Vlan Flapping between ports

[harsha shankariah]

Hi, While I am getting Vlan Flapping on LAN network and seeing regular logs on my core switch - 2. I tried to troubleshoot but not able to succeed. Below is the details attached for understanding and support needed. I have attached network diagram also for better understanding. Vlan 221 is Flapping between port 1/21 and 1/43 Vlan 130 is Flapping between port 1/21 and 1/43

[Mark Malone]

What type of STP are you running on the core switches ?

you may have a spanning-tree loop , are you sure access sw1 and sw2 are not connected directly other than through the core sw2 switch , check show cdp neighbor to be sure a cable hasnt been connected as that could cause a loop

sw1 has 2 links 1 to each core is one of these blocking for vlan 221 ?

whats does this command show if you run it on core sw2

show spanning-tree detail | inc ieee|occurr|from|is exec

on core sw2 run this command to see where the TCN changes are constantly coming from

[harsha shankariah]

Hi Mark, Thanks for the reply and below are my observation and answer to your question. 1. We are running PVST and Access SW 1 and 2 are connected to Hub below and have uploaded diagram in which you will get more clarity. 2. Yes, SW1 has 2 uplinks and root port is towards Core-SW2 and STP blocked port is towards Core SW1. 3. For executing command I am waiting for customer approval Observation: I have physically verified that there are few HUB and access point connected to access switch and also there are other cisco 2960 switches uplinked. As you rightly updated I am suspecting STP loop. When is trace the MAC address its identified these MAC are learnt by Access Point which are connected to these switches. Please find the updated diagram attached for your reference.

[Mark Malone]

yes so if the hubs are set like that shouldn't be a huge problem as there single homed
that command i posted should narrow it down to where the tcns are and show you where the loop is originating if its constantly flapping
there should also be information on the logs on sw2 core relating to stp changes which could help

i would also check that your customer has the switches configured correctly for stp and the priorities are set correctly and nothing irregular stands out from their setup