Vlan and Trunk Port

[bapatsubodh]

hi,

If the following configuration is done on any swicth and it is connected to another switch port insted of any host.

Will it negotiate as a trunk port ? What configuration is required on the second swicth so that it can negotiate as a trunk if it is possible.

int gig 0/0

switchport access vlan 2

Only this much is the configuration is done on first switch.

Please share the experinece.

Thanks

Subodh

[Edison Ortiz]

bapatsubodh wrote:
hi,
If the following configuration is done on any swicth and it is connected to another switch port insted of any host.
Will it negotiate as a trunk port ? What configuration is required on the second swicth so that it can negotiate as a trunk if it is possible.
int gig 0/0
switchport access vlan 2
Only this much is the configuration is done on first switch.
Please share the experinece.
Thanks
Subodh

It can potentially negotiate as trunk if the connected device is also a switch.

The answer depends on the default mode on either switch.

If both switches are 'dynamic auto', then they won't form a trunk

If one of the switches are dynamic desirable', then a trunk will be form. The same holds true for both switches being 'dynamic desirable'.

Here is a sample from a 3560 switch:

Rack1SW1#sh int g0/7 switchport | i Administrative Mode
Administrative Mode: dynamic auto

Regards

Edison

[Giuseppe Larosa]

Hello Subdoh,

default settings for IOS based switches should be dynamic desirable this allows them to start DTP negotiation and to successfully negotiate a trunk.

If VTP is used DTP checks the vtp domain name and allows the trunk to be set if the name matches.

to be honest this is often a source of problems in real world because this can cause a trunk to be built when not desired and it is often overlooked.

to setup a trunk between two switches in different VTP domains you need to use

switchport mode trunk

so that a port trunks unconditionally regardless of DTP negotiation results.

if you want to configure ports that stay in access mode regardless of what is connected to them it is wise to add:

switchport mode access

Hope to help

Giuseppe

[Edison Ortiz]

giuslar wrote:
Hello Subdoh,
default settings for IOS based switches should be dynamic desirable

This default setting isn't the same in all switches. Access layer switches (35xx) default to dynamic auto.

Core|Distribution layer switches default to dynamic desirable.

[Giuseppe Larosa]

Hello Edison,

thanks for your correction.

I had in mind C6500s and some cases happened when they have been moved from CatOS to IOS: unwanted trunks.

This can still happen nowdays if this aspect is not considered.

Hope to help

Giuseppe