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Is this Indicated there's the loop in NETWORKS

with this sample network design that include the 2 x 2960 switches as access and 1 x 3850  for intervlan with SVI routing between more than 40 VLANs , we have issue with Server connected to left 2960 Switch with PC connected to right Switch both these device in same VLAN (50) , and trunk there and allow in trunk , the problem appear recently and the problem is 

1- ping not stable after  some packet we get replay just 8 or 10 after that not reachable !

i try to decrease  the arp time aging  lower than Mac address table aging to avoid the Unicast flooding 

in switch 3850 we see the LED lighting quickly and rapidly

I look around the interface and INPUT ,OUTPUT ,Drop ,broadcast , in some interface in 2960 switch in have some of Packets and 3850 doesn't 

Is that seem that we have a Loop in Network , and from where start to investigate ?

thank you for help  

Mark Malone
VIP Mentor

I dont think you have a loop in that topology but you can check with show spanning-tree detail | inc ieee|occurr|from|is exec  and see if there are constant changes at switch level but your topology doesn't show a looped setup

can you post the interface where the server is connected to interface x/x

can you post the interface where the pc is connected to interface x/x

1- ping not stable after  some packet we get replay just 8 or 10 after that not reachable !

do you see this issue if running a ping from the 3850 too or just from the server to pc ?

Actually  this is the samples for our network that problem happens 

Thank you for replying  and help after i analysis the traffics cominng to core switch we notice there's a loop on switch #31 there's  port down stream that make this issues  and effect almost network

It's clear and easy to find it on access layer by looking to LED interface lights  but in core distribution very hard as the problem  happen in different VLAN and building 

Just remove the cable from switch port everything back to normal 

Thank you again 


whats that cable connected to , it should only cause a loop if its connecting to another switch ? or maybe a faulty nic on a device


Given that network I am interested just as Mark is how a loop has occurred!

Can you elaborate please?



kind regards

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