Ccna routing and Switching Complete Study Guide

sh int gi0/13 switchport

Name: Gi0/13

Switchport: Enabled

Administrative Mode: dynamic auto

Operational Mode: static access

Administrative Trunking Encapsulation: negotiate

Operational Trunking Encapsulation: native

Negotiation of Trunking: On

Okay—we can see that the port is in dynamic auto and that it’s operating as an access 

port. Let’s look into this further:

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Troubleshooting VLAN Connectivity 

871

S2#

sh dtp interface gi0/13

DTP information for GigabitEthernet0/3:

  DTP information for GigabitEthernet0/13:

  TOS/TAS/TNS:                              ACCESS/AUTO/ACCESS

  TOT/TAT/TNT:                              NATIVE/NEGOTIATE/NATIVE

  Neighbor address 1:                       000000000000

  Neighbor address 2:                       000000000000

  Hello timer expiration (sec/state):       17/RUNNING

  Access timer expiration (sec/state):      never/STOPPED

Do you see the problem? Don’t be fooled—it’s not that they’re running in access mode; 

it’s because two ports in dynamic auto will not form a trunk! This is a really common 

problem to look for since most Cisco switches ship in dynamic auto. The other issue you 

need to be aware of, as well as check for, is the frame-tagging method. Some switches run 

802.1q, some run both 802.1q and Inter-Switch Link (ISL) routing, so be sure the tagging 

method is compatible between all of your switches!

It’s time to fix our problem on the trunk ports between S1 and S2. All we need to do is to 

just fix one side of each link since dynamic auto will trunk with a port set to desirable or on:

S2(config)#

int gi0/13

S2(config-if)#