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I need to connect my lab L3 switch (3750) to a remote router (interface 192.168.100.1). So I was enabled a port on a lab that (through cabling and intermediate switches) goes to the router. I checked this port by connecting a PC within 192.168.100.0 network and pinging the router => Ok it works. Problem is if I connect the same port to the switch (on which I have set a switchport access port and an interface vlan on the 192.168.100.0 network).

The interface goes up/up and down/down continuously.

While it's up/up I can see it's a-full/a-1000 and launching several show command on spanning tree I see

#sh spanning-tree vlan 2

Spanning tree instance(s) for vlan 2 does not exist.

#sh spanning-tree vlan 2

Spanning tree instance(s) for vlan 2 does not exist.

#sh spanning-tree vlan 2

VLAN0002
  Spanning tree enabled protocol rstp
  Root ID    Priority    32770
             Address     0021.d7c5.9080
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32770  (priority 32768 sys-id-ext 2)
             Address     0021.d7c5.9080
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi1/0/22         Desg BLK 4         128.22   P2p

#sh spanning-tree vlan 2

Spanning tree instance(s) for vlan 2 does not exist.

#sh spanning-tree vlan 2

Spanning tree instance(s) for vlan 2 does not exist.

so it seems like a spanning-tree problem...but on the other side the port is configured as access, there are no rings for vlan 2 and portfast is disabled.

How to troubleshoot/solve this?

Thanks in advance for any help.

EDIT: configuration for g1/0/22 is simply

switchport mode access
switchport access vlan 2
switchport nonegotiate

I don't have access to the configuration of the other end switch. HOWEVER, I changed nothing, the owner of the other end switch also said he changed nothing BUT now it works (!) as expected.

The port goes in FWD and I can ping the remote router. Mystery.

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  • You really should edit your question to show the configurations for both this switch, and the switch to which you are connecting it, and be sure to indicate which port on each switch is connecting to the other switch.
    – Ron Maupin
    Jul 8 '16 at 6:54
  • How often your interface Gi1/0/22 swaps between up/down states? It seems like switch on other end has spanning-tree bpduguard enable command enabled. Feb 9 '17 at 11:26
  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin
    Aug 8 '17 at 17:01
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Try Rapid Per-vlan spanning tree protocol. I suspect that you are using multiple VLANs in your network. RSTP is not for multiple VLANs networks. I will edit this when I'd some more research.

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Say no switchport on the 3750 interface. Put an ip address directly on the interface.

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  • This is interesting. I always thought that setting an interface vlan for a point to point L3 link is kinda intricate. In case of a point to point L3 link what are the pros/cons of putting the ip address directly on the interface or using the interface vlan?
    – kuma
    Jul 13 '16 at 18:05
  • @kuma this is a good question, worth putting it as an actual question on this site ;)
    – JFL
    Feb 6 '18 at 9:07
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I would recommend PVST, also the reason that it is going up and down is obvious...portfast, you have a misconfiguration that is causing flapping.

Alsoo as they have indicated above, no switchport command applied to the interface would be correct. I would have just applied something like:

interface gi1/0/22
  switchport access vlan 2
  switchport mode access
  spanning tree portfast 
  spanning tree bpduguard enable

Also, here is a helpful reference from the Cisco site which is a comprehensive resource for the 3750 series along with most other major campus level switching products.

http://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/5234-5.html?referring_site=RE&pos=3&page=http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst4000/8-2glx/configuration/guide/stp_enha.html

Lastly,

This article from Cisco is great for understanding all of the fine details of STP, troubleshooting it and configuration.

http://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/10556-16.html?referring_site=RE&pos=2&page=http://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/5234-5.html

Best

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