Re: What do you consider acceptable packet / session modification for a network operator?

[Jared Mauch via NANOG <nanog () lists nanog org>]

> On Dec 26, 2025, at 11:05 AM, Marco Moock via NANOG <nanog () lists nanog org> wrote:
>
> Am 26.12.2025 um 10:52:48 Uhr schrieb Tom Beecher:
>
> > Packet comes in with DF set. Egress interface MTU is too small. ICMP
> > Frag Needed generated, source address is RFC1918 loopback from the
> > router control plane. On the return trip, packet crosses network that
> > (correctly) drops all RFC1918 sourced traffic.
>
> Can't you use NAT (the Cisco ISR devices support nat inside on lo too
> IIRC) or configure the source address for outgoing router traffic?

Some devices you can configure this, but it varies and gets even more interesting when you have a device that may not 
even have an IPv4 address but is processing IPv4 packets.

Eg:

https://www.juniper.net/documentation/us/en/software/junos/cli-reference/topics/ref/statement/autogen-protocols-bgp-group-family-inet-unicast-extended-nexthop.html

I think as the scope of IPv4 public continues to narrow we will see more of this as time goes on.  This was already a 
challenge for IPv6 over a 6PE network, but this is just the flip side of that.

- Jared
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