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What will be the issue if we use rfc1918 private IP address range /31 or /30 in point-to-point link?

[ISP]--------------[Router1]------rfc1918--------[L3 Switch]-------[LAN] 

I heard PMTU can be issue but what if i increase 9000 MTU on point-to-point link?

EDIT:

From ISP we get /24 IP subnet and we are running BGP on Edge router, We don't want to use NAT because our service required public IP direct on server.

Whole reason was using /24 for host subnet and don't use any single IP from it for Point-to-Point link. My question was can i just use private IP /30 between Router and L3 switch?

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  • I don't understand the question. RFC 1918 address space is just like any other IPv4 addressing, except that ISPs will block its use on the Internet.
    – Ron Maupin
    Jun 21 '16 at 15:01
  • I just raise point to find out does it cause any PMTU discovery issue in path because sometime they send ICMP message back for fragment stuff ?
    – Satish
    Jun 21 '16 at 16:13
  • RFC 1918 addressing has nothing to do with that.
    – Ron Maupin
    Jun 21 '16 at 16:18
  • I get it RFC 1918 has nothing to do with MTU but if remote host trying to find out MTU size of network path then it will send packet with Don't Fragment and Router will send ICMP mesg back with failed or pass. If router has private IP then it won't send ICMP back to host to find out MTU size.. I may be wrong but just want all option on table before we say lets do it.
    – Satish
    Jun 21 '16 at 16:25
  • I don't understand. Why do you need the second router with nothing but a link between it and the first router? Does the LAN use RFC 1918 space? is the WAN router using NAT? You really need to explain the whole thing.
    – Ron Maupin
    Jun 21 '16 at 16:28
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If you are only using RFC1918 between your PRIVATE devices on your LAN going from your core to your edge, there is no issue. Depending on your IGP there is nuance to the design, but beyond that, absolutely no problem there.

What you cannot do, is ask or attempt to use RFC1918 between your edge-router and your ISP. RFC1918 is unroutabl on the public internet. If it's not, get a new ISP :)

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  • (additional) ICMP messages generated by an RFC1918 addressed router will have an RFC1918 source address. As that flies across the internet, it's very likely to be dropped along the way. Plus, the use of a private address means the source of the error cannot be pinned down.
    – Ricky
    Jun 21 '16 at 19:59
  • Here is the nice article, why shoun't use rfc1918 in internet path tools.ietf.org/html/rfc6752
    – Satish
    Jun 24 '16 at 18:03
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The situation may vary on the services you have contracted for with your ISP.

If for example your ISP provides a 'normal' Layer 3 WAN connectivity, then your rfc1918 /30 or /31 IP addressing will not work as your WAN facing ip addresses will be unroutable.

But if your ISP provides your WAN connectivity through their managed business mpls vpn, they may provide you with ip addressing from rfc1918 addresses.

Mpls vpn implementations are very common with business ISPs and have been around for quite awhile. And there are costs involved.

https://en.wikipedia.org/wiki/MPLS_VPN

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  • We don't have MPLS any kind of VPN. Its very plain network 10G coming from ISP and terminated on Router1 Edge router for BGP and between ISP and My Router it used Public IP address /30 but from my Edge Router to L3 Switch inside i want to use rfc1918 Private IP address to save Public IP. that is the requirement and its very simple network. my Concern is doing that will break any legacy or any protocol (like PMTU)
    – Satish
    Jun 21 '16 at 18:38

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