Trunk connection issue on SG500XG-8F8T switch

[Sergey Lubenkov]

Hello,

I am trying to configure trunk connection between cisco SG500XG-8F8T and cisco 2960s switches.

However, as soon as I am typing command "switchport mode trunk" on 2960S - the SG500XG-8F8T switch immediately lose connection on all interfaces.

Interface config on switches:

sw(config)#interface gi0/1
sw(config-if)#switchport mode trunk
sw(config-if)#switchport trunk native vlan 500
sw(config-if)#no shut
sw(config-if)#exit

Please advice what could be the cause of problem.

Thank you,

Sergey

[John Saour]

Hi There,

I think you have a native vlan mismatch you have to make sure that vlan 500 configured as native vlan on both switches.

Also you are missing the encapsulation to dot1q 

switchport encapsulation dot1q.

Thanks

[Sergey Lubenkov]

Thanks, but that cannot be the "native VLAN mismatch"...

Please see my Interface config commands: [..switchport trunk native vlan 500..],

Also "encapsulation dot1q " command is not required on Cisco 2960S switch because it's default encapsulation....as for the 500XG switch - it is only one supported encapsulation.

[John Saour]

switches use native VLAN to form trunk connection, DTP, so I meant you need to make sure that you have it on both trunk ports , cisco SG500XG-8F8T and  Cisco 2960S

Also you can try the following command on both trunk ports "switchport nonegotiate". this command will force the port to go to trunk mode immediately without negotiation.

[Sergey Lubenkov]

Hey John,  

I used the same set of commands on both switches:

"sw(config)#interface gi0/1
sw(config-if)#switchport mode trunk
sw(config-if)#switchport trunk native vlan 500" .

Also,  cisco SG500XG-8F8T switch doesn't support DTP.

[Florin Barhala]

This is definitely  a trunk negotiation issue. Is there any command on the SG500 similar to show interface .... switchport or show interface .... trunk ?

This way you could see the default negotiation parameters enabled on the SG500's interface.

[Sergey Lubenkov]

Hi Florin,

The CLI is similar as on 2960, and it shows TRUNK (everything is correct)

Thanks,

 Sergey

[Sergey Lubenkov]

...Finally, I've found the cause of that issue and made it work.

Although, it doesn't make any sense for me but at least it works.

OK, another port  'te2/1' which is access port and connected to the Distribution layer switch had a native VLAN mismatch. I saw that, but didn't pay attention because there is a non-trunk connection.

However, as soon as I changed native vlan for the  'te2/1' access port, everything have started work fine.

 So, the problem is solved, but I would be much appreciate if anybody can explain what is going on .

(How native VLAN mismatch on access port can affect trunk connection on another ports?)

As for me, I tend to think  this is a glitch in OS of SG500XG-8F8T switch.

Thanks

[Tom Watts]

Hi Sergey, it sounds the problem is one switch transmitted a tag frame while the other switch expected untag VLAN frame. Therefore you would increment "ingress discard". The ingress discard is from ingress filtering mechanism which is standard across all vendor platforms that utilize VLANS.