Redundant Switching - 3750v2

[danpfrommer]

Hello,

We have 2 3750v2 connected to 2911 ISR via ethernet cable. The 3750's are configured with stack wise.

I want to add redundancy to the 3750 stack, but am not sure how to accomplish this.

Can I add another ethernet cable to a seperate ethernet interface on the 2911? If so, what is that called and how do I accomplish this configuration.

Are there any documentation on best practice designs for what I am trying to accomplish? Thanks to everyone in adv ance. Truly appreciated...

[Reza Sharifi]

Hi Dan,

Since the 2 3750s are stacked, you can connect one interface from the 2911 to one of the 3750s and the second interface to the other 3750 and create an etherchannel. It would be a layer-3 Etherchannel.

HTH

Reza

[danpfrommer]

Thanks Reza.

What if I had 4 3750's? Would I still connect the first 2 in the stack to the 2911 or choose another switch to connect?

Also, what if the switches were not stackable, how would they be connected to still provide some redundancy? Basically how do you connect all of them physically (layer 1) so you can acheive router on a stick design?

Thanks for answering my questions, Layer 1 is always interesting to me.

[Reza Sharifi]

Dan,

What if I had 4 3750's? Would I still connect the first 2 in the stack to the 2911 or choose another switch to connect?

The Etherchannel can have up to 8 links in it, so with 4 switches you would be ok, but you would need to buy a network module for the 2900 since it comes with 3 on borad ports.

Also, what if the switches were not stackable, how would they be connected to still provide some redundancy? Basically how do you connect all of them physically (layer 1) so you can achieve router on a stick design?

If they are not stacked and you still need redundency, then you need to have two uplinks per switch and also connect the switches together, but then again your 2900 would be a single point of failure.

HTH

Reza

[Amit Singh]

Daniel,

I am assuming that the 3750 stack is connected connected to 2911 using one link only? If you want to add another cable, you can connect that from the 2911 Ethernet port to another 3750 in the stack other than the originial 3750 with the first cable connected. You have few options to add the redundancy :

1. Configure two default static routes each pointing to 2911 ethernet interfaces. The 3750 might load balance the traffic going up to 2911 depening upon the destination session.

2. Run a routing protocol between 2911 and 3750 with L3 ports. This will give you optimum redundancy with fast and auto-convergence on case of a link failure. Also you will have multiple paths for forwarding traffic automatically.

3. If you are not using the native L3 routed ethernet ports of the 2911 and using an Etherswitch module, you can connect 3750's to the etherswitch ports and run a port-channel either L2 or L3 for dynamic load-balancing.

HTH,

-amit singh

[Jignesh Desai]

I have a question along the same line of thought.  I have two 3750's configured in a stack.  I have an uplink from each of the 3750 to two different switches.  My 3750's are distribution switches connecting to 6500 core switches.  The 6500's themselves are interconnected.  My EIGRP topology table on the 3750's shows equal cost paths from each of the uplinks.  Do i need to be concerned about return traffic coming in on the opposite interface as the interface the traffic sourced from? or Will i have equal cost load blancing?

[Reza Sharifi]

You will have per-packet equal cost load blancing, which is default.

[Jignesh Desai]

That's what I thought.  How does the return packet know which route to take given equal cost?

Thanks