> On Dec 26, 2025, at 11:05 AM, Marco Moock via NANOG <[email protected]> > 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 _______________________________________________ NANOG mailing list https://lists.nanog.org/archives/list/[email protected]/message/BJXWZPONG2TKQL43U2PQ6H267I2ULQSF/
