dynamic trunking protocol

Answered Question
May 10th, 2008

hi every body!

i have some questions about DTP.

According to cisco, DTP covers

encapsulation negotiation( isl or dot1q)and whether the link becomes a trunk at all.

Let say we have two swithes:

sw1 and sw2

sw1 is connected to sw2 by f0/1 and sw2 is connected to sw1 by f0/1.

Both are trunking.

sw1 configuration:

sw1(config) interface f0/1

sw1(config-if)switchport

sw1(config-if)switchport trunk encapsulation negotiate.

sw1(config-if)switchport mode trunk.

sw1(config-if)switchport nonegotiate.

Assume sw2 has the similar configuration in place. I turn off the DTP on both switches. With DTP turned off on both swithes, how the encapsulation would be negotiated i.e isl or dot1q between sw1 and sw2?

simply put, when dot1q or isl need to be

negotiated between two switches should they have "DTP" turned on?

I have one more question about trunk link.

Can i use any port from any vlan for trunking? for example if i have switch with vlan 2 and vlan 3 , can i use the port from vlan3 for trunking or the port should be from native vlan?

thanks alot!

thanks alot!

I have this problem too.
0 votes
Correct Answer by cisco_lad2004 about 8 years 8 months ago

There is something not quite right about ur pasted config lines. this is what happens when I try it.

TST-POP1-NPE01(config-if)#switchport trunk encapsulation negotiate

TST-POP1-NPE01(config-if)#switchport mode trunk

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

TST-POP1-NPE01(config-if)#switchport nonegotiate

Command rejected: Conflict between 'nonegotiate' and 'dynamic' status.

In order to turn off DTP, u need to set encapsulation and mode manually. which means, you won't be in a situation where DTP is off and encapsulation not set.

when DTP is on and encapsulation not specified, link will default to Cisco proprietary ISL.

With regards to trunking VLANs, u can trunk VLAN2 and VLAN3 on same port. I hope I understood ur question correctly.

Pls rate all helpful posts

HTH

Sam

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Correct Answer
cisco_lad2004 Sat, 05/10/2008 - 14:02

There is something not quite right about ur pasted config lines. this is what happens when I try it.

TST-POP1-NPE01(config-if)#switchport trunk encapsulation negotiate

TST-POP1-NPE01(config-if)#switchport mode trunk

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

TST-POP1-NPE01(config-if)#switchport nonegotiate

Command rejected: Conflict between 'nonegotiate' and 'dynamic' status.

In order to turn off DTP, u need to set encapsulation and mode manually. which means, you won't be in a situation where DTP is off and encapsulation not set.

when DTP is on and encapsulation not specified, link will default to Cisco proprietary ISL.

With regards to trunking VLANs, u can trunk VLAN2 and VLAN3 on same port. I hope I understood ur question correctly.

Pls rate all helpful posts

HTH

Sam

bvsnarayana03 Sun, 05/11/2008 - 12:12

With the comand "switchport mode trunk", you are manually defining the trunk & this command is not acceptable unless you specify the encapsulation method.

By default all swicthports are left to be detected as trunk. so you are good to go with DTP with the command "switchport trunk encapsulation negotiate" & optionally "switchport mode desireable".

Ports assigned to a VLAN cant be a trunk.

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