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Connecting 2 Cisco Switches

Posted on 2007-11-14
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Last Modified: 2012-06-27
Currently I have 1 Cisco Catalyst 3750G switch, Switch_A.  Switch_A has VLAN 20 and VLAN 30 configured, and each VLAN has servers and workstations connected to it.  Now I wish to connect a Cisco Catalyst 2960G switch, Switch_B to Switch_A.  Switch_B will have only VLAN 30.  Devices connected to Switch_B need to communicate with devices in VLAN 20 and VLAN 30.  In this case, do I have to configure the ports that interconnect the switches? (e.g. trunk?)  Or I only need to connect Switch_B to any port of VLAN 30 in Switch_A via a crossover cable?  Please help.  Thank you.
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Question by:hoggiee
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murthysonic earned 300 total points
ID: 20279791
Here is what I think in my views:
1. Yes, You have to configure the ports the interconnects the switches as TRUNK ports.
2. I suggest you to use Cisco's Virtual Trunking Protocol - which will allow them to share VLAN configuration consistency across the the entire network.
3. Once you interconnect them with cross-cable, make sure the trunk links are always remain in the same VLAN. (This is not applicable in your case as your Switch_B doesn't going to have any other VLAN other than VLAN 30).
4. Assuming that you might already have communication between your VLAN20 and VLAN30 which are on the Switch_A, you can leave the ports on Switch_B to be in the default VLAN (VLAN30).
5. Otherwise, you will also need a router or some sort of device to provide router summarization, and actually route information between VLANs. That just depends on how you plan on using them.

Hope this information is helpful.
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by:mb042
mb042 earned 100 total points
ID: 20279815
You only need the crossover cable. If you use the trunk method, (which gives you more possability for the future) then you will have to configure all you ports on B to VLAN30.
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by:Don Johnston
Don Johnston earned 100 total points
ID: 20280230
You are correct that a crossover cable will be used to connect the two switches. Since there is only one VLAN on the 2960, there is no need for a trunk between Switch_A and Switch_B. However, DTP (Dynamic Trunking Protocol) is enabled by default and will create a trunk between the two switches. There's no harm in allowing the trunk between the two switches. As a result, all VLAN 30 devices on Switch_B will be able to access all VLAN 30 devices on Switch_A. You will need to configure the default gateway on the VLAN 30 devices to the VLAN30 interface on the 3750 (assuming that's how you're providing routing to the VLAN30 deivces).

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Author Comment

by:hoggiee
ID: 20280736
"However, DTP (Dynamic Trunking Protocol) is enabled by default and will create a trunk between the two switches."
This means that the ports interconnecting the switches will be automatically in trunk mode when I connect them using a crossover cable?
Questions:
Do I have to set both ports interconnecting the switches in VLAN 30? How about all other ports in Switch_B? And also is it neccessary to specify the encapsulation type of the trunk link? (ISL? dot1q?)
Thanks.
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