Dynamic access and static access

[nnn_sss22]

Hello,

When i put this command in privilege mode

show interface fa1/3 switchport

then i see two things there

1- Administrative Mode= dynamic auto

2-Operational Mode= static Access

what is mean of both. thanks

Regards

[Reza Sharifi]

Hi,

2-Operational Mode= static Access

This means the port is an access port and not a trunk port

1- Administrative Mode= dynamic auto

This is for DTP negotiation, which means in this mode if this port is connected to another switch and both sides are configured as "dynamic auto" the port will never become a trunk port. 

If the port is connected to an end device, the the administrative mode will be  "static Access" as well.

For more info on DTP negotiations, refer to table-3 in this link:

http://www.ciscopress.com/articles/article.asp?p=2181837&seqNum=8

HTH

[nnn_sss22]

Hello,

I read this article and understand this thing. please see image and tell me my understanding is ok. if i have some dought then clear it. thanks

[Reza Sharifi]

Hi,

Yes, you understanding is correct. Once you set it manually then that is dot1q. If you want it to work dynamically then you would set the port as auto or desirable using command:

switchport mode dynamic auto

or

switchport mode dynamic desirable

Also, note that in the newer IOS versions, there is no more "switch port trunk encapsulation dot1q" any more. The new command is simply "switch port mode trunk" and that is by default dot1q.

In the older IOS versions, Cisco devices supported dot1q and ISL for trunking but Cisco eliminated ISL and now it is only dot1q and that is why the command is just "switch port mode trunk"

HTH

[nnn_sss22]

Hello,

You means, if i am using new switches then i will use single command. 

switchport mode trunk

dont need to put this command  switch port trunk encapsulation dot1q (Automatically this encapsulation protocol will be run ) right?

[Reza Sharifi]

Hi,

dont need to put this command  switch port trunk encapsulation dot1q (Automatically this encapsulation protocol will be run ) right?

Yes, that is correct.

HTH

[nnn_sss22]

Now i am confuse . if we put this command in new switch 

switchport mode trunk 

then automatically dot1q protocol will run.

if we put this command switchport mode dynamic desirable then dtp protocol will run automatically and it will use also dot1q encapsulation protocol.

what is the difference both of them. bcz if you put static entry of trunk or put dynamic desirable, in both case dot1q will be run

[Reza Sharifi]

If you have 2 switches connected together (sw-a and sw-b) if sw-a is configured as trunk and sw-b is configured with switchport mode dynamic desirable sw-b will negotiate with the neighbor and become trunk as well.

HTH

[nnn_sss22]

If we have multiple vlan and we create dynamic desirable link then three protocols will run.

1- 802.1q (for vlan tagging)

2- DTP (for encapsulation)

3- dot1q (this is subprotocol of encapsulation and DTP will use this protocol for encapsulation ). right?

.....................................................................................................................

If we have native vlan on both switch and we create dynamic desirable port then also three protocols will run.

1-802.1q (but not vlan tagging) is it corect? or 802.1q will not run?

2- DTP

2- dot1q

right?

..............................................................................................................

if we have have multiple vlans and create trunk port then two protocols will run,

1- 802.1q

2- dot1q

right?

...........................................................................................................

If we have native vlan on both switch and create trunk port then two protocols will run.

1- 802.1q (there will not be vlan taggs attached )  or 802.1q will not run. please correct it

1- dot1q

right?

...........................................................................................................

If i have something wrong please correct it. thanks

[nnn_sss22]

Hello Reza,

I am waiting for your response. Thanks

[Reza Sharifi]

Hi,

802.1Q is an IEEE standard  and it is the same as dot1q.  

DTP is used for protocol negotiation on the link to make the port a trunk port.

Here is more info on dot1q

https://en.wikipedia.org/wiki/IEEE_802.1Q

HTH

[nnn_sss22]

Hello,

802.1Q it can use as a tagging protocol but 802.1Q does not encapsulate the original frame.

If 802.1Q does not encapsulate the original frame then why we use this command switchport trunk encapsulation dot1q

thanks

[Reza Sharifi]

Hi,

Correct.  What is why in the older IOS, the command was "switchport trunk encapsulation dot1q" and now it is just trunk which is also dot1q since there is no more ISL.

HTH

[nnn_sss22]

Hello

I read this article https://en.wikipedia.org/wiki/IEEE_802.1Q . there are wrote some lines which are below.

802.1Q it can use as a tagging protocol but 802.1Q does not encapsulate the original frame.

If 802.1Q does not encapsulate the original frame then why we use this command switchport trunk encapsulation dot1q. 

If we use new IOS then it is true that no need to put this command but on backend this command is working automatically.

thanks

[Reza Sharifi]

Hi,

Here is good link with additional information explaining both ISL and 802.1q.

IEEE 802.1Q Frame

IEEE 802.1Q uses an internal tagging mechanism which inserts a 4-byte tag field in the original Ethernet frame itself between the Source Address and Type/Length fields. Because the frame is altered, the trunking device recomputes the FCS on the modified frame.

link:

http://www.cisco.com/c/en/us/support/docs/lan-switching/8021q/17056-741-4.html

HTH