Catalyst Switch: No trunks is formed

I have connected two Cisco Catalyst 3560 switches together by a pair of fibre cables. No trunk was formed.

The default interface configuration for Catalyst 3560 is dynamic auto, in Catalyst 3550 the default was dynamic desirable.

Dynamic desirable is actively negotiating a trunk with another cisco catalyst switch, whereas dynamic auto is passively waiting for another cisco catalyst switch to initiate the trunking negotiation. Base on this it is not hard to analyze this:

dynamic desirable + dynamic desirable = trunk

dynamic desirable + dynamic auto = trunk

dynamic auto + dynamic auto = no trunk

Sw2-3560#sh interface trunk

Sw2-3560#sh interface gi0/1 switchport
 Name: Gi0/1
 Switchport: Enabled
 Administrative Mode: dynamic auto
 Operational Mode: static access
 Administrative Trunking Encapsulation: negotiate
 Operational Trunking Encapsulation: native
 Negotiation of Trunking: On
 Access Mode VLAN: 1 (default)
 Trunking Native Mode VLAN: 1 (default)
 Administrative Native VLAN tagging: enabled
 Voice VLAN: none
 Administrative private-vlan host-association: none
 Administrative private-vlan mapping: none
 Administrative private-vlan trunk native VLAN: none
 Administrative private-vlan trunk Native VLAN tagging: enabled
 Administrative private-vlan trunk encapsulation: dot1q
 Administrative private-vlan trunk normal VLANs: none
 Administrative private-vlan trunk associations: none
 Administrative private-vlan trunk mappings: none
 Operational private-vlan: none
 Trunking VLANs Enabled: ALL
 Pruning VLANs Enabled: 2-1001
 Capture Mode Disabled
 Capture VLANs Allowed: ALL

Protected: false
 Unknown unicast blocked: disabled
 Unknown multicast blocked: disabled
 Appliance trust: none

Sw2-3560#sh interface trunk


As shown above no trunk was established.

Configure one of the switch to be dynamic desirable

Sw2-3560#conf t
Enter configuration commands, one per line. End with CNTL/Z.
Sw2-3560(config)#interface range gi0/1-2
Sw2-3560(config-if-range)#switchport mode dynamic desirable
*Mar 1 00:17:58.036: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/1, changed state to down
*Mar 1 00:17:58.045: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/2, changed state to down
*Mar 1 00:17:58.858: %SYS-5-CONFIG_I: Configured from console by console
*Mar 1 00:18:01.056: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/1, changed state to up
*Mar 1 00:18:01.073: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/2, changed state to up

Sw2-3560#sh int trunk

Port Mode Encapsulation Status Native vlan
Gi0/1 desirable n-isl trunking 1
Gi0/2 desirable n-isl trunking 1

Port Vlans allowed on trunk
Gi0/1 1-4094
Gi0/2 1-4094

Port Vlans allowed and active in management domain
Gi0/1 1,100,200,300,400,500,600
Gi0/2 1,100,200,300,400,500,600

Port Vlans in spanning tree forwarding state and not pruned
Gi0/1 1,100,200,300,400,500,600
Gi0/2 none


Trunk is a term used by Cisco, other vendors do not use this term. The idea of trunk is it transports all vlans by default including native vlan 1. User can prune it or set the allowed vlans to traverse across the trunk links.

In Hirschmann switches, the use of trunk is known as tagged link. Vlan that needs to be transported over the switch link has to be tagged with respective Vlan id, Cisco switches did this tagging automatically with switchport mode trunk or by using dynamic desirable and auto combination. Hirschmann switches also have Vlan1 which is the default Vlan, Vlan 1 is untagged by default which behaves the same as Cisco native Vlan 1.

Command example of Hirschmann switch for Vlan


vlan database

vlan 10



interface 1/1

vlan participate include 20

vlan tagging 20



The above tags vlan20 for egress port 1/1.



interface 1/2

vlan participate include 20

vlan pvid 20

the above untagged vlan 20 for egress port 1/2, this effectively is the same as Cisco command:

switchport mode access

switchport access vlan 20


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s