spanning-tree issue

[KDG_work]

hi all

In the same network configuration as the image, the C9300#2 equipment intermittently displays mac flapping logs, and the C9300#1 and C9300#2 become Root Bridges respectively.

At this time, ICMP communication from C9300#2 to adjacent switches does not communicate and does not register information with adjacent switches in Mac Table or ARP Table.

Similarly, ICMP communication from adjacent switches to C9300#2 is not possible.

If you restart C9300#2, the symptoms are taken care of, but I can't proceed with the restart every time, so I want to find the root cause and solve it.

Please help me.

※ Please note that the sensitive information in the attached "show run" file has been processed as #######.

※ STP Mode : rapid-pvst

[MHM Cisco World]

Show spanning tree summary <<- for all SW 

Show spanning tree <<- for all SW

MHM

[KDG_work]

The "show spanning tree summary" setting cannot be checked today, so I only send the "show spanning tree" setting, and it is in normal operation.

The "C9300-2 error" file in the attachment is "show spanning tree" and log at the time of failure.

[MHM Cisco World]

The mac is flapping between G13 and G22 <<- but show spanning there is no G22??

MHM

[KDG_work]

Sorry, I think I took out the g22 port and sent what I tested.
I'm sending it back to you.

#show span
VLAN0001
Spanning tree enabled protocol rstp
Root ID Priority 16385
Address 04bd.97af.d180
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 16385 (priority 16384 sys-id-ext 1)
Address 04bd.97af.d180
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec

Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Gi1/0/13 Desg FWD 200000 128.13 P2p
Gi1/0/14 Desg FWD 20000 128.14 P2p
Gi1/0/15 Desg FWD 20000 128.15 P2p
Gi1/0/21 Desg FWD 200000 128.21 P2p Edge
Gi1/0/22 Desg FWD 20000 128.22 P2p
Gi1/0/23 Desg FWD 20000 128.23 P2p
Gi1/0/24 Desg FWD 20000 128.24 P2p

[MHM Cisco World]

Use CDP to see which device connect to both G13 and G22

MHM

[KDG_work]

I turned off the cdp function as a security vulnerability measure, but I will check the G13, G22 ports as you advised. Thank you.

[MHM Cisco World]

If it other mgmt SW then share SPT from it 

If it unmanaged SW remove it

If it FW interface check mac address is it for VIP of FW 

MHM

[David Ruess]

Hello,

Are any of the devices connected to multiple switches with NIC teaming (port-channel on a PC)?

-David

[KDG_work]

No devices are configured for port channel.

[Elliot Dierksen]

Some devices will attempt to use multiple ports without using a port channel. I think that is where @David Ruess was going with his question.

Another thing that can cause MAC flaps is if there are multiple ports connected between two switches, but the spanning tree priority isn't consistent between them. An example would be SW1P1 -> SW2P2, SW1P2 -> SW2P1. SW1 would send traffic that SW2 would receive on P2, but SW2 would think P1 was the root port so it would block P2. Rather than change priorities, try to make sure you keep the lowest port # on one switch connected to the lowest port # on the other switch.

One other question, why not stack them? The 9300's should certainly be capable of stacking. That would make those two switches a single logical entity which would make spanning non-existent between them.

[KDG_work]

The reason why the stack was not configured is a configuration that the customer does not want, and other system configurations of the customer use it like that, so it is configured as it is now