Switch LOOPING

[jai.s401]

How to confirm if a switch is Looping.

what are the causes of switch loops & how to troubleshoot it.

[balaji.bandi]

A bridging loop or spanning-tree loop caused a network outage. Switch loops due to different ports connected to each other in access level - which causes loops and traffic interruption.

you can enable route guard to minimize the issue :

https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/10588-74.html

[Martin L]

Generally speaking there are 2 types of loops: L3 loops because of routing issues and L2 loops like STP, Broadcast storm,  In both cases, You should see high CPU cycles/utilization when switch is Looping.  L3 loops only happen on L3 switch types, while L2 loops can happen on L3 / L2 switch models.  L3 loops are a bit hard to spot. Basic way to see loops is with trace command.  Even when ping is OK, traceroute shows too many hops and repeating IP on the result.  This is even when routing table is stable and does not change.  Different types of L3 loops are when You see changes in routing table via show ip routing brief or status.  Number of routes in table changes frequently but does not have to for L3 loops to occur.  There is a command to enable ip routing profile to see overtime changes in routing.   Easy way to spot L2 loops is to notice that some or all traffic will be slow, switch performance is going down;  eventually if you do not have any loop-forming protections, traffic should stop before going again a bit then stop again, and on and on we go.   I think command to see CPU utilization is show cpu process;  L3 loops happen because of routing protocols configuration mistakes while L2 loops are because of wiring issues, STP configuration, MAC address spoofing and Security hacks. .  You will need a book on Troubleshooting networks for more details on loops.

Regards, ML
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[Giuseppe Larosa]

Hello @jai.s401 ,

>> How to confirm if a switch is Looping.

all  switch ports LEDs blinking very fast

the switch cannot be accessed remotely or you can access it and it is very slow in answer and the output of

show process cpu sorted 1min

you see CPU usage near to 100% and STP process is the main process causing CPU load.

user traffic can be almost stopped as a bridging loop lead to a broadcast storm:

if there is a L2 loop broadcast frames are multiplied and they eat all the bandwidth.

Most of bridging loops are caused by end users that can connect unmanaged switches to an access switch port  and then they can even close two ports of the unmanaged switch with a cable ....

In other cases there can be an issue with a switch.

The first thing to do is to achieve access to the network  by unplugging some inter switch links or even power off a distribution switch

( this was true in old times before the introduction of VSS and other forms of virtualization stacks and SVL)

Hope to help

Giuseppe