Configure Gbit Interface on 3750

Unanswered Question
Aug 16th, 2008

Hi hi,

Hope someone can help to help clear some doubts and provide information on how to configure the Gbit interfaces on 2 x 3750. I had some doubts about doing it and they are as follows:

a. Trunking up both 3750 via the Gbit interfaces are considered as stacking?

b. There are 2 Gbit ports on each 3750. Do I confirm port A on the first 3750 to port A of 2nd 3750 or i need to connect both ports to link both devices up?

c. I believed I just need to configure the "Interface Range" and "switchport mode trunk" and both devices will be in trunk mode without LACP and PAgP enabled?

d. The connection speed can yield up to 32GB?

Many thanks for any assistance.

Tan

I have this problem too.
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ohassairi Sun, 08/17/2008 - 02:23

a-no. it is a simple cascade every switch is a standa lone unit.

b- as you like more you cascade, more you get bw.

c- "switchport mode trunk" and both ports will be in trunk mode

d-no 32G is only acheived by the special stack cable in the rear.

tanziweigca Sun, 08/17/2008 - 08:59

Hi Ohassairi,

Thanks for the information. Still have some doubts though and hope you can once again provide some assistance.

There are actually 4 plugable SFF transceiver slots on each of the switch and I had purchased the GLC-SX-MM modules (2 for each switch). If I wanna trunk up both switches, I just need to plug into one of the SFF slots instead of 2 right?

Also, instead of using the transceiver to perform trunking, I still think using stacking will also do the task. Stacking will "merge" both the 2 x 3750 I had into one big switch and the configuration will flow from master to secondary. Am I right too?

Many thanks for your help. This is the first time using 3750 switches and had some doubts about it.

Tan

bmcginn Mon, 08/18/2008 - 18:59

Hi there Tan,

If you want to logically trunk both of the switches, and you have 2SFPs for each switch you can do the following:

1. Plug 2 SFPs into one of the switches and 2 SFPs into the other one

2. Connect each SFP from one switch to the corresponding SFP in the other switch with multimode fibre cable.

3. Configure the channel-group on each switch (this assumes you want to have 2 physically separate cables going from switch to switch but you only want 1 logical circuit) you can configure the channel-group with the command "channel-group 1 mode on" in the interfaces on the switch. obviously you can configure both interfaces at the same time using the interface range gi0/1 - 2 command.

4. configure both interfaces to be trunks using the switchport trunk encapsulation dot1q and switchport mode trunk commands.

5. Collectively, trunking the 2 switches with the 2 ports will give you theoretically about 2Gbps each way.

If you stack the 2 switches using the silver stacking cable, you will get about 32Gbps between the switches.

In relation to the 'stacking' cable question, yes you are correct. When stacked, the 3750 switches perform as 1 switch. It will automatically sort that out for you.

I hope that helps..

tanziweigca Mon, 08/18/2008 - 19:07

Hello bmcginn,

Many thanks for your detailed information. I will try out on that and feedback if any problems encountered.

At the meantime, I had actually connected 1 x 3550 and 1 x 3750 together and I did not set any trunking mode in the port at all. It looks like both switches have auto-sensing and both are trunked without any need for configurations. Do you know anything abt it?

Also, the multimode fiber cable, do you know if we can custom make one as the original cost from Cisco is quite expensive.

Many thanks for your help,

Tan

bmcginn Mon, 08/18/2008 - 19:10

you can buy multimode fibre patch cables from your local data and electrical store mate. Its a standard.

tanziweigca Mon, 08/18/2008 - 19:19

Hi bmcginn,

Wow... thanks for your prompt reply. How about the auto-sensing?? Am I right in saying that the 3750 has the feature and no specific trunk commands are needed? I do know that the 3550 comes with it but not too sure about the 3750.

Many thanks once again for your help,

Tan

bmcginn Mon, 08/18/2008 - 19:29

I am not sure about the auto sensing. I have always put the encapsulation type in. Most trunks these days are using dot1q anyway. I am fairly sure that the switch will complain if you put in the command switchport mode trunk without first entering the switchport trunk encapsulation command..

If you want to aggregate the 2 physical links into one logical link you will need to put the channel-group command in, the switch won't autosense that. In fact, spanning-tree will disable one of the links!

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