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?

> This is not a routing problem at all. This is very common.

Indeed, but a filtering problem if routers on the path filter
non-public address ranges.

-- 
Gruß
Marco

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