I inherited a network laid out (very broadly speaking) as follows:

A number of access switches connect to 2 main switches. The Main Switch in the MDF (SW1) connects to the WAN router and is connected to the Main Switch in the IDF (SW2) by 5x Cat6 links.

Is there any way to add redundancy to this network? Running cables is impossible in our current situation but I do have two spare switches. The only real idea i've had is this: enter image description here A third switch would be added in the IDF where some of the Access Switches would connect to. SW3 would use two of the MDF-IDF Cat6 links to link it to SW1 and would be connected to SW2

There are still single points of failure but at least I would halve the number of devices affected by a SW2 failure. It would also allow me to setup some manual MSTP load balancing.

The main function of the network is really to provide a WAN connection; local service were non existent when I arrived. Thats beginning to change but WAN still remains the focal point. Therefore would it make more sense to use a cable for SW2-WANRouter?

Thoughts on the matter greatly appreciated!


This is looking like the next iteration of the topology change and tries to incorporate forward expansion by including the servers I plan to add.

enter image description here

Two switches get added, one into the MDF (SW3) and one into the IDF (SW4).

-3 links from SW1-SW2
+1 link SW1 - SW4
+1 link R1 - SW2
+1 link SW2 - SW4
2x links each between SW1-SW3 and SW2-SW4
2x Servers in the IDF, both with 1 link to SW2 and SW4

To get this to work, IVR will have to be configured on SW2 (currently only on SW1) and multiple instances of VRRP added to both SW1 and SW2. I've got a feeling that I'm going to have a few hours of cursing spanning tree with this config!

Unfortunately all access switches have to be treated as hosts due to there being no option to run extra cable to them.

  • I think the solution by @tonton gerrit is spot on. To generalize you can always consider a fat tree topology with K=2 in this case. Or a leaf spine fully meshed network
    – john
    Commented Apr 11, 2017 at 21:01

2 Answers 2


Redundancy Suggestion

Do you have enough router ports available to do this ? You would still have your five trunks between the two buildings, but an outage on SW1 wouldn't bring down the whole LAN anymore.

You would need to finetune loadbalancing.

Ideally you could maybe add a second uplink from switches 1-5 to SW4, as well as from 6-10 to SW2. And maybe an extra portchannel between SW1-SW3 and SW2-SW4 :

Suggestion 2

Bottom line : with the same capacity between the two buildings, you only have one single point of (total) failure left : the router.

  • I could have enough if I removed unused WAN links but each time we move (this network floats) we use different links. That means manually changing configuration and replacing links each time and its not something I particularly want to do.
    – eto.ttnflt
    Commented Aug 14, 2016 at 8:26
  • I do have 1 port spare on the Router which I could link to SW2. Note: SW1 & SW2 are good L3 switches. All access and possible spares are less powerful therefore I want to keep as much on 1-2 as possible. One is HP and the other Juniper (forward planning by my predecessor) so are there any vendor neutral alternatives to GLBP?
    – eto.ttnflt
    Commented Aug 14, 2016 at 8:50

I think you're on the right track adding the extra device to the IDF.

If SW2 ever fails, you could also have a port pre-configured on SW3 so that you could quickly, manually fail patch it across and restore service to access switches 1-5 while SW is replaced.

I would also look at placing another switch in the MDF, connected between SW1, SW3 and the WAN router

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