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Very recently I picked up a Cisco 3550 just to play around with networking and to familiarize myself with Cisco's command line configuration due to the fact that I will be working an electrical engineering internship in which I will have to deal with various networking issues (yes, I agree that a CSNA would be much better suited for this task, but since its part of the job requirements, I'm willing to learn about it).

What I was wondering, is whether or not my 3550 can act as a router. I know its a L3 switch, but does that mean it can take the place of a router?

EDIT: Since it seems possible to set my 3550 to act as a router, could you guys possibly point me to some documentation that might help me set this up? I think I need to use the ip route command, but I'm not entirely sure if that is the right approach.

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4 Answers 4

A 3550 will do great routing packets and even using BGP, but as mentioned by jgoldschrafe it won't hold complete BGP tables (that is, every route announced on the internet by any AS, with a CIDR larger than /25).

There's nothing stopping you from using 3550 devices to BGP announce your networks, and use default routes to transit providers. Thanks to the Cisco Express Forwarding technology, it will actually do more PPS than it's cousins ISR 2800 serie, according to the cisco specifications anyway.

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The term "L3 Switch" describes a switch that is also a router. A layer 3 switch should be a serviceable, but likely limited, router.

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Most layer 3 switches, including the Cisco 3550, do not have TCAMs large enough to allow the routing engine to hold Internet-scale routing tables. It will do a fine job of simple LAN routing, but you're probably not going to get away with running BGP on it.

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If you 3550 has an SMI image installed, the switch will be Layer 2 only, and won't support routing. If it has an EMI image installed, it will support Layer 3 switching. 3550s will route, but lack the functionality of a dedicated router (such as support for advanced routing protocols), so don't expect to be able to do NAT and eBGP on any scale, if at all.

Try putting "ip routing" in the configuration - if the switch supports this, you have an EMI image. If not, you need to download an EMI image from Cisco.com and make sure you have a licence for the software.

After enabling IP routing, create at least one SVI, and VLANs for the SVI. Place ports in VLANs as required, and create a static route, if necessary, using "ip route X.X.X.X Y.Y.Y.Y Z.Z.Z.Z".

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eBGP is possible. –  3molo Apr 1 '12 at 12:09
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