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New Member

Trunking on older switches

I am doing a Lab for school with a CCNP kit I bought from Amazon.com. I have a Catalyst 2950 switch and 2 Catalyst 3750 with ver 12.2 IOS. I am doing Lab 2.3.2.3 (Configuring Rapid PVST+, PortFast and BPDU Guard) in the Scaling Networks Lab manual. I am using the 2950 as Switch 1. When I get to the trunking config I get an error that says I can not configure trunk because it is encapsulated AUTO. What do I need to do to disable this? Or can I? These are used switches but I deleted the vlan.dat I was not looking at the screen and was just typing the commands for the lab and so it did it on F1/0/1 and i didn't notice until I got to F1/0/3 and it did it again. Here is the error I get? I was not in Cisco when these switches were taught so I do not know all the commands for these old switches. Can anyone help?

t1/0/1, changed state to up
S1(config-if)#switchport mode trunk
Command rejected: An interface whose trunk encapsulation is "Auto" can not be configured to "trunk" mode.
S1(config-if)#switchport trunk native vlan 99
S1(config-if)#int f1/0/3

*Mar 1 00:16:04.765: %LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan1, changed state to up
S1(config-if)#no sh
S1(config-if)#sw
S1(config-if)#switchport
*Mar 1 00:16:12.315: %LINK-3-UPDOWN: Interface FastEthernet1/0/3, changed state to up
*Mar 1 00:16:13.321: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet1/0/3, changed state to up
S1(config-if)#switchport mod
S1(config-if)#switchport mode tr
Command rejected: An interface whose trunk encapsulation is "Auto" can not be configured to "trunk" mode.
S1(config-if)#

  • LAN Switching and Routing
Everyone's tags (3)
4 REPLIES
Bronze

Hello,try this:switchport

Hello,
you must select if you want to use DOT1Q or ISL encapsulation. ISL is cisco proprietary.

Change encapsulation type with 1st command and then 2nd command will work


switchport trunk encapsulation dot1q
switchport mode trunk

New Member

Thanks Milos so it would be

Thanks Milos so it would be like the dot1q commands for a router since a layer 3 switch acts like a router

Bronze

catalyst 2950 supports only

catalyst 2950 supports only DOT1Q trunking. So it is selected by default.

But 3750 supports both ISL and DOT1Q trunking, so you must select one of them.

New Member

Hi you need to set the

Hi you need to set the encapsulation to DOT1Q.

Under interface config mode

switchport trunk encapsulation dot1q

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