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L3 switch VS. Router and switch

Discussion in 'Networking' started by jamesb2, Apr 30, 2015.

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  1. jamesb2

    jamesb2 Thread Starter

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    Would like to get some opinions on this. I have a Dell N3000 switch and a Cisco 2901 router. Somebody recommend to me to consolidate the two devices and let the N3000 switch and route. My question is, is there are performance increase? What is the downfall of bundling the two devices? What is the upside?
     
  2. CleaverX

    CleaverX

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  3. zx10guy

    zx10guy Trusted Advisor Spam Fighter

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    Layer 3 switches are used all the time in enterprise networks where some people would think a router would be used. Both devices do what layer 3 devices do...which is route packets. The differences are in the details of the particular device. In general, layer 3 switches are preferred in campus and data center deployments due to the high port count and cheaper cost per port (especially when you're talking about high speed interfaces such as 10, 40, and 100 Gig....with 25 and 50 Gig soon to be released). Also in some cases the performance of layer 3 switches may be superior to a full on router such as latency. In many data centers, the network admins are very concerned about how much time a packet takes to go from port to port. Many high speed layer 3 switches can move data from port to port in about 600 nanoseconds and sometimes less....this includes being routed.

    Where a full on router is beneficial is when you're dealing with WAN circuits, tunneling, and routing protocols. Because layer 3 switches are designed for a specific purpose, they don't have the large routing table spaces found in dedicated routers. Also many routers are better at processing routing protocol traffic such as OSPF, EIGRP, IS IS, and the big one BGP. When routing decisions and topology changes happen, these full on routers can converge faster than many layer 3 switches...if they have the capability of supporting said routing protocol.

    To blur the line even more, firewalls can be layer 3 routing devices (and most are configured this way); some routers support transparent mode which means they operate strictly at the layer 2 level. Firewalls can route and many of them can support routing protocols. But these devices are meant to be security devices and concentrate on traffic dissection rather than moving packets as fast as they can.

    I'm very familiar with the N3000 series switch. In fact, I have a N3024P in my home network providing PoE services. I don't see any problems with using an N3000 as your internal router but this is a guarded endorsement as I don't know what your network looks like. To give you example, I have 3 Layer 3 switches acting as routers in my home network. One is a Juniper EX4200, the other is a Dell S4810, and the last is a Cisco 3560E. They are all sharing route tables with each other via OSPF. I have some Cisco routers in my network which have a very defined purpose; they're mostly there for proof of concept work/experimentation.

    Some vendors such as Cisco sort of blurred the line with how a layer 3 switch would be used in a network. The Swiss Army knife....Catalyst 6500 series blurred that line. This layer 3 switch allowed one to do regular layer 2 switching, layer 3 switching, and will support some WAN circuits through the use of add in line cards like the Flex WAN card. The 6500 also supported other services modules such as the FWSM firewall, CSM/ACE load balancers, and intrusion detection services with their NAM module. How I know about this is I've worked with this switch in a number of various environments and have used some of those available option modules.

    I don't know if what I've said answered your question. Or just created more confusion. I guess the answer is...it depends. And an answer can only be formulated based on what is going on in your network.
     
  4. jamesb2

    jamesb2 Thread Starter

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    Thanks for the input guys. I was talking to some people around the building and the logic is three fold 1) that there is one point in and out from the switch to the router as two separate devices 2) the backplane of the switch is faster than the router and 3) store and forward vs cut-through. Everything is working correctly right now. I am just seeing what tweaks I can do to the network to increase the speed.

    I am not using any routing protocols (RIP, IGRIP, etc) it's a very flat network without may frills.
     
  5. zx10guy

    zx10guy Trusted Advisor Spam Fighter

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    Is there a specific performance issue you're trying to solve?

    What's the layout of your network? What's the purpose of the router if you have one flat network? Are you routing among multiple VLANs?

    While cut through has a performance advantage over store and forward, you have to be aware of the type of traffic and loads on the network. If you have a lot of large data frame type traffic and this overwhelms the buffers of said switch operating in cut through mode, you can easily drop frames and lose data requiring retransmissions with performance degradations. While store and forward isn't as fast as cut through, you can be assured the frames are transmitted. You can invoke mechanisms like flow control to offer up a safety net for cut through switches. But some times interop between manufacturers isn't very seamless.
     
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