Cisco recommends an explicit trunk configuration of "desirable" at both ends. In this mode, network operators can trust syslog and command line status messages that a port is up and trunking, unlike "on" mode, which can make a port appear up even though the neighbor is misconfigured. In addition, "desirable" mode trunk provides stability in situations where one side of the link cannot become a trunk or drops trunk state.
Is this the preferred trunk mode?
The above is taken verbatim from one of the catalyst presentations and yes "desirable" is the recommended trunk mode when connecting two Cisco switches.
I ran into a problem getting a 6509 running Catos to trunk to a cat3548XL running cisco IOS using the desirable configuration on the 6509. The trunk cam up initially but then began flapping. It wasn't until I configured the trunk on the 6509 as ON instead of desirable that it finally stabilized.
The 2900/3548XL doesn't support DTP (Dynamic Trunk Protocol) so it will not be able to create a dynamic trunk with another switch. Has to be hard-coded. Also, the 2900/3548XL doesn't do PaGP which is used for Etherchannel.
I ran into the same problem when I was configuring a 3524xlpwr to my 6509. When the 6509 was in desirable I could not get a link. I switched the 6509 to on and got connectivity.
I had the same issue with an 4006 switch. Looks like some switches only support 802.1q and the other end has to be negotiate 802.1q. for the trunk to be up and not flapp.
You can use the command "sh port capabilities mod/port#" to see what features and their modes are supported by different hardware.
Console> (enable) sh port capabilities 1/1
Trunk encap type 802.1Q,ISL
Trunk mode on,off,desirable,auto,nonegotiate
Broadcast suppression percentage(0-100)
Flow control receive-(off,on),send-(off)
Fast start no
QOS scheduling rx-(none),tx-(none)
CoS rewrite yes
ToS rewrite DSCP
Inline power no
COPS port group 1/1-2
Link debounce timer yes
We can use *Desirable* trunk mode.
Trunk mode drop can be due to the misconfiguration or mismatch in the configuration in either of the sides. To make tunk to work both the sides should agree upon the same trunk configuration.
Check the vlans added in the trunk , duplex & other configuraions which should match both the sides.
hope it helps....