CCIE Pursuit Blog

August 8, 2007

Minimum Configuration For ISL Trunk

Filed under: Cisco,Cisco Certification,IOS,Switching,Tech Tips — cciepursuit @ 9:48 am

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

Task: Configure an ISL trunk between sw1 and sw2 on using the minimum configuration.

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

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

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 both switchports as DTP dynamic desirable
5) Configure one switchports as DTP dynamic desirable and the other as DTP  dynamic auto(default)

Since hardsetting the prot to trunk requires two lines (“switchport trunk encapsulation isl” and “switchport mode trunk”) while configuring an interface to use DTP dynamic desirable only requires one line (“switchport mode dynamic desirable”), we should use option 5 :

sw1:
sw1(config-if)#do sh run int fa0/13
Building configuration…

Current configuration : 69 bytes
!
interface FastEthernet0/13
 switchport mode dynamic desirable
end

sw1(config-if)#do sh int fa0/13 trunk

Port        Mode         Encapsulation  Status        Native vlan
Fa0/13      desirable    n-isl          trunking      1
—–output truncated—–

sw2:
sw2#sh run int fa0/13
Building configuration…

Current configuration : 34 bytes
!
interface FastEthernet0/13
end

sw2#sh int fa0/13 trunk

Port        Mode         Encapsulation  Status        Native vlan
Fa0/13      auto         n-isl          trunking      1
—–output truncated—–

This gets even easier when one or both of the switches are 3550s.  Since the DTP default on 3550s is “dynanmic desirable” you don’t need to configure anything at all in order bring up an ISL trunk.  DTP will actively attempt to establish a trunk when using “dynamic desirable” and will use ISL as the default trunking encapsulation.

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1 Comment »

  1. […] between sw1 and sw2 and to do the same between sw1 and sw3.  Simple, except that they threw in the dreaded “minimal configuration” restriction: “Use the MINIMUM CONFIGURATION POSSIBLE to accomplish this […]

    Pingback by Internetwork Expert Volume II: Lab 3 - Section 1 « CCIE Pursuit — December 18, 2007 @ 10:18 am | Reply


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