CCIE Pursuit Blog

August 8, 2007

Minimum Configuration For 802.1q Trunk

Filed under: Cisco,Cisco Certification,IOS,Switching — cciepursuit @ 10:28 am

Building on this post, let’s explore a favorite type of lab task –  the dreaded ”minimum configuration”” 

Task: Configure an802.1q trunk between sw1 and sw2 on using the minimum configuration.

sw1 (3560) fa0/13—————–fa0/13 sw2 (3560)

Our options are as follows:

1) Hardset both switchports to trunk
2) Hardset one switchport to trunk and set the other to use DTP dynamic auto(default)
3) Hardset one switchport to trunk and set the other to use DTP dynamic desirable
4) Configure one switch to use DTP dynamic auto with dot1q encapsulation and set the other to use DTP dynamic desirable.
5) Configure one switch to use DTP dynamic desirable with dot1q encapsulation and set the other to use DTP dynamic desirable.
6) Configure one switch to use DTP dynamic desirable with dot1q encapsulation and set the other to use DTP dynamic auto(default).

Looking at our configuration options, we should focus on solutions that use default settings (zero configuation):

1) Hardset one switchport to trunk and set the other to use DTP dynamic auto(default)
2) Configure one switch to use DTP dynamic desirable with dot1q encapsulation and set the other to use DTP dynamic auto(default).

The problem is that both configs require two lines of config on one side of the trunk:

option 1:
interface FastEthernet0/13
 switchport trunk encapsulation dot1q
 switchport mode trunk

option 2:
interface FastEthernet0/13
 switchport trunk encapsulation dot1q
 switchport mode dynamic desirable

You could count characters and choose option 2 🙂 or you could present both choices to the proctor and get guidance from him.

This gets very easy if one or more of the switches is a 3550.  Since the default DTP config for a 3550 is dynanmic desirable, you simply need to configure “swithport trunk encapsulation dot1q” on one of the 3550s and leave the other alone(configure the 3550 and leave the 3560 alone in the case of trunking between a 3550 and a 3560).

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