Distribution Switching

[GFrazier]

This one is interesting:

I.

have a client that has an MX100 and three MX220’s.  These switches are in different areas of the building - Sw1 (MDF room with router and firewall) uplinked to MX100.  The other 2 switches are uplinked to Sw1.  Mind you, from what I understand, these are Layer 2 switches.

in this scenario, what would be the best approach?  I figured to just use the MX100 as the acting distribution switch since it’s doing all the routing anyways. Uplink the other 2 switches to the MX100.  WWYD? 

[Bruce]

As always (annoyingly), it depends. 

If your network is generally a flat Layer 2 network where the communications is generally between clients on a single VLAN, then having the two switches connected to the one makes sense, as a lot of the traffic won’t need to hit the MX (and so reduces the load on the MX).

If however you have a well structured network with separation between server and clients so that the majority of traffic goes through the MX (as it’s a acting as a Layer 3 gateway) then it probably makes little difference as the traffic is traversing the MX to reach the other VLANs anyway. 

Generally I’d take the approach you currently have, MX to MS, then that MS to the other MSs, but if you using the MX for Layer 3 (since the MS220s are Layer 2) I don’t think there’s much difference either way.

[GFrazier]

Hi Bruce,

The network is on several VLANs setup on the MX - it is acting as the gateway.  With the equipment i have to work with, what would be your suggestion being that it is not a flat layer 2 network with a single VLAN?

[Bruce · Accepted solution]

I can’t see it mattering too much. If the MX is the only Layer 3 device then all inter-VLAN traffic is going to traverse the MX anyway. If most of your traffic is inter-VLAN then it’s probably not going to make much difference. If your trunk between the MS and MX is heavily utilised then you may see some benefit having three paths from the MX to the M (I.e. one to each MS). But that said, if you’ve got that much traffic on the trunk to the MX then you’re likely pushing the limits of the MX100.

As you can tell from my blabbering, there is no right or wrong answer when you have Layer 2 switches, with an MX as your Layer 3. It’s more what works for you. (Personally I’d go MX to one MS, then that MS to the others - at some point int the future you may introduce a Layer 3 switch, or make use of some of the other MS features between the switches).

[GFrazier]

Thank  you very much - that was helpful.

I do think we may be pushing the limits to the MX.  We are urging for an upgrade to the devices anyways - more ports are needed and better devices.  We are looking at an MS225-48LP model (layer 3) and MX250.  I will make sure I use the layer 3 features on the "core" switch.  

[GreenMan]

 key point, to add to what has already been said:   MXs do not participate in Spanning Tree, beyond simply forwarding BPDUs within the same broadcast domain;   any loops you create must be resolved by the switching, so think very carefully about the loops you are creating, where the root bridge lies and what links are going to be discarding.

https://documentation.meraki.com/MX/Networks_and_Routing/MX_Layer_2_Functionality#MX_Handling_of_Mul...

[GFrazier]

@GreenMan 

THANK YOU for the info!! 

I assumed that the MX had some sort of STP feature being that it can act as a switch to some extent.  I was going to connect the MS’s directly to the MX (separate uplinks) just as it has in the diagram from the link you sent me (thanks).  but looks like this is not such a good idea after all.

We are expecting to upgrade all equipment to:  MX250, MR225-48LP’s (L3 switches).  Even with that setup (separate uplinks directly to MX), looks like I would have to do some extra configuring for it to work correctly.

Looks like I will just need to use the traditional setup:  MX --> MS (for distribution).   As a matter of fact, we may need 3 MS’s in the MDF - the other switches are in different locations of the building.  

I would also like you guy’s opinion on this as well... we will be upgrading and expanding their network soon:

SW1, 2, 3 in MDF

SW4 & 5 in IDF closet on another floor

SW6 & 7 in IDF closet on another floor

My thoughts were to do this:

-Setup Sw1 for Layer 3 switching, while relay DHCP to MX250.

--SW1 Distribution Switch, uplinked to MX250

--Sw2 & 3 Stacked and uplinked to Sw1

--Sw4 & 5 Stacked and uplinked to Sw1

--Sw6 & 7 Stacked and uplinked to Sw1

I have never utilized DHCP relay for switching - do you think this is a good idea to let the MX handle this?  I want to make sure I optimize the LAN as much as possible.  I want to be sure I am configuring DHCP correctly.

If this helps, all servers will be in the Cloud (Azure) - even our printing service.  We will have VLANs separating traffic between Guests, Members, and Staff. 

[Bruce]

@GFrazier the lack of STP support on the MX is good to remember, but if you only have a single link to it from each switch then it doesn’t make too much difference - if you have multiple links you need to consider the spanning-tree topology.

With your plans, it generally sounds good, but a couple of things to consider:

1. Why not stack all three switches in the MDF (assuming they’re located together, and the same model). You could use 3x MS225 if you only need basic Layer 3 capabilities - they’ll support up to 16 VLAN interfaces and 16 static routes. You can then use the stack as your Layer 3 gateways.
2. Having all switches in a stack in the MDF would allow you to dual connect the MX and the IDF switch stacks to provide improved redundancy - in physical stack you can do cross switch LACP aggregation (this is only to the MS, the MX doesn’t support LACP so you need to rely on STP for redundancy).
3. Relaying DHCP to the MX should be fine; the MS225 only supports DHCP relay (and Azure doesn’t allow DHCP servers).

[GFrazier]

@Bruce 

I really appreciate your input, Bruce.  I will make sure to keep this in mind and configure accordingly.  

Thank all you guys for your help on this.

Originally, I was doing networking more in depth, but I moved away from it for a few years as a Telecom Manager for the State (GA Dept of Public Health).  I am now back to doing this and looks like I have gotten rusty on a few things.  I "discovered" the Meraki product and it has surely made networking life easier than it used to be. 

I know I will be posting here quite a few times in the future for tips and information - Hopefully, you all will not tire of giving me advice here and there.