Helpful Hints about INE’s virtual lab environment:

• Save your configuration changes frequently in the event that you get locked out of a device or lose connectivity
• If you are suddenly logged out of the GNS3 lab environment and log back in again, you may see the prompt, “Another GNS3 instance is running, continue? (Y/N)”. Select “Yes”

• If you ever lose console access to one (or more) of your devices you will need to reboot the Ubuntu Virtual Machine that is hosting your GNS3 lab environment. To do this,

  • Minimize your GNS3 window
  • Open the “Terminal” app within the Ubuntu device

  

  • Type “reboot” within the Terminal window
  • Wait 20-30 seconds and then click the “Reconnect” option to log back in.

Solutions:

In the previous task, we configured all the trunk ports as static trunks, which can be replaced with Cisco's DTP (Dynamic Trunking Protocol). DTP has three modes: 1) auto, 2) on, and 3) desirable. Most switches utilize a default interface mode of "Auto" which isn't visible in the output of "show running-config" but can be seen in the output of "show interface x/y switchport"

In this lab you needed to manually set the two interfaces on Switch-1 to "dynamic desirable" mode in order to fulfil the lab requirements. Because you reset the interfaces on Switches-2 and 3 (connecting to Switch-1) to their default configurations, they were set back to dynamic auto mode (by default) and thus were able to respond to the incoming DTP requests from Switch-1 and form VLAN Trunks.

    Sw1:
    interface Gig1/0
     switchport mode dynamic desirable
     switchport trunk encapsulation negotiate
    !
    interface Gig3/0
     switchport mode dynamic desirable
     switchport trunk encapsulation negotiate

    Sw2:
    interface Gig1/0
     switchport mode dynamic auto
     switchport trunk encapsulation dot1q

    Sw3:
    interface Gig3/0
     switchport mode dynamic desirable
     switchport trunk encapsulation dot1q

Verification

We can verify the negotiated trunk by using the show interfaces trunk and show interfaces switchport commands in the privileged exec mode. Below are the command outputs for negotiated trunk verification.

    Sw1#show interfaces trunk

    Port        Mode             Encapsulation  Status        Native vlan
    Gi1/0       desirable        n-isl          trunking      1
    Gi3/0       desirable        n-isl          trunking      1

    Sw1#show interface gig1/0 switchport
    Name: Gi1/0
    Switchport: Enabled
    Administrative Mode: dynamic desirable
    Operational Mode: trunk
    Administrative Trunking Encapsulation: negotiate
    Operational Trunking Encapsulation: isl
    Negotiation of Trunking: On
    [snip]

    Sw1#show interface gig3/0 switchport
    Name: Gi3/0
    Switchport: Enabled
    Administrative Mode: dynamic desirable
    Operational Mode: trunk
    Administrative Trunking Encapsulation: negotiate
    Operational Trunking Encapsulation: isl
    Negotiation of Trunking: On
    [snip]

    Sw2#show interfaces trunk

    Port        Mode             Encapsulation  Status        Native vlan
    Gi1/0       auto             n-isl          trunking      1

    Port        Vlans allowed on trunk
    Gi1/0       100,200


    Sw2#show interface gig1/0 switchport
    Name: Gi1/0
    Switchport: Enabled
    Administrative Mode: dynamic auto
    Operational Mode: trunk
    Administrative Trunking Encapsulation: negotiate
    Operational Trunking Encapsulation: isl
    Negotiation of Trunking: On

    Sw3#show interfaces trunk

    Port        Mode             Encapsulation  Status        Native vlan
    Gi3/0       desirable        n-isl          trunking      1

    Port        Vlans allowed on trunk
    Gi3/0       200


    Sw3#show interface gig3/0 switchport
    Name: Gi3/0
    Switchport: Enabled
    Administrative Mode: dynamic desirable
    Operational Mode: trunk
    Administrative Trunking Encapsulation: negotiate
    Operational Trunking Encapsulation: isl
    Negotiation of Trunking: On

In the above output, we can see some additional parameters like n-isl, which means that the ISL encapsulation is negotiated by DTP. By default, Cisco switches use ISL as the trunking encapsulations when using DTP. In this task, we have the additional method of verifying trunking parameters using the show interface switchport command. It reveals the additional components of a trunk port such as Administrative mode, Operational mode, status of negotiation, etc.