[c-nsp] Private IP in point to point link on internet

Mike mike-cisconsplist at tiedyenetworks.com
Tue Jun 21 01:10:01 EDT 2016

On 06/20/2016 07:52 PM, Satish Patel wrote:
> This is weird question but i thought let me get opinion from you guys.
> We have following scenario
> [ISP]------<Public-IP>-----[Router]-------<Private-IP>-------[L3
> Switch]------------[Hosts]
> In above diagram we get /24 subnet from ISP for hosts, Now i want to
> configure routed network between [Router] and [L3 switch] so can i use
> Private IP address (rfc1981)  like instead of Public
> IP address, to save public IP address in point to point link. what
> would be the disadvantage i will get doing that?

Well, if I understand your question, you want to put private IP's on the 
point-to-points to save ip addresses. The only primary side-effect you 
will have is for path mtu discovery - if the router or l3 switch needs 
to fragment a packet for example (or send back any other icmp messages 
like host unreachable or the like), it's likely to use the interface 
address 'facing' the destination which would be your private IP's. These 
are dropped by many firewalls and such, which effectively breaks path 
mtu discovery and such. Depending on your equipment, you could assign 1 
public IP to a loopback interface on the router and to the l3 switch, 
which usually will cause that device to use that address as the source 
for any icmp messages it generates.I am not %100 certain of the fine 
details (would love someone to point out an authoratitive doc that 
explains this for cisco), but I have used this method for exactly this 
reason and it does seem to work.


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