Solved: Trunking configurations during upgrade

Ken S · ‎10-21-2021

Hello,

What VLANs should be passed if the following configurations are on a 3750 to 2960x?

3750 (version 12.2(25)SEB4)

interface GigabitEthernet1/0/10
switchport trunk encapsulation dot1q
switchport mode trunk

2960x (version 15.2(7)E4)

interface GigabitEthernet1/0/49
switchport trunk allowed vlan 1,45,46,70,90,91,95,96,120,150,151
switchport mode trunk

on a 3850 to 2960x

3850 (version 16.12.05b)

interface GigabitEthernet1/0/10
switchport mode trunk

I did have switchport trunk encapsulation dot1q on the 3850, but that is not allowed on it.

2960x (version 15.2(7)E4)

interface GigabitEthernet1/0/49
switchport trunk allowed vlan 1,45,46,70,90,91,95,96,120,150,151
switchport mode trunk

I appreciate any assistance.

mlund · ‎10-22-2021

Hi

I agree with Reza and Balaji, it is best to have same config on both side.

But as an answer to your question, the vlans 1,45,46,70,90,91,95,96,120,150,151 is the ones that will be function in this configuration.

If there are vlan on 3850 side that is not allowed on 2960 side, they will be sent over the trunk to 2960, but 2960 will drop them because they are not in the allowed list.

/Mikael

View solution in original post

Reza Sharifi · ‎10-21-2021

Hi,

On the 3750, match the config that is already on the 2960X as follows:

interface GigabitEthernet1/0/10
switchport trunk encapsulation dot1q
switchport mode trunk

switchport trunk allowed vlan 1,45,46,70,90,91,95,96,120,150,151

HTH

Ken S · ‎10-22-2021

Thank you @Reza Sharifi. I did want to make the question more complex. If the configuration stands as is, only the VLANs listed would be allowed correct?

It both sides had switchport trunk encapsulation dot1q, then all VLANs would be allowed through?

Reza Sharifi · ‎10-22-2021

Hi Ken,

only the VLANs listed would be allowed correct?

Yes, and that is best practice. You only want to allow what is needed and no more.

It both sides had switchport trunk encapsulation dot1q, then all VLANs would be allowed through?

That is correct.

HTH

Ken S · ‎10-22-2021

Except when I cutover the 3750 to 3850, it didn't seem the listed VLANs were being sent over. The phones and APs on the 2960X didn't come back up.

Thank you for the clarification on the switchport trunk encapsulation dot1q.

Reza Sharifi · ‎10-22-2021

Ken,

Except when I cutover the 3750 to 3850, it didn't seem the listed VLANs were being sent over. The phones and APs on the 2960X didn't come back up.

If you don't see the vlans sent over, check to make sure you actually have created all the vlans. Do a "sh vlan" and you should see what vlans you have created and what ports are assigned to each vlan.

HTH

Ken S · ‎10-22-2021

Interesting note, but I do believe the VLANs were created on both switches due to VTP. I actually had this 3850 connected to the network and trunked to my office switch to upgrade it. Also, I believe I was able to see another switch come up with phones, only when I added switchport trunk allowed vlan #,#,#.

I am sure that matching the allowed VLANs on both sides of the switches will work, but why didn't the original configuration for the 3850 and 2960X work? Do I need both

switchport trunk encapsulation dot1q

switchport mode trunk

to send VLANs over the trunk or just

switchport mode trunk?

Reza Sharifi · ‎10-22-2021

Do I need both

Depends on the IOS. In most older IOS, you had to specify "switchport trunk encapsulation dot1q" because you could use dot1.q or ISL as trunking (ISL is a Cisco proprietary protocol that is not being used anymore). On most newer IOS versions, dot1.q is the default, and the only trunking protocol and therefore you don't need to specify it anymore.

As for using VTP, I personally do not recommend it as it can be more harmful than helpful especially when you only have a few vlans to worry about.

HTH

Ken S · ‎10-22-2021

I am assuming that the IOS-XE has dot1q on my default and I am sure I verified that either on the 3850 I am working with or another one. So essentially I would say I do have both

switchport trunk encapsulation dot1q

switchport mode trunk

but the devices on the 2960X didn't come up.

VTP is another story. Thank you for the feedback.

Reza Sharifi · ‎10-22-2021

but the devices on the 2960X didn't come up

If all the vlans are present and assigned to the correct ports (show vlan) then check VTP, you may want to put VTP in "transparent mode" and see if the issue gets resolved. VTP is a pain.

HTH

balaji.bandi · ‎10-21-2021

is this 2 setup or 1 setup

I see you looking to connect 3750 to 2960 and 3850 to 2960 ?

always should match bot the side config.

if you like to control the vlan

you should go with both the side :

switchport trunk allowed vlan 1,45,46,70,90,91,95,96,120,150,151

If you like to allow all vlan then :

both the side: switchport mode trunk (should allow all vlan)

On 3850 new IOX XE code so that should be ok. you just configure the required vlan along with trunk should work

any issue post any logs.

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Ken S · ‎10-22-2021

Thank you @balaji.bandi. This is an upgrade. A 3750 is connected to a 2960X, but I want to replace the 3750 with the 3850.

That is what I read about matching both sides. It is good for many things, keeping good documentation is one of them.

When I cut over the 3750 to 3850, the interfaces with just switchport mode trunk didn't bring up the 2960X side. This is why I am asking this question. I could not add switchport trunk encapsulation dot1q to the 3850 as I think it might be removed.

I don't have any logs, but if this is a bug, I would be happy to work with Cisco.

balaji.bandi · ‎10-22-2021

I do not believe this is a bug :

you can verify this command, some command default in the interface

show interface GigabitEthernet1/0/10 capabilites

you see example output

Model: WS-C3850-48P
Duplex: full
Trunk encap. type: 802.1Q
Dot1x: yes

Since you mentioned outage. Now you need to test offline with out cutover

you can run 3750 and 3850 same time connecting to Cisco 2960 switch, test it before you do cutover, i do not see any Loops herer, so you should be good for testing standalone.

BB