From: CCIE KH49279 (ccie_lab@inetiq.com)
Date: Mon Mar 13 2006 - 13:59:50 GMT-3
Geert,
You are correct at least with the behviour as it applies to the 3550.
I am not sure about other switch types, but on the 3550 the defulat
behaviour is for DTP to attempt to establish a trunk.
In this case SW1 is being forced into trunking mode and not to generate DTP
frames using the 'switchport mode trunk' and 'switchport nonegotiate'
interface commands.
This can be seen in the following outputs I have generated below. You can
see how this occurs by looking at the Administrative modes for each
interface. SW1 is set to trunk, and use ISL, while SW2 is set to dynamic
desirable, and negotiate trunking. Because of this particular configuration
SW1 operational mode stays set to trunk, while SW2's operation modes changes
to 'static access', which is the defualt behaviour of an interface on 3550
that cannot succesfully negotiate a trunk.
Here is the configurations.
SW1 -
interface FastEthernet0/13
switchport trunk encapsulation isl
switchport mode trunk
switchport nonegotiate
SW2 -
interface FastEthernet0/13
switchport mode dynamic desirable
Here is the output for the switch interfaces:
sw1#sh int fa0/13 switchport
Name: Fa0/13
Switchport: Enabled
Administrative Mode: trunk
Operational Mode: trunk
Administrative Trunking Encapsulation: isl
Operational Trunking Encapsulation: isl
Negotiation of Trunking: Off
sw2#sho interfaces fa0/13 switchport
Name: Fa0/13
Switchport: Enabled
Administrative Mode: dynamic desirable
Operational Mode: static access
Administrative Trunking Encapsulation: negotiate
Operational Trunking Encapsulation: native
Negotiation of Trunking: On
HTH,
Wayne
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Geert Nijs
Sent: Monday, March 13, 2006 9:21 AM
To: ccielab@groupstudy.com
Subject: Trunk Modes theory
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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