trunk port

[arvindai]

If we put ports connecting two switches trunk and member of a vlan will the two switches pass traffic for all the vlans.

 

[VinceWhirlwind]

If the switchport configuration is something like:

!
interface GigabitEthernet2/0/51
switchport trunk encapsulation dot1q
switchport mode trunk
!

then traffic on all VLANs that are configured on *both* of the switches will pass.

If you have the following:

!
interface GigabitEthernet2/0/51
switchport trunk encapsulation dot1q
switchport trunk allowed vlan 1,2,3
switchport mode trunk
!

Only traffic for vlans 1,2,3 will pass.

 

[arvindai]

The interface configuration is as below

interface GigabitEthernet0/24
description Cross Connect to adjacent Switch
switchport access vlan 412
switchport trunk encapsulation dot1q
switchport trunk native vlan 412
switchport mode dynamic desirable
media-type internal
channel-group 2 mode desirable

 

[VinceWhirlwind]

Who configured that dog's breakfast?

Get rid of the "switchport access vlan 412" - assuming your design calls for a dot1q trunk, that bit is nonsense.

"switchport mode dynamic desirable" - why? Your design either calls for this to be a dot1q trunk or it doesn't - leaving it as "dynamic" is pointless. Change it to mode "trunk".

"switchport mode dynamic desirable" - why? You either want an etherchannel or you don't - that configuration is a recipe for unwanted effects, like the etherchannel not coming up because you don't realise one side isn't negotiating. Just configure it "on".

 

[arvindai]

I understand the configuration is not up to mark however I wanted understand if this port will pass traffic for all vlan or not and what effect this has when this port is member of a specific vlan.

 

[VinceWhirlwind]

OK, line by line:

This defines the VLAN tagging format:
"switchport trunk encapsulation dot1q"
(as opposed to ISL, the cisco-proprietary version of VLAN tagging.

This defines which VLAN is *not* tagged on the trunk:
"switchport trunk native vlan 412"
(if you don't have this line in there, then VLAN1 is assumed)

This defines which VLANs are allowed on the trunk:
"switchport trunk allowed vlan 1,2,3"
(if you don't have this line in there, then all VLANs are allowed)

This defines it as a VLAN trunk:
"switchport mode trunk"
(as opposed to "Access", which means there is no tagging and only one VLAN).

Bear in mind that the only VLANs passed by the trunk will be VLANs that are actually configured on the switch.

 

[arvindai]

this still leaves my question unanswered, is the configuration I pasted will work for trunk port and pass all vlan traffic that is configured on the switch. If not what would be the issue.

 

[vipergg]

Do you want it just a trunk or a trunked etherchannel ? The way you have it is set up for a trunked etherchannel (multiple ports aggregated together . If you just want a straight one port trunk just get rid of the "channel group command... and it will work fine allowing all vlans..

interface GigabitEthernet0/24
description Cross Connect to adjacent Switch
switchport access vlan 412
switchport trunk encapsulation dot1q
switchport trunk native vlan 412
switchport mode dynamic desirable
media-type internal--> have never seen this
channel-group 2 mode desirable

 

[arvindai]

Is etherchannel configuration restricting communication for all the vlans and Is the 'switchport trunk native vlan' necessary for the trunk configuration connecting switches. what effect this command puts in and what if I don't use this command.

 

[VinceWhirlwind]

On the etherchannel question, the answer is no.
On the second question, I answered that previously: the "native" VLAN is untagged on the trunk. If you leave the line out, then VLAN1 is by default the untagged VLAN.