VLAN TRUNKING/Access Ports

Unanswered Question
Mar 30th, 2008

This is on Page 209 of BCMSN 4th Edition Self-Study Guide CCNP Learning by Froom, Sivasubramanian, Erum. The following is the configuration given in the review questions of Chapter 4 (Ex 4-31, Pg 209):

int Gi3/9

mtu 9216

no ip address

switchport

switchport access vlan 5

switchport trunk encapsulation dot1q

end

And the following questions multiple choices are given:

Question: Which statement is true regarding the interface?

a) The interface is a member of VLAN 5 and may negotiate to trunk port.

b) The interface may negotiate to an ISL trunk with a Native VLAN of 5.

c) The interface may negotiate to an 802.1Q trunk and operate with a Native VLAN of 1

d) The interface will not neotiate to a trunk port because it is configured in access VLAN-5

Isn't the answer "d", but the book says its "a". Could anyone clarify why?

Also, looking at the configuration why would there be encapsulation on an access port. Isnt access port part of only one VLAN?

And DTP messages are sent on trunk ports only when the mode is set to NEGOTIATE, coorect?

I have this problem too.
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i-kendall Sun, 03/30/2008 - 16:34

OK, this is my shot at this, and if wrong I am sure I will be corrected ;-)

a) The switchport command makes it a member of vlan 5, and the trunk will negotiate as this is the default behaviour.

b)No - it is a dot1q trunk not ISL

c) no - I think the access vlan 5 makes the native vlan 5

d) no it will still negotiate to be a trunk, but with vlan 5 as the untagged (native) vlan

Trunk ports are members of many vlans, but only one can be untagged, any others must be tagged. I think there is some exception, but this is generally true. You put an encapsulation for when it negotiates to be a trunk.

The switchport mode command says if it is a trunk or negotiates. The default is to negotiate. As there is no mode command it WILL negotiate, depending on what the other end of the link does.

i-kendall Sun, 03/30/2008 - 16:41

OK, this is my shot at this, and if wrong I am sure I will be corrected ;-)

a) The switchport command makes it a member of vlan 5, and the trunk will negotiate as this is the default behaviour.

b)No - it is a dot1q trunk not ISL

c) no - I think the access vlan 5 makes the native vlan 5

d) no it will still negotiate to be a trunk, but with vlan 5 as the untagged (native) vlan

Trunk ports are members of many vlans, but only one can be untagged, any others must be tagged. I think there is some exception, but this is generally true. You put an encapsulation for when it negotiates to be a trunk.

The switchport mode command says if it is a trunk or negotiates. The default is to negotiate. As there is no mode command it WILL negotiate, depending on what the other end of the link does.

sundar.palaniappan Sun, 03/30/2008 - 18:10

Iain,

Your response was correct :)

Answer A is the correct choice. Yes, the port is an access port in vlan 5 but potentially could become a dot1q trunk if the other end of the connection is set as a trunk port. AnswerD would have been the correct choice if the command 'switchport mode access' had been configured on the port which would have disabled dynamic negotiation of the trunk.

HTH

Sundar

royalblues Mon, 03/31/2008 - 00:27

Lain,

Just one difference... the Switchport access vlan 5 does not make the native vlan as 5. The native vlan would still be vlan 1 unless explicitly configured with the command

switchport trunk native vlan 5

switchport access vlan 5 would make the port fallback to vlan 5 as an access port if for some reason it loses its trunk status

Narayan

i-kendall Mon, 03/31/2008 - 02:43

Narayan,

Thanks, I suspected that, but couldn't remember if the access command took precidence or if you needed to do both. Thanks for the correction, we all learn something :-)

Regards, Iain

Dcoffshore Tue, 04/01/2008 - 14:36

Hi,

Look at the command carefully, "switchport access vlan 5", here we have not changed the mode which can be changed by "switch mode access"

so the port will remain the trunk port, now the question is why we are using the switchport access valn 5 when already a trunk is created the answer is simple : the port will remain trunk but if trunk fails the vlan 5 traffic can still be forwared by the switch, we have just given th redundancy to vlan 5, so that its information is not lost during the trunk failure,

Thanks

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