[nsp] adding another class C network switched by 3524XL

Stephen J. Wilcox steve at telecomplete.co.uk
Mon Mar 31 15:29:56 EST 2003

On Mon, 31 Mar 2003, Dave [Hawk-Systems] wrote:

> >> >Be aware, however, that communication between machines on the same
> >> >Ethernet but on different IP subnets will travel via your service
> >> >provider's router, thus causing a possible bottleneck at the 100 Mbps
> >> >port towards their equipment.
> >>
> >> I am (at this point in time) not critically concerned about clogging up that
> >> link.  However, could we bridge the two networks in the switch to avoid this?
> >> Since machines from both IP addresses connect directly to the switch I would
> >> suppose we would set up VLANs or something then provide a bridge
> >between the two
> >> VLANs...  a little beyond my Cisco expertise though.
> >
> >You are confusing your Layer2 and Layer3 functions throughout this.. as the
> >devices are all in the same VLAN (L2) they are already all bridged.
> >
> >Your problem is your IP addresses (L3) are in different subnets, to traverse
> >between subnets you must use a router (L3) which your SP is providing.
> The confusion may also be stemming from this swtich being only L2 and the 3550
> we have slated to replace it being capable of L3 (or so the documentation
> reads).  Would things be different with the 3550 (L3) being able to
> intelligently route the IP traffic between the two networks without involving
> the SP router or requiring the installation of an additional router into the
> rack?

I have had bad experience with the G-L3 Cat switches, I'd advise you get a 
separate router such as a 2610 or 3620 to work with your current L2 switch 
rather than the all in one box. 

Things arent different tho, they're exactly the same except your switch now also 
has the ability to route. But the two functions are separate, you still 
configure your L2 and VLANs separately to your L3 and IP routing. 

In fact you might say that if you think a switch+router is confusing putting
them both in the same box and then trying to figure out whether your command
refers to a L2 or a L3 item can only be more confusing!! :)


> >Have a good long think and read about L2 (ethernet, VLANs) and L3 (IP,
> >routers),
> >it might help to clear things in your head,
> Sleep too. :)  Thanks.

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