Multiple networks on Cisco 2811 -> 3750 stack

[smorrissey88]

Hi all,

I'm (incredibly new to networking and) trying to wrap my head around how to create multiple networks on a single switch stack and router.

What I currently have is a Cisco 2811 router that is connected to the outside via GigabitEthernet0/0. I then have network 10.10.10.0/24, with a DHCP pool, on GigabitEthernet0/1. I believe this is referred to as "router on a stick"?

GigabitEthernet0/1 connects to a HP ProCurve switch (port 1). Port 24 on the ProCurve is a trunk port that connects to GigabitEthernet1/0/1 on my Cisco 3750 stack.

This is all working great. I can plugin to any switchport on the ProCurve or 3750 and get an IP address on the 10.10.10.0/24 network and can successfully connect to the outside (woo!). This is all being done on the "default VLAN". The reason I even have a ProCurve there is because my 3750 stack only has 12 total GigabitEithernet ports (SFP), the rest are FastEthernet, while the ProCurve has 24 gigabit ports.

Now what I wanted to do was create another network, 10.10.20.0/24, where my servers, NAS, etc would sit. My thought was I'd be able to do something similar: take one of the FastEthernet ports on the 2811 router, create another DHCP pool, plug it into port 2 on the ProCurve switch, assign port 2 on the ProCurve to be VLAN 20, create the same VLAN on the 3750 stack, and config some ports on it to be VLAN 20 access ports.

But obviously that's wrong and my understanding of the issue is wrong. This was made obvious when I plugged into a VLAN 20 access port on the 3750 and didn't get an IP via DHCP. So rather than trying to troubleshoot that I'd like to better understand the right way to handle this.

I've been doing some reading and can't seem to wrap my head around how to properly tackle the issue I'm trying to solve here, which is basically that I want to create another network that can exist on the same stack of 3750's as my current 10.10.10.0 network (thus wanting to create a VLAN (vlan20) for devices on that network), while also being able to get DHCP from the 10.10.20.0 pool on the 2811 and NAT to the outside.

[Mark Malone · Accepted solution]

Hi for router on a stick with multiple vlans you need to use sub-interfaces , this doc below explains it well , the dhcp pools can be on the switch or the router that makes no difference but its important if your using this design to have sub-ints on the router side and trunk on the switch side to allow multiple vlans be pushed between router and switch  , switch is purely  layer 2 in this setup router is layer 3 so this is how they need to operate , routers dont support trunks so sub-ints are required

your other option is to create layer 3 setup between router and l3 3750 switch routed port with an IGP running and route between each device but router and stick would be easier to start off with

www.orbit-computer-solutions.com/How-to-Configure-Router-on-a-Stick-InterVLAN-Routing.php

http://ciscobasics.blogspot.ie/2010/02/configure-router-on-stick.htm

[smorrissey88]

Hi Mark,

Thanks for the reply! I'd been trying the sub interfaces on the 2811 with little success but it's now clearer to me why.

I'm guessing that my 4 port FastEthernet HWIC card in the 2811 router doesn't allow for sub interfaces -- which makes sense as those are L2 switchports if I understand correctly.

When I used a single GigabitEthernet interface from the 2811 and created 2 sub-interfaces off of it (gi0/1.10 and gi0/1.20), setup the trunk ports properly, etc. it worked like a charm.

[Mark Malone]

Glad it helped and yes your right L2 ports will not allow sub-ints only l3 routed ports allow this and usually for the specific reason of connecting back to layer 2 trunked ports that allow many vlans

[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.

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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
I'm guessing that my 4 port FastEthernet HWIC card in the 2811 router doesn't allow for sub interfaces -- which makes sense as those are L2 switchports if I understand correctly.

Correct, but, BTW, there are also routed FE port HWIC cards too.  They (logically) behave just like the built-in Ethernet ports.

Re: ". . . how to properly tackle the issue . . .", as Mark already noted, since the 3750 is a L3 switch, normally you would route between it and the ISR.  You don't actually need to use an IGP, but you often would.

The advantage of routing on the 3750, is the 3750 has much, much (much) more routing throughput than a 2811.  (A 2811 is good for about up to 40 Mbps, while the 3750 handles multiple gig, easily.)  I.e. You only want the 2811 to need to deal with off-site routing, inter-VLAN routing should be done by the L3 switch.

PS:

BTW, if you have a 3750 stack, it's possible to multi-member Etherchannel between the 2811 and the 3750 stack.  You would do this to take advantage of the redundancy offered by the 3750 stack.

(Oh, and if your wondering, if you only have two routed ports, how do you access the outside, you define another VLAN for that and do what you're now doing on the router, i.e. inside and outside subinterfaces.)

[smorrissey88]

Hi Joseph,

The limited throughput on the 2811 is something I hadn't considered. I'm currently using gig0/0 as the external interface and gig0/1 and the sub ints are natting, which works great however I've only been accessing the internet over wireless, I haven't tried a hard-wired connection yet. But based on what I'm hearing the gig port connected to the outside can only do ~40Mbps? I ask because my external line is capable of nearly 100Mbps. 

But limiting inter-vlan routing to 40Mbps seems like a show-stopper so I'll definitely look into doing it on the 3750 stack. I've never done that before -- do you know of a good resource that has sample configs of a 3750 with multiple VLANs that forwards to a router if needed? I tend to learn best by looking at configs.

Cheers!

[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

Sorry, don't have a handy example of configuring routing on the 3750.  It's not much different from your 2811.  You need to enable IP routing, and then you can route between routed interfaces or SVIs on the 3750.

Where it gets a bit more complicated is routing between the 2811 and the 3750.  Again, not difficult if you're used to setting up multiple router topologies.