Implication if I omit 'switchport trunk encap'

Answered Question
Aug 13th, 2009

Imagine my environment is all Cisco switches. If my intention is to assure that dot1q is my encapsulation protocol when on trunking mode, is it necessary to do:

fa0/1

switchport mode trunk

switchport trunk encap dot1q <===

switchport trunk allow vlan (...)

(...)

In my environment the line 'switchport trunk encap dot1q' is not present in switches. I just want to confirm that is not necessary and DTP would negotiate encapsulation dot1q by default, always? From reading the documentation so far, it is not clear to me if certain platforms could pick ISL as a default, in case 'trunk encapsulation dot1q' is not specified.

I have this problem too.
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Correct Answer by Leo Laohoo about 7 years 4 months ago

Some Layer 2 switches, such as the 2950 and 2960 series switches, support 802.1Q only therefore the commands "switchport trunk encapsulation dot1q" will not be accepted.

Hope this helps.

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ivarnhagen Thu, 08/13/2009 - 21:22

Hi,

DTP is disabled by specifying either "switchport mode access" or "switchport mode trunk". "switchport mode dynamic auto" is default and not shown in the running config...this enables DTP.

If the switch offers the switchport trunk encap dot1q command, you should use it to specify dot1q (ISL is default). If this command is not present, the switch only supports dot1q, thus making the command unnecessary( since dot1q is the only option).

hth

news2010a Thu, 08/13/2009 - 21:46

So if I do:

1)

switchport mode trunk

or:

2)

switchport mode trunk

switchport nonegotiate

In both scenaros above DTP would be disabled. Configurations 1) and 2) are the same then?

ivarnhagen Thu, 08/13/2009 - 22:35

Hi,

I stand corrected...the actual forwarding of DTP frames is turned off by the switchport nonegotiate command.

If you statically configure permanent trunking mode with switchport mode trunk DTP is still active and negotiates to convert the link to a trunk.

Here is the info straight from Cisco (see Usage Guidelines):

http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_25_sea/command/reference/cli3.html#wp1948171

http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_25_sea/command/reference/cli3.html#wp1989134

Correct Answer
Leo Laohoo Thu, 08/13/2009 - 21:43

Some Layer 2 switches, such as the 2950 and 2960 series switches, support 802.1Q only therefore the commands "switchport trunk encapsulation dot1q" will not be accepted.

Hope this helps.

Edison Ortiz Fri, 08/14/2009 - 06:07

As Leo pointed out, the reason you don't see the 'switchport trunk encapsulation dot1q' command in some switches is due to the lack of ISL support on those switchports.

If the switchport has only one trunking option, it is implied that is using dot1q protocol.

If the switchport supported ISL and dot1q, you will be unable to set the switchport 'mode trunk'.

A error message such as the one below would come up:

Switch(config-if)#sw mo tr

Command rejected: An interface whose trunk encapsulation is "Auto" can not be configured to "trunk" mode.

Switch(config-if)#

You can also find out the switchport capabilities on trunking support with the following command:

Switch#sh int f0/1 cap

FastEthernet0/1

Model: WS-C3560-48TS

Type: 10/100BaseTX

Speed: 10,100,auto

Duplex: half,full,auto

Trunk encap. type: 802.1Q,ISL

Trunk mode: on,off,desirable,nonegotiate

Channel: yes

Broadcast suppression: percentage(0-100)

Flowcontrol: rx-(off,on,desired),tx-(none)

Fast Start: yes

QoS scheduling: rx-(not configurable on per port basis),tx-(4q2t)

CoS rewrite: yes

ToS rewrite: yes

UDLD: yes

Inline power: no

SPAN: source/destination

PortSecure: yes

Dot1x: yes

HTH,

__

Edison.

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