Default is "dinamyc desirable" . that will cause a trunk to be automatically created if 2 switches with default configuration are connected.
This is from a cisco 3550.
DSW1(config)#default interface fa0/48
Interface FastEthernet0/48 set to default configuration
DSW1(config)#do sh run int fa0/48
Current configuration : 69 bytes
switchport mode dynamic desirable
this from a 2950
Enter configuration commands, one per line. End with CNTL/Z.
ASW1(config)#default interface fa0/16
Interface FastEthernet0/16 set to default configuration
ASW1(config)#do sh run int fa0/16
Current configuration : 34 bytes
ASW1(config)#do sh int fa0/16 switchport | i Mode
Administrative Mode: dynamic desirable
Operational Mode: down
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)
Capture Mode Disabled
Nope. It was an old 12.2S-EWA running on a 4500 and I was linking some 2940s that were running 12.1. I was pretty shocked when I saw it, but that is when I realized that "defaults" can change, regardless of what a book says. Had I not seen it with my own eyes, I would still have the understanding I got from my CCNA studies.
Think about it, many years ago IP classless was not the default and you had to enter that command on a router if you were going to do classless rotuing. Now, you don't have to because its a default behavior. Why is that? Yep, the IOS version is what sets those defaults. As newer routers shipped with a newer IOS, the defaults on the gear change. There are several other examples, but this is just one. Now, it could be that auto desireable is the default on newer gear with newer IOS versions, but 5 years ago, that was not the case.
Not sure. My point is that the statement saying that all switches have the default of dynamic desirable is inaccurate because I have personally seen the 4500 with the default of dynamic auto.
It is ios that sets defaults, so in the case of the 4500, at one time the default was dynamic auto. I am talking like 5 to 7 years ago.
You are right that older IOS like on the 3550's, 2950 switches defaulted to dynamic desirabe. But many administrators then considered this a security risk.
On later models the default was changed to dynamic auto which is a bit more secure than dynamic desirable. For instance the 3560's model switches started coming out with dynamic auto as default.
As this blog post from 2007 states there was at some point a mixture of 3550's and 3560's in the ccie lab, which was a bit fun since the 3560's could easily trunk to the older 3550's but could not automatically trunk with eachother.
Thanks Chemillo! I knew it was out there, but again it was several years ago. Perhaps with newer switches the statement may be correct. Not sure that the current CCNA has IOS versions that have 5 to 7 year old IOS versions in mind.
This is actually a good example of why I just stick with manually configuring a trunk or access port and not depend on dynamic trunking.