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Cisco switch trunk question

Posted on 2006-06-22
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Last Modified: 2012-05-05
Hi

Just a quick question on trunking on Cisco 2950/1900 switches.

Lets say I have a 2600 series router which is connected to a 2950 switch which in turn is connected to another 2950 switch.

Port f0/1 on both switches hold hosts which are on VLAN1. Port f0/2 on both switches hold machines on VLAN2.

InterVLAN routing has been set up on the 2600 series router using subinterfaces.

Unless I configure the ports connecting the router/switch and switch/switch as trunk ports, will this will not worl? Or are they set up by default as trunks?

Same applies to 1900 series switches.

Thx
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Question by:Dilan77
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6 Comments
 
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lrmoore earned 250 total points
ID: 16958946
Yes, both links must tunk. The 2950 is set for auto trunking so if you don't change a ports default settings it should recogonize the vlan tags and trunk them.
The 1900 - it's so old I don't remember, but I don't think they support VLAN's at all unless you have the Enterprise Edition..and trunk ports must be set manually.
1900 only supports ISL trunking
2950 only supports dot1q trunking
Ditch the old 1900
 
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by:harbor235
ID: 16959013
The 1900 is also only 10M for the host ports, some models have 100 for uplinks. I would not throw the switch away, just change the trunking method to ISL.

switch:
switchport trunk encapsulation isl

2600:
interface FastEthernet 1/0.1
encapsulation isl 1
interface FastEthernet 1/0.2
encapsulation isl 2

harbor235 ;}
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by:lrmoore
ID: 16959056
The 2950's only support dot1q trunking, so why would you toss them in favor of the 1900/isl ?

>I have a 2600 series router which is connected to a 2950 switch
The only choice for the subinterfaces is encap dot1q

The only thing you can use the 1900 for is an extension of one VLAN or the other, but not both.
 
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by:Dilan77
ID: 16959511
Thanks for the replies. It's not a real life situation, but a question about the CCNA which I'll be hopefully taking in a couple of weeks.

So, if I get a simulation where I have to link two 2950's switches together (or a 2950 switch/router), I don't have to configure the interface for trunking since it's on by default?

For the CCNA, 1900's can support trunking but it doesn't state whether it's on by default or not.

Is trunking needed whenever there is a link between switch/switch or switch/router, or only if different VLAN's are involved?
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by:harbor235
ID: 16961829
Aah, thats weird, 2900 models support ISL but the 2950 does not, i didn't know that.

Trunking is not needed to connect switches, however, if you would like to make multiple vlans availble between switches then a way to switch them between devices is to trunk the vlans. Trunking adds a dot1q or ISL header identifying the associated vlan of that particular frame. That way you are not limited to connecting hosts for a particular vlan to one switch.

harbor235 ;}
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by:lrmoore
ID: 16964985
Agree that trunking is only needed if you define vlans and those vlans need to be carried between switches or to a router or other device that understands vlans (some server NIC's)

If you plug two 2950's together, they just work. Everything is automagic.
If you then define vlans on one switch, then they are automatically trunked across to the other
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