Trunks and access ports

Hey Guys,

I just have a question in regards to trunk ports.

So I've always understood it that Switches activly try to trunk with other switches when you plug two switches together. How come this doesn't show up in #sh int trunk
This would imply to me that they have not trunked?

Also I did a test and I made the ports between two switches an access port and connected them via a crossover and traffic still got through, how is this possible? I'm just a bit confused by this.

Note that I did this in packet tracer and havn't attempted this on the live equipment yet...

thanks

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Comments

JeanM

Looks like ports by default don't trunk in PT. If you switch the port to trunk then it will show.

switchA(config-if)#switchport mode trunk
%LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/24, changed state to down
%LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan1, changed state to down
%LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/24, changed state to up
%LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan1, changed state to up
switchA(config-if)#exit
switchA(config)#exit
switchA#
%SYS-5-CONFIG_I: Configured from console by console
sh int tr
switchA#sh int trunk
Port Mode Encapsulation Status Native vlan
Fa0/24 on 802.1q trunking 1
Port Vlans allowed on trunk
Fa0/24 1-1005
Port Vlans allowed and active in management domain
Fa0/24 1
Port Vlans in spanning tree forwarding state and not pruned
Fa0/24 none
switchA#

SwitchB#
%LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/24, changed state to down
%LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan1, changed state to down
%LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/24, changed state to up
%LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan1, changed state to up
SwitchB#sh int tr
SwitchB#sh int trunk
Port Mode Encapsulation Status Native vlan
Fa0/24 auto n-802.1q trunking 1
Port Vlans allowed on trunk
Fa0/24 1-1005
Port Vlans allowed and active in management domain
Fa0/24 1
Port Vlans in spanning tree forwarding state and not pruned
Fa0/24 1
SwitchB#

Chitownjedi

alliasneo wrote: »

Hey Guys,

I just have a question in regards to trunk ports.

So I've always understood it that Switches activly try to trunk with other switches when you plug two switches together. How come this doesn't show up in #sh int trunk
This would imply to me that they have not trunked?

Also I did a test and I made the ports between two switches an access port and connected them via a crossover and traffic still got through, how is this possible? I'm just a bit confused by this.

Note that I did this in packet tracer and havn't attempted this on the live equipment yet...

thanks

Still studying all this stuff myself, so here is my shot at answers.
1. The interfaces are not set to dynamic desirable so they aren't trying to actively create a trunk when they are connected. They are probably set to no negotiate or dynamic auto which would respectively not allow a trunk, or only try to trunk if being asked to be another switch. I believe you can check what administrative mode is with a show interface fa "interface number" switchport

Also if the encapsulations don't match they will not trunk.

2. The switches have a native vlan, which allows traffic to flow between the switches without a trunk being needed, trunks are needed to pass non native vlan traffic. If you did not create separate vlans and place all of the active interfaces into those vlan, those still in the native vlan should be able to have traffic flow over the crossover cable without much ceremony. Experts correct me if I'm WRONG!!!

lantech

I think the ports on newer switches are defaulted to dynamic desirable which means they should automatically negotiate the trunk.

dead_p00l

Some one please correct me if I'm wrong. All ports by default are access ports with membership to VLAN1. so if you plug to switches back to back they will communicate via the default VLAN. In order to pass multiple VLANs would require trunk ports which would have to be configured as such.

lantech

See table 8-7 of this document. It states that the default interface mode of the 2950 is dynamic desirable.

Cisco 2950

vishaw1986

Hey Alliasneo

Trunk :- we use trunk link to carry MULTIPLE VLAN traffic .

Access:- Access Port can carry single VLAN traffic .

So , when we connect two switches through access port e.g access port vlan 2 on both end , then host in VLAN 2 in one switch can communicate to host in VLAN 2 in another switch .

And when we connect the two switches through trunk port then its access is turned off .

DTP (dynamic truncking protocol ) is used for trunking with mode

Dynamic desirable : Ask for negotiations
Dynamic Auto : Listen to another end . (with both end auto no trunk )
On : Forces the link into permanent trunking .

or we can set the trunk through unconditional manner by command :

switchport mode trunk .