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In our rented office, we have a router (BT smart hub) connected to a large switch (model unknown). The two are connected using three ethernet cables, thus:

-----------------------
|  1  2  3  4  Router |  
-----------------------
   |  |  |  |
   |  |  |  ------[Mobile signal booster]
   |  |  | 
------------------------------------
|  1  2  3  4 5 6 7 8 9 10  Switch |
------------------------------------

Can anyone please explain the advantage this offers? I always assumed that a switch was connected to a router using a single cable (as here).

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  • Disable spanning tree protocol on the switch and see what happens. If your network stays up, then none of the connections is redundant. If it crashes and burns, then you have a redundant connection.
    – Mike Scott
    Mar 14, 2017 at 20:37

3 Answers 3

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32

The switch is probably a managed switch on which multiple VLANs are configured. The three cables between the router and the switch are used to provide inter-VLAN routing.

Another possibility is that the multiple cables are used for link aggregation (e.g. LACP) to increase throughput.

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  • I normally think of the switch traffic as being pretty fast compared to the other side of the router. In my home office scenarios like that, I can't see how aggregation would help. (FWIW, I am not a network guru by any stretch).
    – donjuedo
    Mar 14, 2017 at 19:19
  • @fernando.reyes, Thanks for the reply. But wouldn't the intranet traffic go through the switch? Or are there multiple routers in the larger scenario you're describing?
    – donjuedo
    Mar 14, 2017 at 22:33
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    @donjuedo For all we know, it could be router on a stick with link aggregation to allow for higher inter-VLAN throughput.
    – Todd Wilcox
    Mar 15, 2017 at 0:51
6

I couldn't find the full specs of the router, but from the little there is I am not sure it supports anything fancy on its ethernet side. It could be just a mistake or stupidity.

In theory and as a general answer it could be different VLANs like @dr01 said, load balancing, allowing x3 of the capacity of a single ethernet link, or fall back in case one port fails.

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2
  • I have that router at home. It doesn't do anything special, it's just your standard home router.
    – Tim
    Mar 15, 2017 at 9:30
  • actually in this setup if one ethernet port totally dies another one will be used...
    – Rsf
    Mar 15, 2017 at 13:11
4

As this is a rented office do you share the connection with other tenants ?
It is not uncommon to have a separate outlet for each customer on the router.

The switch receives each of those connections on a separate port and keeps them internally separated by using VLAN's. Depending on to which office the various other connections on the switch go these connections are placed in the same VLAN as the router-link for that customer.
(The alternative approach to that would be to hook up a dedicated switch for each customer to each router outlet, which is easier to configure, but probably more expensive.)

Other than that link-aggregation comes to mind, but that doesn't make sense as the BT router and the internet connection would not profit at all from the increased bandwidth as the internet is in general much slower than a single link between router and switch.
(And as Rsf already mentioned, it is doubtful the BT router would technically be able to support it. Most SOHO routers don't.)

That leaves a simple mistake as option number 3.

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5
  • Thanks for this. As a test I'm going to pull out cables 2&3 and basically see who screams first. There should be no VLANs and if there is its being done without our knowledge.
    – EvilDr
    Mar 14, 2017 at 19:39
  • @EvilDr Good luck and please let us know what happened. I'm curious :-)
    – Tonny
    Mar 14, 2017 at 20:29
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    @EvilDr That's not the right test. Disconnect #2 or #3, not both!
    – Loren Pechtel
    Mar 14, 2017 at 22:40
  • @LorenPechtel The right test would be to try all possible combinations, including disconnecting #2 and #3 at the same time. :-)
    – Nobody
    Mar 15, 2017 at 14:48
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    @Nobody True, but you start with one cable. If you do them both and people yell you still learned nothing about whether they are both essential. Any one person should yell at most once.
    – Loren Pechtel
    Mar 15, 2017 at 22:24

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