link 2 switches - trunk?

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Sep 6th, 2008
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Might be a simple question to you, but I have a 3750 which is full and multiple VLAN's. I have bought a 2nd 3750, is it possible to extend a VLAN on the first switch and create more VLAN's?

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Doesn't really matter if u stack it or not, you can do it without that as well. All you have to do is, assign a port on sw-2 to a vlan defined in sw-1, and configure the link between sw-1 and sw-2 as trunk link.. Simple :)

Stacking the switch is used only when you want to see multiple switches as 1 switch, meaning, using 1 management IP (Stack master's IP) you can manage mutliple swtiches.

If you want to learn more about stacks, here is the link.

plz rate helpful posts.



whiteford Sat, 09/06/2008 - 07:07
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You first optioned seems to make sense, do I set the trunk as a dot1q on each side?

Also can I some how connect to the 2nd switch on a L3 from the LAN for management?

lamav Sat, 09/06/2008 - 09:57
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You're asking very basic questions that can easily be answered with a little reading on Cisco's website or a CCNA book...just to let you know, in case you wanted to research on your own.

To answer your question, it doesn't matter what kind of trunk you establish, as long as both trunk ports on each end are using the same trunking protocol. Dot1q is an option and so is ISL (I do believe the 3750s support ISL, it is Cisco proprietary, after all).

The purpose of the trunk is to extend (span) a vlan across to another switch. A trunk port, by default, will carry traffic from all configured vlans, unless you prune the trunk using certain commands.


interface gigabitethernet 1/0/1

description dot1q trunk to switch 2

switchport mode trunk

switchport trunk encapsulation dot1q

-- All vlans allowed across trunk --

OR to deny access/prune certain vlans --

switchport trunk allowed vlan 1,5,6

As for the management interface, the answer is yes, of course. You can create a L3 interface on the second switch that you can telnet/SSH to for remote access.


interface vlan 10

description management_vlan

ip address

ip default-gateway (that would be the L3 interface address on your uplinked L3 switch).



glen.grant Sat, 09/06/2008 - 12:47
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If you have the money just buy the stack cable then you don't have to worry about trunking anything and you only have 1 logical switch to manage and configure . They plug into the back of the 3750's , you will see 2 stack ports. Everything that is configured on the first switch is automatically created on the second second switch including vlans. think of it as configuring like a single 4500 chassis switch.

whiteford Sat, 09/06/2008 - 23:38
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That sounds like a great idea, so it will "see" the 2 switches as one?

This could hepl on another issue actually. I have another 3750 which is very important and live and if this fails I have a spare one underneath which I manually update via the console as it has the exact config as the live one. If the live switch fails then I can just switch the cables, could a serial cable help be here and manage online?

I ssh to the live switch and manage all the VLAN's on there.


lamav Sun, 09/07/2008 - 04:56
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I am assuming that your desire to create a trunk to carry mutltiple vlans between two 3750 switches is for the purpose of introducing some redundancy into your network.

If that is indeed the case, I don't think that stacking 3750s is a viable design solution. For one, a dependency is created between the 2 switches -- I have seen instances in which the entire stack had to be rebooted as a result of some types of failures. Also, you will only have one data path between switches (albeit, it will be an extension of the 32Gbps backplane, but nonetheless just one path), as opposed to creating 2 or 3 dot1q trunks and bundling them in an etherchannel. That would cost about $75 and take about 10 minutes to configure.

I don't think I have ever seen a design solution in which stacking is used to solve a redundancy requirement. Stacking was meant more for the purpose of increasing port density and ease of management (the one logical switch phenomenon).




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