Switch Restarting issue

[aslulam555]

haii,

I have three switches in my network, I configure the vlans in cisco ws 3850 and i tagged these all vlans in to other two switches is cisco sg 500,The uplink was connected from the each port of cisco ws 3850 to other 2 switches, The network is perfectly but the problem is when i restart the cisco sg 500 switch the communication other two switches also lose, Pls give me a solution for this

[Georg Pauwen]

Hello,

Make sure the 3850 is the root for all your Vlans. On the 3850, configure:

spanning-tree vlan x root primary

where x is the Vlan

[aslulam555]

The problem is for only one cisco sg 500 switch when i restart the other
switch the communitation of other switches is not gone##- Please type your
reply above this line -##
--
*Regards,*

*Aslam N S*
*+971508152171*

[Georg Pauwen]

Hello,

from the 3850, post the output of:

show spanning-tree vlan x

for all Vlans...

[balaji.bandi]

This looks for me spanning tree issue, but good to verify the config, please post the 3850 config for reference.

If you have not set the STP priority it default to - The default priority for switches is 32768.

So suggest to keep the lower priority for required VLAN , 3850 to be root bridge, like 8192 something and test.

you can also do other way @Georg Pauwen suggested.

[Andrew Khalil]

@aslulam555

It’s simply because the switch that is restarted is considered the spanning-tree root for the network, to fix this issue you can configure the following in the 3850 switch:

#spanning-tree vlan x root primary

or to minimize the priority for each vlan by using the following command: 

#spanning-tree vlan x priority 0

(x represents each configured vlan, so you should repeat that command for each vlan or to issue “,” in between vlans if it’s accepted in your iOS)

you may wonder why the network decided to elect that restarted switch to be the root? The answer is that the election of the root follows the following sequence:

1- The least configured priority, if it’s not configured like in your case,

2- Then the least MAC address, the most probably that this restarted switch has the lowest value of MAC address that’s why it was elected to be the root, you can check that by issuing the following command on the 3850:

#show spanning-tree vlan x

and then you will see the roots MAC is the address of the restarted one and the bridges MAC address as well will be the address of the 3850 itself, while if you issue the same command on the restated switch you Will find:

“this bridge is the root” Which’s simply describing what’s mentioned above!

Please don’t forget to rate all helpful responses and mark solutions!

Bst Rgds,

Andrew Khalil