Allowed vlan remove 2-19 ...., found after reboot on SG350 Cisco Switch

[AhmadZ]

Hi guys,

i have configured an SG350 Cisco switch, with 10 ports on it as trunk ports, these ports are connected to an ip phone and then to pc.

what have i done in details, is that i configured the ports with this config:

switchport trunk allowed vlans all

and i have vlan 10 and 20 with dhcp, 10 for data and 20 for voice, all works perfectly, i get vlan 20 on the ip phone and vlan 10 on the pc, but  after a reboot to the switch, the pc wasnt getting ip and was checking the port running config, and i found this additional command that i didnt add

switchport trunk allowed vlan remove 2-19,21-4094

how is this coming automatically, am i missing something? or it is a bug?

Thanks in advance

[marce1000]

 - Hmm, to rule out situations of irregular management or sometimes that was forgotten (in both directions) between managing the running and startup config. Safe a good and or intended running-config again. Reboot ->Then check if the problem persists.

 M.

[AhmadZ]

every time i do anything on the switch, i do a wr at the end, so i am actually saving everything, why is this happening at every reboot?

[Richard Burts]

You have asked multiple times "why is this happening" and we are trying to find an answer to that question. I suggested starting over and you tell us this is not feasible because there are users already active in this network. I am wondering if the users are already active but there is this problem, then how well are they working?

But for the moment I accept that you can not start over again for vlans 10 and 20. So is there a possibility to use a few ports on the switch that are currently not used, and to start the process of configuring 2 vlans (perhaps 15 and 25) and trunks and as you do this every time you make a configuration change that you do a show run and identify at what point the unwanted line shows up in the config?

[AhmadZ]

after every reboot, to get things working i make a command on the interfaces that have the problem

switchport trunk allowed vlan all

and this gets everything back to normal, but i dont want this to happen everytime on a reboot

[Richard Burts]

I understand that you do not want this to happen every time that the switch reboots. Until we can figure out what is causing this we can not know what to do to prevent it. It appears that there is something about vlan 10 that the switch does not consider valid. I have made 2 suggestions about how to investigate this. If you do not want to do either of those approaches perhaps if you provide more detail from the configuration we might be able to find something in it.

[AhmadZ]

i have read other discussions, they say we have to use the vlan remove except for our vlans, i have tried this command:

switchport trunk allowed vlan remove 2-9,11-19,21-29,31-4094

these are my vlans, 1,10,20 and 30.

i restarted the switch and still the same problem,

but i realized something different in the discussions configuration, they also use for example :

switchport trunk native vlan 99

what will this command do?, and how to use it in my situation

[Richard Burts]

You ask what does the command switchport trunk native vlan 99 do and that is fairly easy to answer. First let us do some remembering of basic concepts about trunks:

- the default for a switch port is to be an access port and an access port carries a single vlan on it.

- a trunk is a switch port that carries multiple vlans.

- when a port is carrying multiple vlans then each frame has a tag that indicates which vlan it belongs to.

- for compatibility with devices that do not understand and do not support frames with tags one vlan sends frames with no tag and this vlan is called the native vlan.

- by default the native vlan (which carries no tag) is vlan 1. But it is possible to make some other vlan into the native vlan.

- the command that you ask about specifies that for this interface frames in vlan 99 will not have tags and frames in all other vlans will have tags.

You also ask how you might use this command in your situation. That is harder to answer without knowing more about your environment. But at this point I do not believe that this command would help solve the problem that you are experiencing.

You are indicating that you are experimenting with the switchport trunk allowed vlan remove command. That is an interesting approach. I would suggest that you experiment instead with changing the switchport trunk allowed vlan and instead of specifying all just specify that you want to allow 10, 20, and 30

[Richard Burts]

The symptoms described suggest that there was something about vlan 10 that the SG350 did not like (or did not accept). I would suggest this approach to troubleshooting:

- restore interface configuration to default.

- restore vlan configuration to default.

- configure vlans 10 and 20. 

- configure interfaces as trunk.

- check running config and verify that the unwanted statement is not present.

- save running config to startup config.

- check startup config and verify that the unwanted statement is not present.

- reboot the switch.

- check the running config.

[AhmadZ]

i can't reconfigure because there are users already on the network, and can't cut their work, but why is this problem?

[Samer R. Saleem]

Hi Ahmed,

try another thing, allow the vlans you need only instead of "All"

save configs and reboot, see if this keeps the configs.

[AhmadZ]

thanks, will try this, but i just want to know, why is this problem happening?

[AhmadZ]

i tried this before, and still the same problem

[Richard Burts]

The discussion suggests that the problematic command gets inserted when the switch reboots (things are working normally, the switch reboots, and the problematic command is in the config). Can you verify that this is the case? Can you check the startup config and verify that the command is not there, and then reboot and see if the command is there after boot?

I also find it interesting that now you tell us that you have vlans 10, 20, and 30. In the beginning of the discussion we understood that you had 2 vlans and one worked and one had the problem. Now is it the case that one works and two have the problem?

[AhmadZ]

i checked everything is alright, it's just when i reboot the switch, i get these commands added automatically, all is working all vlans are working, but the ports have the trunk mode, are having the problems