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

Stephen J. Wilcox steve at telecomplete.co.uk
Mon Mar 31 14:58:37 EST 2003

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

> >> Since the datacenter we host our equipment in applies the gateway IP
> >> for our current block to their end, we just route to it via eth0...  I
> >> am assuming that to serve the second class C we just have the
> >> datacenter advertise the YYY.YYY.YYY.4 address on the same interface on
> >> their end along with the XXX.XXX.XXX.1 address, and we just have each
> >> server use the appropriate gateway address, and the switch will route
> >> everything out the eth0 port once it leans that both gateway IP
> >> addresses are there?
> >
> >Your switch does not route.  It is only forwarding Ethernet frames.
> >You need to change its default gateway only if you renumber out of
> >your XXX.XXX.XXX.0/24 netblock.  As long as your service provider
> >properly configures YYY.YYY.YYY.4/24 on the same interface (as a
> >secondary IP address, for example) no further configuration is needed
> >on your part.
> Assumed such, jut didn't want to be caught unaware of anything at 1am  :)
> >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.

If you really want host-host communication within that VLAN between hosts in the 
different subnets (L3) you could configure secondary IP addresses (L3) so they 
can talk directly without needing a gateway (router).

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


> Thanks for the comments received to date.
> Dave
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