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Re: layer 3 switch debate

  • From: Stephen J. Wilcox
  • Date: Fri Sep 27 03:56:38 2002

On Thu, 26 Sep 2002, ip dude wrote:

> When designing an all IP network requiring mostly Ethernet interfaces, the
> logical conclusion is to specify layer 3 switches (instead of routers). The
> cost per port and functionality requirements make a layer 3 switch the
> perfect choice. 

I dont see this logical conclusion? My home network is all ethernet and my
Netgear hub does a fine job!

First questions would be how large is this network in terms of interfaces and
traffic flows and how is it distributed. If its small use cheap L2 switches, if
its on one or a couple of sites with not many hosts why do you need L3 in there
at all, stick to L2.. 

If its larger and more distributed then you need to aggregate up anyway so I'd
imagine its cheaper to use plain L3 routers connecting the L2 LAN across
intersite WANs

Needs more quantifying to find any conclusion but I dont see that an "all IP
network" requires a L3 switch network!

> However, the rule of thumb in the IP community is that routers are superior
> to layer 3 switches and should be utilized instead, especially when
> considering core type functionality.

I think we have a terminolgy issue here.. assuming a L3 switch is a device which
uses routing decisions to influence a switching process then you get this on
current Cisco routers.. the L3 makes the routing decision on the first packet in
the stream but then additional frames are switched. But that aside I think you
mean what vendors call "L3 switches or L4 switches" which are like a L2 switch
but go into higher layer protocols to influence the switching decision and
perform other features most commonly load balancing.

> Does this rule of thumb still apply considering the modern layer 3 switches
> available? If not, why? What makes a layer 3 switch sub-standard to a pure
> router? Any quantitative analysis you could provide would be greatly
> appreciated.

Most commonly seems to be interoperability, the switches do their own job fine
in their own isolated environment but they cant act as a "ISP router".. in my
experience then tend to have odd bugs and behave slightly unexpectedly when say
for example routing OSPF or BGP. Altho this is probably a chicken and egg - if
more people tried to use them perhaps the vendors would fix the code!

> I realize your answer may depend on device position within the network. I am
> comparing a router to a layer 3 switch as a core routing device, an EBGP
> border router and access device. Remember, my network is comprised of mostly
> Ethernet interfaces (FW, GE) and the occasional DS1 and DS3 interface.


> Any opinions would be great.
> Cheers!
> Asand Bijaka 
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