From: Mark Lasarko (mlasarko@co.ba.md.us)
Date: Mon Mar 13 2006 - 13:12:52 GMT-3
Greetings Geert,
on my version... 12.2(25)SED
"switchport mode dynamic desirable"
will be the default w/o any other commands.
Considering we only have Vlan1, the default will look something like this:
Port Mode Encapsulation Status Native vlan
Fa0/x desirable n-isl trunking 1
Now, we add the commands you mention:
(corrected by the config in regards to syntax and order of operation)
!sw1
interface FastEthernet0/15
switchport trunk encapsulation isl
switchport mode trunk
switchport nonegotiate
!sw2
interface FastEthernet0/15
switchport mode dynamic desirable
~~~~~~~~~~~~~~~~~~~~~~~~
Now,
Rack1SW1#sh int trunk
Port Mode Encapsulation Status Native vlan
Fa0/15 on isl trunking 1
Port Vlans allowed on trunk
Fa0/15 1-4094
Port Vlans allowed and active in management domain
Fa0/15 1
Port Vlans in spanning tree forwarding state and not pruned
Fa0/15 1
Rack1SW1#
AS2511#2
[Resuming connection 2 to sw2 ... ]
Rack1SW2#sh int trunk
Rack1SW2#
(It appears SW2 does not see a trunk ???)
! Let's look at the switchports for more detail
Rack1SW1#sh int fa0/15 trunk
Port Mode Encapsulation Status Native vlan
Fa0/15 on isl trunking 1
Port Vlans allowed on trunk
Fa0/15 1-4094
Port Vlans allowed and active in management domain
Fa0/15 1
Port Vlans in spanning tree forwarding state and not pruned
Fa0/15 1
Rack1SW1#
AS2511#2
[Resuming connection 2 to sw2 ... ]
Rack1SW2#sh int fa0/15 trunk
Port Mode Encapsulation Status Native vlan
Fa0/15 desirable negotiate not-trunking 1
Port Vlans allowed on trunk
Fa0/15 1
Port Vlans allowed and active in management domain
Fa0/15 1
Port Vlans in spanning tree forwarding state and not pruned
Fa0/15 1
Rack1SW2#
Looks like sw2 is "not-trunking" :)
As a matter of fact, I don't even see any CDP neighbors on this connection!
(That surprised-confused me a bit) - I enabled two other ports and the CDP
info was there so that's impacted by this config as well from what I can
tell.
Please share what books and docs suggest this trunk will actually work?
Best,
~M
>>> "Geert Nijs" <geert.nijs@simac.be> 03/13/06 10:20 AM >>>
All,
I am studying for my recertification and although i do understand the
different trunking modes, there has been a large discussion between me
and
a colleague for the following specific situation:
First Situation
SW1 - Fa0/1
switchport mode trunk
switchport trunk mode isl
switchport trunk nonegotiate
SW2 - Fa0/1
swichport mode desirable
Is an ISL trunk being formed or NOT ?
Trick: nonegotiate makes the port NOT send any DTP packets.
I always thought that "desirable" NEEDED DTP packets from the other side
to form a trunk (so: ON, AUTO or DESIRABLE)
So i would say: NO, their is no trunk.
Several books and docs seem to suggest that in this case, a trunk will
be formed. I am unable to test right now.
So what will happen ?
regards,
Geert Nijs
Service Engineer
CCIE #13729 <http://www.cisco.com/en/US/learning/le3/ccie/index.html>
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