1 ?
disable Disable stack port
enable Enable stack port
The first number 1 would indicate the switch number in the stack and the second number 1 after the port is the Stackwise port number you want to shut. Make a note of which switch and port you shut as it will not show up in the config or the show outputs which could prove tricky when you want to reenable it..
You can determine the status of the ports using the command below but not how is just shows the port as down. There is no indication that it has been manually shut down.
Switch#show switch stack-ports
Switch # Port 1 Port 2
-------- ------ ------
1 Ok Ok
2 Down Ok
3 Ok Down
On thing to note about Stackwise is that it utilises Bidirectional flow. "Packets are allocated between two logical counter-rotating paths. Each counter-rotating path supports 16 Gbps in both directions, yielding a traffic total of 32 Gbps bidirectionally. The egress queues calculate path usage to help ensure that the traffic load is equally partitioned. When a break is detected in a cable, the traffic is immediately wrapped back across the single remaining 16-Gbps path to continue forwarding."
Effectively Stackwise cables are always in use (they do not leave one link in standby) if you remove one of the cables in the loop you reduce the total throughput to 16Gbps.
You can determine if the stack is running in Full or Half mode by using the following command.
Switch#show switch stack-ring speed
Stack Ring Speed : 16G
Stack Ring Configuration: Half
Stack Ring Protocol : StackWise
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