Design Question

[tohoken]

I would like some advice on setting up a routing plan for a small business. We use about 50 physical switches and about 25 VLANs. We currently use 2 3550 switches in a collapsed core model that perform routing and are setup using HSRP. I would like to build a stack of about 4 3750G switches and was wondering the following: from a design perspective, would it be better to configure 2 of the switches in the stack to run HSRP an take care of the campus routing, or would it be better to use a two dedicated routers and set them up using HSRP in a router on a stick configuration to handle campus routing? Thanks for any advise.

Ken

[Richard Burts]

Ken

In general I believe that you are better off to do the routing in the 3750 switches than to do router on a stick. With router on a stick all traffic between VLANs must go from the switches to the router and from the router back to the switches. The link between the switches and the router might become a bottleneck.

For a network such as you describe the routing performance of the 3750 is fine. I think using the 3750 for routing is the better design choice.

I am puzzled about your statement about running HSRP on 2 routers in the stack. If you build 2 stacks you could do this, but I do not believe that you can run HSRP between members of the stack.

HTH

Rick

[tohoken]

Rick

Thanks for the information. I was leaning this way also, just wanted some confirmation. I am new to the 3750 stacking technology so I have a question. Do all the switches in the stack have the same config? Meaning if one that is the active router fails, another switch takes on that role? If this is true then HSRP is not needed since all the switches will be backups of each switch in the stack.

Thanks,

Ken

[Richard Burts · Accepted solution]

Ken

Essentially each switch in the stack shares the config and if the master switch fails then another switch in the stack will take over. So HSRP is not needed (and I suspect is not possible).

HTH

Rick

[Joseph W. Doherty]

Rick is correct, HSRP isn't needed with a stack of 3750s, but it is supported. E.g. If you have a single 3750, you can run HSRP with a WAN router.

[tohoken]

So if I am understanding you correctly, every switch in the stack will have the routing protocol enabled and configured so it can take over if needed? Also, without HSRP and a virtual IP that each router uses, how will the switches take on the IP address used for the end user's default gateway?

[Richard Burts]

Ken

Each switch in the stack would respond using the IP address assigned to the stack. The user default gateway is the address assigned to the switch interface for that VLAN and that address is shared by each router in the stack.

HTH

Rick

[tohoken]

Rick,

Thanks for the clarity, it makes sense now. One last question, I have a few users that sit on VLAN1, not a good practice I know, so would the default gateway of VLAN1 be the IP address of the stack?

[Richard Burts]

Ken

The default gateway for users in VLAN 1 would be the address assigned to the switch interface for VLAN 1. Most of us would understand that to be the management address for the stack.

One possible clarification: your question seems to imply that there is a single address that is the stack address. That would be true for a layer 2 switch which can have only a single IP address configured on a virtual (VLAN) interface. But it is not true for a layer 3 switch (or a stack). If the layer 3 switch/stack has 3 VLAN interfaces then the switch/stack has 3 IP addresses.

HTH

Rick

[tohoken]

Rick,

Thanks for all the information. So I am fine for routing if a switch goes out and all devices will continue to be routed, with the exception of the devices plugged into the failed switch. So for redundancy, if I have a stack of 4 3750s that act as a aggregation point for my incoming fiber uplinks, I should connect a fiber link to switch one in the stack and another fiber link to any switch besides switch one so I have physical redundancy also?

[Richard Burts]

Ken

Yes if you want/need layer 2 redundancy then a connection into switch 1 and another connection into some switch other than 1 would provide layer 2 redundancy. And the switch stack addresses layer 3 redundancy.

HTH

Rick

[Richard Burts]

Joseph

Your point is well taken that HSRP is supported between the stack and some other device. My comment about not supported was in the context of the original post which suggests that he is asking about:

to configure 2 of the switches in the stack to run HSRP an take care of the campus routing

HTH

Rick

[Joseph W. Doherty]

Rick,

From the very high quality of your posts, I figured you knew, but I also try to consider other readers of these posts. Which is why I both agreed with you, and also mentioned a stack, but didn't want others to think the feature wasn't there at all and why you might still need to use it when working with a single 3750.

[Richard Burts]

Joseph

I certainly agree with you that we need to consider other readers of these posts. And I acknowledge that someone might read my post without understanding the context and get the incorrect understanding that HSRP is missing in the stack. I appreciate your filling in a gap that I may have created.

HTH

Rick

[Joseph W. Doherty]

Agree with Rick, stack of 3750s is much better.

If possible, you might also consider connecting high bandwidth hosts directly to that stack, e.g. servers. If your plans for 4 3750Gs don't provide sufficient ports to do that, also consider adding more 3750Gs to the stack.

Since the stack provide redundancy, you only need one HSRP instance.