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

Brian Johnson brianj at nvc.net
Mon Mar 31 08:43:53 EST 2003

I have a question pertaining to L3 and L4 switches and this conversation. ;)

If one were to install a L3 or L4 switch and had multiple logical IP
segments on the same Physical Segment, would the switch then become the
default gateway and actually separate the logical segments into physical

- Brian J.

> -----Original Message-----
> From: cisco-nsp-bounces at puck.nether.net 
> [mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of Dave 
> [Hawk-Systems]
> Sent: Monday, March 31, 2003 8:25 AM
> To: Stephen J. Wilcox
> Cc: Niels Bakker; cisco-nsp at puck.nether.net
> Subject: RE: [nsp] adding another class C network switched by 3524XL
> >> >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?
> >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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