L3 routing on 3750 switches

[dtran]

Hi all !! Hope everyone is doing well !!

I have a small campus network using 3750 stackable switches  and a 3725 router (see diagram below). Currently the 3725 router is handling inter-vlan routing for the campus and it looks like it's not able to handle the amount of traffic we're pushing. The router CPU sometimes hits above 90% due traffic load. What I would like to accomplish is move L3 process over to the 3750 MDF stack and the IDF1 stack. I am thinking creating SVI's on both MDF stack and IDF1 stack, run HSRP between the two stacks and may be do load balance traffic between the two stack as well.

Does anyone foresee any issues / concerns with what I am trying to accomplish ?

Thanks all in advance !! I appreciate any inputs / suggestions !!

D.

[Reza Sharifi · Accepted solution]

Hi,

3750 is Ethernet only, you can't terminate any WAN circuit on it and does not support GRE/ IPsec tunnels, if you don't need any of theses features then 3750 doing the routing will work just fine.  Make sure to uplink the access layer switches to both stacks and also install the correct license and image.

HTH

[dtran]

Hi Reza !! Thanks for your help !!

Currently all my access layer switches are uplinked to the MDF stack. Do you see any issues with that ?

This campus is about a mile away from my data center. The diagram below shows how it's connected to the data center. Do you foresee any issues / concerns ? The end goal is to move L3 process to the 3750 stack and each stack will have an L3 fiber link to the Nexus 7010 as seen below.

[Reza Sharifi]

Currently all my access layer switches are uplinked to the MDF stack. Do you see any issues with that ?

So, if all your switches uplink to the MDF stack, what is IDF stack providing?

Also, the 7ks are doing VRRP or HSRP right?

Reza

[dtran]

Hi Reza !!! See my response to your questions below.

So, if all your switches uplink to the MDF stack, what is IDF stack providing?

Beside IDF1 you see in the diagram I have a couple more IDF's that are uplinked to the MDF stack that I did not include in the diagram. The IDF stacks are purely L2 switches. My end goal is to move L3 over to the IDF1 stack and MDF stack to handl inter-vlan routing for this campus and leave the other IDFs the way they are.

Also, the 7ks are doing VRRP or HSRP right?

The 7Ks are doing HSRP.

Please let me know if you foresee any issues / concerns with this setup !!

Thanks Reza !! your inputs / suggestions are much appreciated !!

D.

[Joseph W. Doherty · Accepted solution]

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Posting

As a 3745 likely will struggle with FastEthernet, so doing L3 on 3750s make much sense.  (You might still want to retain the 3745 if you need to do WAN routing, as also noted by Reza.)

Normally you rarely run HSRP on 3750 stacks, as redundancy is provided by the stack.  (For redundancy, insure you have a complete stack ring [not shown as such on your posting] and consider setting up MEC between the two IDFs and the MDF.)

Also as the stack ring usually supports much more bandwidth then links between stacks, you generally don't want to "load balance" between stacks with HSRP.

Routing on either/both IDF might make sense if there's L3 East-West traffic on that device/stack.  If the traffic is North-South you can leave them L2 although you might want to insure VLAN edge ports do not extend beyond a single device/stack and you prune the VLAN trunks.

[dtran]

Hi Joseph !!! Your inputs / suggestions are much appreciated !!!

My thought behind running HSRP between the two stacks is just in case if I lost power to one stack I still have to the other stack to handle inter-vlan routing for me. I have a couple other IDFs that are uplinked to the MDF stack that I didn't show in the diagram. My end goal is the move L3 over to the IDF1 stack and the MDF stack and leave the other IDFs the way they are.

Thanks Joseph !!! Let me know if you foresee any issues / concerns with the setup !!!

D.

[Leo Laohoo]

My thought behind running HSRP between the two stacks is just in case if I lost power to one stack I still have to the other stack to handle inter-vlan routing for me.

Redundant power to the stack of switches, i.  e.  switch 1 goes to power 1, switch 2 goes to power 2, switch 3 goes to power 1, switch 4 goes to power 2, etc.

[Joseph W. Doherty]

Disclaimer

The  Author of this posting offers the information contained within this  posting without consideration and with the reader's understanding that  there's no implied or expressed suitability or fitness for any purpose.  Information provided is for informational purposes only and should not  be construed as rendering professional advice of any kind. Usage of this  posting's information is solely at reader's own risk.

Liability Disclaimer

In  no event shall Author be liable for any damages whatsoever (including,  without limitation, damages for loss of use, data or profit) arising out  of the use or inability to use the posting's information even if Author  has been advised of the possibility of such damage.

Posting

Ok, if power loss to the whole stack is an issue, with the topology you have in your second posting, it would be better changing the link between the two stacks to L3.  Why?  Well suppose that link fails.  If you have HSRP in both stacks for the same subnets, both stacks will have the active gateway for the same subnets and also advertise those same subnets to the upstream routers.  I.e. routing will be confused; from the Nexus routers, for host on subnet X, which downstream stack should it send the packet to?

An alternative approach, if you run both stacks with same gateways, would be MEC between them.  That will make the foregoing unlikely, but if you try to load balance, you can set yourself up for unicast flooding.

So, if you're going to enable routing on a stack, then it should have a L3 link to any other L3 device.  Avoids both of the above issues and also allows traffic to find the shortest path to its destination.

PS:

If you're unable to provide different power for each stack member, as Leo suggests, but do have two power circuits, you might also want to consider using a RPS unit.