We have 2 network rooms in our building and each are connected via a fiber link. One room is very tiny and is where our Comcast Internet comes into the building. I want to place our Cisco ISA570 into the other network room which is much larger and easier to work in so I must get the Comcast Internet over to the large network room and into the ISA570.

This is my plan in the small network room: - configure my switch so port 1 is VLAN 1. - plug in the Comcast cable modem to port 1.

This is my plan for the large network room. - configure my switch so port 1 is VLAN 1. - plug the Cisco ISA570 into port 1.

Since both of the switches are connected over the fiber link, I assume VLAN 1 traffic would egress the small room switch and arrive at the large room switch still tagged as VLAN 1 and would egress port 1 on that large room switch into the ISA570. And the reverse is also true.

Does this make sense? Am I doing the right thing here?

  • If you can provide the make/model of the switches you are using, we could narrow down our answers to be a bit more specific. Commented Jul 10, 2013 at 2:22
  • Plan seems to work just fine, I'd be just bit uncomfortable adding two new SPOF to the topology. But maybe you have redundant uplink via other provider, at which point it's not very relevant worry.
    – ytti
    Commented Jul 10, 2013 at 5:54
  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin
    Commented Aug 8, 2017 at 14:24

3 Answers 3


If you are using cisco gear configure the interswitch ports as trunks (never know when you may want to pass other things between the rooms). Using VLAN 1 in cisco land is considered bad form, so create say vlan 10 in both cisco boxes, in the switch in the small room the Comcast box is connected to an access port with vlan 10, then trunk vlan 10 between the cisco devices. Then anything on configured on vlan 10 in the large room goes directly to Comcast, you can use other vlans for things you don't want internet access to.

  • Strictly speaking nothing wrong in VLAN 1, what is usually bad form is running untagged traffic in core links. Native VLAN is usually untagged and is usually 1, but both are configurable. Although making native VLAN tagged removes lot of tools from you.
    – ytti
    Commented Jul 10, 2013 at 5:50
  • 1
    VLAN 1 is (or was) used by certain protocols (usually invisibly to the admin) on Catalyst switches. If a problem (i.e. traffic flood) occurred on VLAN 1 it could potentially create a problem. This isn't an issue with more recent switches / software revs but best practice still tends to be to just avoid it altogether.
    – rnxrx
    Commented Jul 12, 2013 at 3:16
  • I think that "bad form" is a good description. There's nothing wrong with it per se, but custom is to use it for the native VLAN, and using it like this violates the Rule of Least Surprise. Commented Aug 9, 2013 at 2:05

Assuming that you manage the fiber yourself (dark fibre, your own runs...etc), then yes - this should be possible as long as you have the proper configuration on both sides of the Cisco ISA570.


This will work as you've outlined, however if there are any other devices on Vlan 1 on either switch they will be "outside" of the ISA570. It will be better to create a new VLAN (999 for example) and place port 1 of each switch in that VLAN. You'll also need to ensure that each switch is configured to carry the new new VLAN between the two. This will logically connect the cable modem to the Internet/WAN port of the ISA.

CableModem --- Port1|Switch1|PortX <--fiber--> PortX|Switch2|Port1 <----> Internet Port|ISA

  • 1
    If there is the possibility of ever having more than one public IP on your side of Internet drop, I recommend defining a global VLAN as the "connected to the WAN drop" VLAN; I typically set this to 256 because of some admins' penchant for matching the VLAN number to the third octet of private IP addressing, and when I have my way I run red cabling on it as a reminder. This can be very helpful when you're doing development or troubleshooting work and need an "outside" IP, and it makes it easy to filter the VLAN from most of the network. Commented Aug 9, 2013 at 2:08

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