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switchport question

Hi. If a port on a switch is not configured with a switchport mode access command and only with a switchport access vlan 10 for example, what exactly are the repercussions of this particular config on the port?

Thanks in advance.

1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Bronze

Re: switchport question

The Administrative mode will be set to 'dynamic desirable or dynamic auto' depending on the switch.

You can verify this status with the 'show interface fx/x switchport' command.

A dynamic desirable mode switchport will automatically trunk with another switch while a dynamic auto mode switchport will only trunk with another switch if this remote switch is set to dynamic desirable.

Setting the access-vlan without entering the 'mode access' will associate that port to such vlan when the switchport negotiates an access mode. If you connect to another switch, a trunk will be made, based on the requirements explained above, and the trunk native vlan will be used which by default is Vlan 1.

So in short, avoiding the 'mode access' can potentially create connectivity issues as you are leaving the switch to negotiate some important parameters.

HTH,

__

Edison.

6 REPLIES

Re: switchport question

It might possibly treat it as a trunk port allowing only VLAN 10, just a guess though, firing up a switch now to see...but I did research this question.....

From:

http://www.cisco.com/en/US/products/hw/switches/ps700/products_tech_note09186a008015bfd6.shtml

"Note: If the output of the show interface switchport command displays the port as a trunk port even after you configure the port as an access port with the switchport access vlan command, issue the switchport mode access command in order to make the port an access port."

Community Member

Re: switchport question

Hi. Thanks for your response. I also checked a live switch and discovered that it is not defaulted to a trunk port when i ran the sh int trunk command. I think it in fact defaults to an access port.

Re: switchport question

You can also set it to trunking, then set it sw ac vl xx (10 in this case) and it will show as a trunk port, but also as a access port for vl 10...weird.

S1(config-if)#sw mo tr

S1(config-if)#sw ac vl 10

S1(config-if)#do sh int fa0/3 sw

Name: Fa0/3

Switchport: Enabled

Administrative Mode: trunk

Operational Mode: down

Administrative Trunking Encapsulation: dot1q

Operational Trunking Encapsulation: dot1q

Negotiation of Trunking: On

Access Mode VLAN: 10 (VLAN0010)

Community Member

Re: switchport question

Yes it is weird. But i think since it is a trunk then it should forward all vlans by default. So i guess the trunk characteristic of the port supersedes the access mode function since a trunk is like a multi-access port.

Hall of Fame Super Bronze

Re: switchport question

The Administrative mode will be set to 'dynamic desirable or dynamic auto' depending on the switch.

You can verify this status with the 'show interface fx/x switchport' command.

A dynamic desirable mode switchport will automatically trunk with another switch while a dynamic auto mode switchport will only trunk with another switch if this remote switch is set to dynamic desirable.

Setting the access-vlan without entering the 'mode access' will associate that port to such vlan when the switchport negotiates an access mode. If you connect to another switch, a trunk will be made, based on the requirements explained above, and the trunk native vlan will be used which by default is Vlan 1.

So in short, avoiding the 'mode access' can potentially create connectivity issues as you are leaving the switch to negotiate some important parameters.

HTH,

__

Edison.

Re: switchport question

Excellent explanation, .... thank you.

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