Cisco C2960X is in spanning tree loop with serial server

Taj2021 · ‎02-06-2025

Hello I need some help. I have a Cisco C2960X switch and a Quatech ESE-100D serial server and cant seem to get them to connect. below is my config. My computer is plugged into port 28 and ip set to 192.168.10.2 and the Quatech serial server is plugged into port 27 and its ip is set to 192.168.10.12. If I connect the server and Quatech directly, it works fine but soon as the switch is involved I get a constant link up and down. I think this is link flap. I would love to hear some opinions. Thank you!

spanning-tree mode rapid-pvst
spanning-tree loopguard default
spanning-tree extend system-id
!
!
!
!
!
vlan configuration 2
no ip igmp snooping
vlan internal allocation policy ascending
!
vlan 2
name PRIMARY

!
interface GigabitEthernet1/0/27
description to Quatech Terminal Server
switchport access vlan 2
switchport mode access
switchport nonegotiate
ip access-group SymphonyNetwork in
spanning-tree bpduguard enable
spanning-tree guard root
!
interface GigabitEthernet1/0/28
description to Dell Server
switchport access vlan 2
switchport mode access
switchport nonegotiate
ip access-group SymphonyNetwork in
spanning-tree bpduguard enable
spanning-tree guard root

!
interface Vlan2
ip address 192.168.1.200 255.255.255.0
no ip redirects
no ip unreachables
no ip proxy-arp
!
!
ip tcp synwait-time 10
no ip http server
ip http banner
ip http authentication local
ip http secure-server
ip http timeout-policy idle 600 life 86400 requests 86400
ip ssh time-out 60
ip ssh version 2
ip ssh server algorithm mac hmac-sha2-256 hmac-sha2-512
ip ssh server algorithm encryption aes256-ctr aes192-ctr aes128-ctr
ip ssh client algorithm mac hmac-sha2-256 hmac-sha2-512
ip scp server enable
!
ip access-list extended Management
deny icmp any host 192.168.10.157 fragments
deny ip any any option any-options
deny ip 239.8.0.0 0.0.255.255 any
deny ip any any log-input
ip access-list extended SymphonyNetwork
permit ip host 192.168.10.12 any log-input
permit ip host 192.168.10.2 any log-input
deny icmp any host 192.168.2.157 fragments
deny ip any any option any-options
deny ip 239.8.0.0 0.0.255.255 any
deny ip any any log-input
!
!
end

marce1000 · ‎02-08-2025

- Can you post logs from this spanning tree loop,

M.

-- Each morning when I wake up and look into the mirror I always say ' Why am I so brilliant ? '
When the mirror will then always repond to me with ' The only thing that exceeds your brilliance is your beauty! '

Richard Burts · ‎02-08-2025

I notice some fairly Cisco specific features in the configuration such as spanning-tree mode rapid-pvst. When you introduce a non Cisco device I suggest that you make your configuration more general and less Cisco specific. And along that line I wonder about some restrictions such as switchport nonegotiate and wonder if there were more flexibility if it might improve the situation.

And while I do not believe that it relates to the posted problem I question the configuration of layer 3 functions (such as ip access-group SymphonyNetwork in) on layer 2 interfaces.

HTH

Rick

Taj2021 · ‎02-10-2025

If I connect the Serial Server to the management port, the serial server comes up immediately but it does not on port 27. I have tried three different spanning-tree modes and they all have the same result. I have also removed switchport nonegotiate and that didn't help either. I have also reset the switch to factory settings and that didn't help either. I can get the serial server working with the cisco switch, if I connect the serial server directly to my laptop first (serial server makes handshake with laptop) then unplug lan cable from laptop and plug it into the cisco switch. As long as I don't power cycle the cisco switch or the serial server the connection stays up. Soon as I power cycle either, I loose the connection.

Note: I do not have this issue when I use the serial server with Cisco C1000 switch with the default config.

Taj2021 · ‎02-10-2025

I get a constant link flap but here is what I see:

Aug 16 02:41:25: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet1/0/27, changed state to down
Aug 16 02:41:26: %LINK-3-UPDOWN: Interface GigabitEthernet1/0/27, changed state to down
Symphony-Switch#sh spanning-tree detail
Aug 16 02:41:29: %LINK-3-UPDOWN: Interface GigabitEthernet1/0/27, changed state to up
Aug 16 02:41:30: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet1/0/27, changed state to up
Symphony-Switch#sh spanning-tree detail

VLAN0002 is executing the ieee compatible Spanning Tree protocol
Bridge Identifier has priority 32768, sysid 2, address 38ed.18b3.5e80
Configured hello time 2, max age 20, forward delay 15
We are the root of the spanning tree
Topology change flag not set, detected flag not set
Number of topology changes 4 last change occurred 00:03:14 ago
from GigabitEthernet1/0/27
Times: hold 1, topology change 35, notification 2
hello 2, max age 20, forward delay 15
Timers: hello 1, topology change 0, notification 0, aging 300

Port 25 (GigabitEthernet1/0/25) of VLAN0002 is designated forwarding
Port path cost 19, Port priority 128, Port Identifier 128.25.
Designated root has priority 32770, address 38ed.18b3.5e80
Designated bridge has priority 32770, address 38ed.18b3.5e80
Designated port id is 128.25, designated path cost 0
Timers: message age 0, forward delay 0, hold 0
Number of transitions to forwarding state: 1
Link type is point-to-point by default
Bpdu guard is enabled
Root guard is enabled on the port
BPDU: sent 1208, received 0

Port 27 (GigabitEthernet1/0/27) of VLAN0002 is designated listening
Port path cost 19, Port priority 128, Port Identifier 128.27.
Designated root has priority 32770, address 38ed.18b3.5e80
Designated bridge has priority 32770, address 38ed.18b3.5e80
Designated port id is 128.27, designated path cost 0
Timers: message age 0, forward delay 9, hold 0
Number of transitions to forwarding state: 0
Link type is point-to-point by default
Bpdu guard is enabled
Root guard is enabled on the port
BPDU: sent 2, received 0

paul driver · ‎02-10-2025

Hello

@Taj2021 wrote:
If I connect the server and Quatech directly, it works fine but soon as the switch is involved I get a constant link up and down.

Is your server performing any tagging?
at first instance suggest remove the acl, change the vlan assiocation of those ports to another vlan, disable any ip routing and test again...

conf t
no ip routing
int ran 1/0/27-28
switchport access vlan 1
no ip access-group SymphonyNetwork in

Please rate and mark as an accepted solution if you have found any of the information provided useful.
This then could assist others on these forums to find a valuable answer and broadens the community’s global network.

Kind Regards
Paul

Taj2021 · ‎02-11-2025

Hello, I tried this and did not work. I know that mac address gets tagged once I make the connection by connecting the Quatech to my laptop directly and then to the switch. The connection stays up and then the MAC address gets registered but if I power cycle the switch or the Quatech the mac address disappears. The Quatech is just not able to make the initial handshake for some reason. Don't have this issue with when I plug the Quatech on the management port on the cisco switch. The Quatech connects right away.

Joshqun Ismayilov · ‎02-10-2025

@Taj2021

You can check port state ##show spanning-tree interface GigabitEthernet1/0/27 detail (you can share details / if possible.)
also / Some industrial devices (like the Quatech ESE-100D) might not support auto-negotiation properly.
Change interface setting and test again
#interface GigabitEthernet1/0/27
#speed 100
#duplex full

Thanks!

Taj2021 · ‎02-11-2025

Hello, I thought the same thing and tried that but didn't work. I reset the switch to start from scratch to see if just working with the Quatech issue would be easier. I am working with this config below and still have no luck. Note: The Quatech only works on speed 10 and 100 and does not on 1000. I tried both but the Quatech did not connect to the switch.

Building configuration...

Current configuration : 4418 bytes
!
! Last configuration change at 08:23:01 UTC Thu Aug 17 2000
! NVRAM config last updated at 08:19:34 UTC Thu Aug 17 2000
!
version 15.2
no service pad
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname Switch
!
boot-start-marker
boot-end-marker
!
!
no aaa new-model
switch 1 provision ws-c2960x-48fps-l
!
!
!
!
!
!
vtp mode off
!
!

!
spanning-tree mode pvst
spanning-tree extend system-id
!
!
!
!
vlan internal allocation policy ascending
!
vlan 2
name PRIMARY
!
vlan 10
name MGMT
!
vlan 666
name UNUSED
state suspend
!
!
!
!
!
!
!
!
!
!
!
interface FastEthernet0
no ip address
!
interface GigabitEthernet1/0/1
switchport access vlan 10
switchport mode access
!
interface GigabitEthernet1/0/2
!
interface GigabitEthernet1/0/3
!
interface GigabitEthernet1/0/4
!
interface GigabitEthernet1/0/5
!
interface GigabitEthernet1/0/6
!
interface GigabitEthernet1/0/7
!
interface GigabitEthernet1/0/8
!
interface GigabitEthernet1/0/9
!
interface GigabitEthernet1/0/10
!
interface GigabitEthernet1/0/11
!
interface GigabitEthernet1/0/12
!
interface GigabitEthernet1/0/13
!
interface GigabitEthernet1/0/14
!
interface GigabitEthernet1/0/15
!
interface GigabitEthernet1/0/16
!
interface GigabitEthernet1/0/17
!
interface GigabitEthernet1/0/18
!
interface GigabitEthernet1/0/19
!
interface GigabitEthernet1/0/20
!
interface GigabitEthernet1/0/21
!
interface GigabitEthernet1/0/22
!
interface GigabitEthernet1/0/23
!
interface GigabitEthernet1/0/24
!
interface GigabitEthernet1/0/25
!
interface GigabitEthernet1/0/26
!
interface GigabitEthernet1/0/27
switchport access vlan 2
speed 100
duplex full
!
interface GigabitEthernet1/0/28
!
interface GigabitEthernet1/0/29
!
interface GigabitEthernet1/0/30
!
interface GigabitEthernet1/0/31
!
interface GigabitEthernet1/0/32
!
interface GigabitEthernet1/0/33
!
interface GigabitEthernet1/0/34
!
interface GigabitEthernet1/0/35
!
interface GigabitEthernet1/0/36
!
interface GigabitEthernet1/0/37
!
interface GigabitEthernet1/0/38
!
interface GigabitEthernet1/0/39
!
interface GigabitEthernet1/0/40
!
interface GigabitEthernet1/0/41
!
interface GigabitEthernet1/0/42
!
interface GigabitEthernet1/0/43
!
interface GigabitEthernet1/0/44
!
interface GigabitEthernet1/0/45
!
interface GigabitEthernet1/0/46
!
interface GigabitEthernet1/0/47
!
interface GigabitEthernet1/0/48
!
interface GigabitEthernet1/0/49
!
interface GigabitEthernet1/0/50
!
interface GigabitEthernet1/0/51
!
interface GigabitEthernet1/0/52
!
interface Vlan1
no ip address
!
interface Vlan2
ip address 192.168.2.100 255.255.255.0
no ip route-cache cef
!
interface Vlan10
no ip address
shutdown
!
!
ip http server
ip http secure-server
!
!
!
!
line con 0
line vty 0 4
login
transport input ssh
line vty 5 15
login
transport input ssh
!
!
end

Switch#
Aug 17 08:26:45.385: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet1/0/27, changed state to down
Aug 17 08:26:46.388: %LINK-3-UPDOWN: Interface GigabitEthernet1/0/27, changed state to downsh spanning-tree interface vlan 2
no spanning tree info available for Vlan2

Switch#
Aug 17 08:27:48.426: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet1/0/27, changed state to down
Aug 17 08:27:49.425: %LINK-3-UPDOWN: Interface GigabitEthernet1/0/27, changed state to downsh spanning-tree interface gigabitEthernet 1/0/27

Vlan Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
VLAN0002 Desg LIS 19 128.27 P2p

Switch#
Aug 17 08:27:58.904: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet1/0/27, changed state to down
Aug 17 08:27:59.908: %LINK-3-UPDOWN: Interface GigabitEthernet1/0/27, changed state to downsh spanning-tree interface gigabitEthernet 1/0/27 detail
Port 27 (GigabitEthernet1/0/27) of VLAN0002 is designated listening
Port path cost 19, Port priority 128, Port Identifier 128.27.
Designated root has priority 32770, address 38ed.18b3.5e80
Designated bridge has priority 32770, address 38ed.18b3.5e80
Designated port id is 128.27, designated path cost 0
Timers: message age 0, forward delay 14, hold 0
Number of transitions to forwarding state: 0
Link type is point-to-point by default
BPDU: sent 0, received 0
Switch#

Richard Burts · ‎02-11-2025

I find this line from the posted output to be interesting

Port 27 (GigabitEthernet1/0/27) of VLAN0002 is designated listening

So Port 25 is participating in Spanning Tree and Port 27 is not. It may not be the only issue, but pretty clearly Spanning Tree is at least part of the issue.

HTH

Rick

Richard Burts · ‎02-11-2025

Each of the posted configs have included this "spanning-tree mode rapid-pvst". I suggest that you try a different spanning tree mode.

HTH

Rick

Taj2021 · ‎02-11-2025

Hello Rick,

I have tried spanning-tree mode pvst and the MSTP and it didnt work. Please see comment from ‎02-11-2025 08:17 AM

Richard Burts · ‎02-11-2025

I had looked through the discussion as I prepared my suggestion but apparently not as carefully as I should have, since I missed that specification of just pvst. I continue to believe that the issue is something specific about Cisco spanning tree and that device. But unfortunately I do not have any better suggestions at this point.

HTH

Rick