Switches loop

NORBERTO RIVERAE · ‎05-16-2014

Hi, I have a new fiber connection to close my network, but when I tray to connect my Build 3 switch with my Build 2 switch, it does a loop and I need to unplug the fiber, this is the configuration on that ports

Build 2, WS-C4503 switch

interface GigabitEthernet1/2
switchport trunk encapsulation dot1q
switchport mode trunk
speed nonegotiate
spanning-tree portfast disable

Build 3, WS-C3560G-48TS-S

interface GigabitEthernet0/50
switchport trunk encapsulation dot1q
switchport mode trunk
speed nonegotiate
spanning-tree portfast disable

I attahced a diagram ilustrated the new conection between build 2 and build 3.

I hope you can help me with my configuration

Leo Laohoo · ‎05-16-2014

Ummmm ... You really are not providing much information. From the diagram, the 3560CG has another link going to another switch (upper right-hand corner and down to another one above your 4503). So I really can't make any comment until we know what the config of those two boxes are.

If you are so much concerned about the loop, which rightly you should, then you may have to consider temporarily disabling one of the two links.

NORBERTO RIVERAE · ‎05-19-2014

Hi Leo

This are the fiber ports configuration from the build 3 switch (WS-C3560)

1.- To core switch upper right corner (WS-C4506)

     interface GigabitEthernet0/49
    switchport trunk encapsulation dot1q
      switchport mode trunk
      spanning-tree portfast disable

2.- Same rack WS-C3560, under yellow circle

     interface GigabitEthernet0/51
   switchport trunk encapsulation dot1q
   switchport mode trunk

3.- To production area in build 3 (WS-C3560) left lower corner

interface GigabitEthernet0/52
switchport trunk encapsulation dot1q
switchport mode trunk

4.- This is the port that I want to connect to build 2 Ws-C4503, but does a loop

      interface GigabitEthernet0/50
   switchport trunk encapsulation dot1q
   switchport mode trunk
   speed nonegotiate
   spanning-tree portfast disable

The configuration from the core switch (WS-C4506) to Build 3 are:

interface GigabitEthernet2/5
switchport trunk encapsulation dot1q
switchport mode trunk
spanning-tree portfast disable

And there is the configuration from Build 2 WS-C4503 that has a loop

interface GigabitEthernet1/2
switchport trunk encapsulation dot1q
switchport mode trunk
speed nonegotiate
spanning-tree portfast disable

The other port from Build 2 that connect to other WS-C4503 are

interface GigabitEthernet1/1
   switchport trunk encapsulation dot1q
   switchport mode trunk
   spanning-tree portfast disable

I Attached partial LAN diagram with connections descriptions

Leo Laohoo · ‎05-19-2014

You know what I noticed about your interface configuration? You've allowed ALL VLANs across.

Exactly what VLANs are you trying to traverse from Office Plant #3 to Office Plant #2 (and back)? If you really want, you can configure allowed-vlan on the interface.

NORBERTO RIVERAE · ‎05-20-2014

Hi Leo

I will tray this on weekend, only that I have time to test it, and does not affect production, I will tell you about the results