Crossover or straight-through for trunk?

Unanswered Question
Sep 26th, 2008


For a trunk between a 2950 and 3550 do I need a crossover or straight-through?

I have plugged a straight-though in FE0/1 of the 2950 to 3550 and I get no lights come up on the interface.

If I plug my laptop into either then I do.


interface FastEthernet0/1

switchport mode trunk


interface FastEthernet0/1

switchport trunk encapsulation dot1q

switchport mode trunk

also why does the 3550 have "switchport trunk encapsulation dot1q" and the 2950 hasn't and can't as there is no option for this?


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Overall Rating: 3.5 (2 ratings)
Jon Marshall Fri, 09/26/2008 - 12:16

You need a crossover cable to connect 2 switches together unless your switches support auto-mdix.

2950 doesn't have the switchport trunk encapsulation dot1q command because it only supports 802.1q so there is no need for the command.

The 3550 supports both 802.1q and ISL.


whiteford Fri, 09/26/2008 - 12:35


I didn't use a crossover between a 3560 and a 3550 so they must use auto-mdix, I guess the 2950 can't.

1.) So all I need is "switchport mode trunk" for the trunk?

2.) What does switchport trunk encapsulation dot1q add to the trunk as I want to interVLAN between the 2 so the some of the VLANs can talk to each other.

Jon Marshall Fri, 09/26/2008 - 12:52

1) On the 2950 yes.

2) It tells the switch what type of encapsulation to use for the vlan tagging. There are 2 types

ISL - which is Cisco proprietary

802.1q - which is IEEE standard.

They each add a vlan tag to the frame although they do it in very different ways. When a switch can do both types as in the 3550 it's best to tell it which one to use.


whiteford Sat, 09/27/2008 - 06:09

So are all VLAN's sent across the trunk - from one switch to the other and visa versa so both switches know about all VLAN's?

Jon Marshall Sat, 09/27/2008 - 09:50

They are by default unless you manually clear the vlan off the trunk which you can do ie.

switchport trunk allowed vlan ...


Jon Marshall Sat, 09/27/2008 - 09:59

Quick addition to previous post.

All vlan traffic is sent across when the same vlan is on both switches. As for vlan information you typically use VTP and either

1) put all your switches into vtp transparent mode which requires you add vlans to each switch

2) set up one (or 2 for redundancy) of your switches to be vtp server and the rest vtp clients. You can then add/modify/delete vlan information on the vtp server and it will get sent to all vtp clients in the same VTP domain.


whiteford Sat, 09/27/2008 - 11:29

Hi, I guess vtp transparent mode is the kind of the manual/controlled mode I'm getting use to, however this VTP server/client mode interests me, I will need to test this is a lba I think, I could use one switch as the server and one as the client.

I'm not sure if the VTP server and client can be mixed across the switches.

I'm struggling o find out if these switches support auto-mdix.

akharmanbiz Sun, 09/28/2008 - 10:08


you must use the cross-over cable for connect the switch to switch.

pc to sw = straight

router to router = cross-over

router to pc = cross-over

in the newst 2950 switches isl dont support and default encapsulation is dot1.q and this command isnt require for configure.


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