Looping in Layer 2 switch 2960

[prince.p]

Hi friend,

I want to know to to find network looping in Layer 2 Cisco 2960 series switch

We are using two Cisco 2960 series switch in this we connected firewall dmz interface in one of the switch interface.i want to how how to find looping in our Cisco switch.can any one please helpe to find

[Julio E. Moisa]

Hi

You could verify the logs and the spanning-tree behavior, below you will find a guide line to find L2 loops:

https://supportforums.cisco.com/document/54376/spanning-tree-loop-troubleshooting-and-safeguards

I don't think the firewall could created a layer 2 loop, Im assuming it is working as routed firewall, could you please provide more details how it is affecting your network?

:-)

[prince.p]

Hi Julio,

We are using sonicwall firewall.our wan IP address is 182.75.***.****/28.in this last 10 IP address we assigned for DMZ Zone and from firewall DMZ interface, to Cisco switch 2960 series we connected cat6 cable in interface Gi0/29.most of the time our sonicwall CPU utilization is showing high. when i talk with sonicwall customer care they said we should use l3 switch for connecting DMZ interface . and if i connect DMZ wan interface in L2 switch .it will get looping in the network so i need to find any looping in our network.

Tri-Sw1#sh spanning-tree detail | i ieee|occurr|from|is exec

VLAN0001 is executing the ieee compatible Spanning Tree protocol
Number of topology changes 1809 last change occurred 00:20:51 ago
from GigabitEthernet0/48
VLAN0002 is executing the ieee compatible Spanning Tree protocol
Number of topology changes 15 last change occurred 1w6d ago
from GigabitEthernet0/1
VLAN0003 is executing the ieee compatible Spanning Tree protocol
Number of topology changes 10 last change occurred 3w0d ago
from GigabitEthernet0/21
VLAN0004 is executing the ieee compatible Spanning Tree protocol
Number of topology changes 10 last change occurred 3w0d ago
from GigabitEthernet0/21
VLAN0005 is executing the ieee compatible Spanning Tree protocol
Number of topology changes 10 last change occurred 3w0d ago
from GigabitEthernet0/21

[Mark Malone]

whats connected to this port 1800 changes last one 20 minutes ago ?

Number of topology changes 1809 last change occurred 00:20:51 ago
from GigabitEthernet0/48

[Julio E. Moisa]

There are a lot of changes on the port 48, have you made any change recently?

[prince.p]

We didn't made any changes in that port.from that port we connected with another Cisco switch.

[Mark Malone]

is that changing constantly like that in stp ?

is it trunk port both sides or access port between the switches , you shouldn't be seeing that change constantly if its a switch to switch setup , are those 2 switches looped on another 2 ports at all in SHOW CDP NEI  , that could cause it to flap at l2

[prince.p]

both the switch we configure 

interface GigabitEthernet0/48

switchport trunk allowed vlan 1-5

switchport mode trunk.

[Mark Malone]

is the port still flapping in stp ,  is the timer still resetting itself constantly for g0/48 ?

[prince.p]

now the port status is 

Tri-Sw1#sh spanning-tree detail | i ieee|occurr|from|is exec
VLAN0001 is executing the ieee compatible Spanning Tree protocol
Number of topology changes 1862 last change occurred 00:26:18 ago
from GigabitEthernet0/48
VLAN0002 is executing the ieee compatible Spanning Tree protocol
Number of topology changes 15 last change occurred 2w0d ago
from GigabitEthernet0/1
VLAN0003 is executing the ieee compatible Spanning Tree protocol
Number of topology changes 10 last change occurred 3w1d ago
from GigabitEthernet0/21
VLAN0004 is executing the ieee compatible Spanning Tree protocol
Number of topology changes 10 last change occurred 3w1d ago
from GigabitEthernet0/21
VLAN0005 is executing the ieee compatible Spanning Tree protocol
Number of topology changes 10 last change occurred 3w1d ago
from GigabitEthernet0/21

[Mark Malone]

still flapping is it another cisco switch connected to it off port 48 , get onto that switch and run the same command again see where the change is originating from on that switch

[Julio E. Moisa]

Have you see any alert on the logs or high CPU on the switches?

[Julio E. Moisa]

Could you please provide the config of both ports? Have you defined what switch will be the root and the backup root?

[prince.p]

Hi friend ,

below i mention both switch configuration

switch 1

Tri-Sw1#sh spanning-tree detail | i ieee|occurr|from|is exec
VLAN0001 is executing the ieee compatible Spanning Tree protocol
Number of topology changes 2100 last change occurred 00:01:57 ago
from GigabitEthernet0/48
VLAN0002 is executing the ieee compatible Spanning Tree protocol
Number of topology changes 31 last change occurred 02:24:01 ago
from GigabitEthernet0/48
VLAN0003 is executing the ieee compatible Spanning Tree protocol
Number of topology changes 22 last change occurred 02:24:01 ago
from GigabitEthernet0/48
VLAN0004 is executing the ieee compatible Spanning Tree protocol
Number of topology changes 22 last change occurred 02:24:01 ago
from GigabitEthernet0/48
VLAN0005 is executing the ieee compatible Spanning Tree protocol
Number of topology changes 22 last change occurred 02:24:01 ago
from GigabitEthernet0/48

swithch 2

sw-2#
sw-2#sh spanning-tree detail | i ieee|occurr|from|is exec
VLAN0001 is executing the ieee compatible Spanning Tree protocol
Number of topology changes 63 last change occurred 00:19:08 ago
from GigabitEthernet0/43
VLAN0002 is executing the ieee compatible Spanning Tree protocol
Number of topology changes 9 last change occurred 02:42:08 ago
from GigabitEthernet0/17
VLAN0003 is executing the ieee compatible Spanning Tree protocol
Number of topology changes 7 last change occurred 02:42:08 ago
from GigabitEthernet0/17
VLAN0004 is executing the ieee compatible Spanning Tree protocol
Number of topology changes 7 last change occurred 02:42:08 ago
from GigabitEthernet0/17
VLAN0005 is executing the ieee compatible Spanning Tree protocol
Number of topology changes 7 last change occurred 02:42:08 ago
from GigabitEthernet0/17

[prince.p]

From Gi0/48 we connected to another Cisco 2960 series switch