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Switchport Mode Access question

After looking through security documents it is suggested that you specify what type of port a switchport will be (access or trunk).  Right now all of my switches are set to auto-negotiate, and it looks like a majority of my ports are access ports.  If a switchport is set to auto-negotiate and is negotiated as a trunk will that show up under the "show interfaces trunk" command, or does that only show ports that are specified as trunks?  I just want to make sure I don't set a switchport as access if it's actually a trunk.  Thank you!

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Accepted Solutions
Cisco Employee

Re: Switchport Mode Access question

Hi Ben,

As other friends have noted already, the default factory setting of ports on lower-end Cisco Catalyst switches is a dynamic mode. The dynamic mode allows a port to adapt to its peer's configuration. There are two submodes to the dynamic mode: the desirable mode prefers the formation of a trunk and has a higher priority, while the auto mode prefers the formation of an access link and has a lower priority. When two dynamic ports negotiate the operating mode of a link, two desirable ports end up creating a trunk, two auto ports end up creating an access link, and a desirable + auto combination also ends up in a trunk link because the desirable mode is more preferred. If any of the two ports is configured statically, the other dynamic port adapts itself to the static port's mode.

Confusion is sometimes caused by the fact that different switch platforms default to different dynamic modes. The 2950 and 3550 Catalysts, for instance, default to dynamic desirable mode. The 2960, 3560 and 3750 default to dynamic auto mode. Therefore, connecting different types of switches may end up in links being differently negotiated, however, always to the rules described above.

Best regards,

Peter

4 REPLIES
New Member

Switchport Mode Access question

Hi  Ben,

Yes it will show as a trunk when you so sh int trunk.

To hard core switch as access ports use the command

switchport mode access

Regards

Mahesh

Cisco Employee

Re: Switchport Mode Access question

Hi,

You need to look for DTP negotiation as said.

Example:

SW1--F0/1 ----------F0/1---SW2

If I configured the one side to be desirable and other side to be auto the port negotiate and bring up the trunk.

SW1(config-if)#int f0/1

SW1(config-if)#switchport mode dynamic desirable

SW1#

%SYS-5-CONFIG_I: Configured from console by console

sh int tr

Port        Mode         Encapsulation  Status        Native vlan

Fa0/1       auto         n-802.1q       trunking      1

Port        Vlans allowed on trunk

Fa0/1       1-1005

Port        Vlans allowed and active in management domain

Fa0/1       1

Port        Vlans in spanning tree forwarding state and not pruned

Fa0/1       none

SW1#

SW2#sh int tr

Port        Mode         Encapsulation  Status        Native vlan

Fa0/1       auto         n-802.1q       trunking      1

Port        Vlans allowed on trunk

Fa0/1       1-1005

Port        Vlans allowed and active in management domain

Fa0/1       1

Port        Vlans in spanning tree forwarding state and not pruned

Fa0/1       1

HTH

Regards

Inayath

********Plz rate if this info is helpfull **************

New Member

Re: Switchport Mode Access question

By default all port are configured as switchport dynamic desirable mode hence they always try to negotiate with other side.

Best idea would be just change those port as access  port by giving below command

Switchport mode access

Thanks,

HP

Cisco Employee

Re: Switchport Mode Access question

Hi Ben,

As other friends have noted already, the default factory setting of ports on lower-end Cisco Catalyst switches is a dynamic mode. The dynamic mode allows a port to adapt to its peer's configuration. There are two submodes to the dynamic mode: the desirable mode prefers the formation of a trunk and has a higher priority, while the auto mode prefers the formation of an access link and has a lower priority. When two dynamic ports negotiate the operating mode of a link, two desirable ports end up creating a trunk, two auto ports end up creating an access link, and a desirable + auto combination also ends up in a trunk link because the desirable mode is more preferred. If any of the two ports is configured statically, the other dynamic port adapts itself to the static port's mode.

Confusion is sometimes caused by the fact that different switch platforms default to different dynamic modes. The 2950 and 3550 Catalysts, for instance, default to dynamic desirable mode. The 2960, 3560 and 3750 default to dynamic auto mode. Therefore, connecting different types of switches may end up in links being differently negotiated, however, always to the rules described above.

Best regards,

Peter

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