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

Garry gkg at gmx.de
Sun Jul 17 06:27:40 EDT 2016

On 16.07.2016 13:11, Brandon Orwell wrote:
> GigabitEthernet0/0 contains an IP address that is connected to our
> provider/BGP drop;
Why use part of your /24 or larger to do the connection to your ISP 
with? This is not how it should be done ...
> 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
Don't. Just don't. If you have overlapping network ranges, all kinds of 
things can - and will - go wrong, even if you trick the router into 
making it work ... ask your provider to assign a transfer network 
outside of your /24 or whatever for the uplink (e.g. something between a 
/29 to /31), you'll save yourself a lot of headaches.

N.b.: You could do something like that with VRF routing, which your 
router would need to support.
> 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?
Yes, 1:1 NAT would work, too ... but this adds to the strain of the 
router, as it has to rewrite every single packet it forwards, instead of 
just pushing it through with CEF.


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