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

Niels Bakker niels=cisco-nsp at bakker.net
Mon Mar 31 11:33:38 EST 2003

* dave at hawk-systems.com (Dave [Hawk-Systems]) [Mon 31 Mar 2003, 07:29 CEST]:
> Currently have a Class C(XXX.XXX.XXX.0/24) with .1 as the network
> gateway going out port eth0 on the 3524XL switch which is received by
> datacenter equipment(router or switch, who knows) which has the class C
> mapped and advertised to the net and so forth.  Simple setup from our
> end, just plug in the .1 as the gateway and the switch takes care of
> the rest.
> With a recent takeover we are having to drop some systems from another
> company into the same rack for a period of time while we move over
> services and sites etc...  They are currently served on a different
> class C(YYY.YYY.YYY.0/24) and similarly use .4 as the gateway (which
> was a linux router box of some sort where the class C was advertised to
> from their ISP uplink).
> 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?
> No fancy VLANs or anything...  just basic switching.
> Does that figure correctly?

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.

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.

	-- Niels.


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