[c-nsp] How to use IPs from the same network on two separate interfaces?

Brandon Orwell xt at codeslum.org
Sat Jul 16 07:11:53 EDT 2016

I am new to Cisco in general and have the following setup:

GigabitEthernet0/0 contains an IP address that is connected to our
provider/BGP drop;
GigabitEthernet0/1 contains various IP addresses assigned ending in .1 for
the IP prefixes we announce to the Internet; this is also the Interface
that all of our devices are plugged into via our switch <-->
GigabitEthernet0/1 connection.

We want our primary IP address on GE0/0 to be on our primary assigned IP
network, such as a.b.c.252 while keeping .1 on GE0/1 for a gateway for our
local machines/equipment. Currently when traceroutes or the like come into
the network, they get responded to with the IP assigned from our uplink to
GE0/0. We'd like this instead to be an IP from our own assigned prefix for
identification/notification/consistency sake. (We'll keep the IP from our
uplink as a secondary IP).

When I try to assign .252 to GE0/0 and .1 to GE0/1, I get the usual errors
about the subnet already existing on another interface.

How do I go about doing this? How do I assign IP in A.B.C, such as
A.B.C.252 to GE0/0 as the primary IP address while assigning A.B.C.1 to
GE0/1 so that clients on the network can use it as a gateway?

Or, is there a way to allow those on the LAN to use IPs on GE0/0 even
though they're plugged into GE0/1, without borking anything too bad? And
more to this point even more so, and going a bit off the rails but curious
to know if it's possible - if we have three distinct prefixes assigned from
RIPE, and wanted to have traceroutes and what-have-you respond with the .1
IP from the beginning of each subnet to the requestor, would this be
possible? I am imagining something to do with NAT and matching response
with a given source IP for certain ICMP packets (but this wouldn't work
for, say, UDP traceroute?).  Any ideas there either?

Thanks in advance!


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