TRUNKING CONFUSION

[seamus00111]

Is there such a thing as a DEFAULT mode on a trunk interface?

I've notice that different iOS versions can default to different  modes!

[johnlloyd_13]

hi,

yes, there's only 802.1Q trunking on newer switches/IOS.

ISL has already been deprecated for quite some time now.

you would see some 'older' switches (like the cat 4500/6500) have ISL trunking support.

[seamus00111]

John, thanks for that.  

I'm studying for the 300-115 switch exam and you'll understand that when the books say that the Ports should default to "Dynamic Desirable" and "Static Access", things become confusing.

Have a look at below. Depending on the iOS, the following do not Default to the same mode/state.  

Switchport

Administrative mode

Negotiation of Trunking

(C3725-ADVENTERPRISEK9-M), Version 12.4(15)T7, RELEASE SOFTWARE (fc3)…..GNS3

ESW2#show int switchport

Name: Fa1/0

Switchport: Enabled

Administrative Mode: static access

Negotiation of Trunking: Disabled

(C3560-HBOOT-M) Version 12.2(25r)SEC, RELEASE SOFTWARE (fc4)……..packet tracer

Switch#show int switchport

Name: Fa0/1

Switchport: Enabled

Administrative Mode: dynamic auto

Negotiation of Trunking: On

I've also seen the switchport, in default mode being "Disabled".

Given the above facts, should I assume therefore that the safest bet when configuring trunk links is to turn off DTP or just configure both sides with the "Switchport mode trunk" command.

[InayathUlla Sharieff · Accepted solution]

Hi,

Its recommended to turn off the DTP on the trunk links.

Here is the explanation on the DTP which can clear your all doubts on it, in case if you still have do let us know.

http://packetlife.net/blog/2008/sep/30/disabling-dynamic-trunking-protocol-dtp/

HTH

Regards

Inayath

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[Peter Paluch · Accepted solution]

Dear friends,

Please allow me to join.

seamus: The experiment with the 3725 router is not very relevant. Switching modules installed into routers may have different default settings or they may not support some of the usual switch features at all. It is always best to go by the default behavior of standalone Catalyst switches.

The default settings have changed over different versions of Catalyst switches. For older 2950 and 3550, the default was dynamic desirable. For more recent 2960, 3560, 3750, the default is dynamic auto. For the super-duper recent 3650 and 3850 Catalysts, the default is also dynamic auto.

Regarding the "static access", this is a confusing name.The "static access" not only means that the interface operates as an access port but also that the access VLAN is fixed (static). There is also a dynamic access mode that derives the access VLAN based on the attached host's MAC address. It is a very old functionality based on so-called VLAN Query Protocol (VQP) and VLAN Membership Policy Server (VMPS). It is not being used today as it has been replaced by 802.1X authentication that can also provide dynamic VLAN membership, but I just thought you might be interested to know this. Note that only when using the VMPS/VQP, we talk about "dynamic access" mode because the configuration was switchport access vlan dynamic. You can learn more here:

http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst2950/software/release/12-1_22_ea11x/configuration/guide/scg/swvlan.html#wp1375288

Best regards,
Peter

[seamus00111]

Thank you Inayath, that was a good article on DTP. Appreciate the help.