routing using 3550 switch

Unanswered Question
Oct 30th, 2008

our ISP has terminated 2MB BW using Optical network terminal ( ONT ) there is one ethernet interface on ONT & one IP address is given to it that is since i need pool of 16 IP so the ISP guys told me to install a router with two ethernet interface one will connected to ONT with routable static IP & on other ethernet interface i will route my IP pool to connect my all other devices .

the obove scenario works fine , but i have 3550 switch too can i apply above scenario using 3550 layer three functionality so no need to use router . cam some one help me in this kind of configuration.


I have this problem too.
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John Blakley Thu, 10/30/2008 - 06:29

Others may want to help or put me on the right track, but here's my thought.

The port that connects to your modem/router for the ISP will need to be configured as a routed port. If g1/1 is connected then:

int g1/1

no switchport

ip address

Then you'll create your vlan and SVIs for your workstations.

vlan 100 (creates your vlan)


int vlan100 (creates your SVI)

ip address

Your gateway on the workstations would be set to, and you'd have a default route of your ISP's gateway on the switch.

ip route (whatever your gateway is).

My disclaimer: I've never done it this way, and I've never seen a 3550. I have a 3750 and ip routing is enabled. If you want to have intervlan communication, you can enable ip routing on the switch, and then all of your hosts in separate vlans can see each other, but you'd have to control traffic using acls.

Others may have better suggestions or be able to correct me where I'm wrong. :-)



Jon Marshall Thu, 10/30/2008 - 17:15

John is correct in that you could do this with a 3550 switch. My only query would be about the public addressing. Are all your devices going to be using this public addressing or are you using private addressing and using NAT. If you are using NAT a 3550 will be no use to you - in fact only a 6500 would be any use for NAT.

Apart from NAT routers also have other advantages eg their QOS configuration is a lot more flexible.

There is nothing wrong with using the switch in this way but you may find yourself limited for some things.


pkaretnikov Thu, 10/30/2008 - 19:40

The above is good, just make sure you add the 'ip routing' command under global config since the 3550 is a switch first, router second. If you don't it won't route between vlans and/or any routed ports you set up.

Also you can set up on a vlan and then put a port in that vlan rather than changing the port to a routed port. This might be useful if you might ever have to place anything else in that subnet.


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