A few weeks ago I got into an interesting discussion about the potential harm caused by unnumbered IPv4/IPv6 interfaces.

Ignoring for the moment the vendor-specific or media-specific implementation details, these two arguments usually pop up in the first 100 milliseconds (assuming engineers involved in the discussion have some hands-on operational experience):

• You cannot do hop-by-hop telnet/SSH in case your routing protocol breaks down;
• Traceroute will display box addresses (loopback addresses) instead of interface addresses.

The first argument is obviously valid in IPv4 world and no longer true in IPv6 world – link-local addresses are always available on IPv6 interface and you can use them to do hop-by-hop telnet/SSH (admittedly in a bit clunky way but nonetheless). However, do keep in mind that if you use any centralized authentication/authorization scheme and the routing protocol breaks, you won’t get into the box no matter what.

Speaking of traceroute: it might give you wrong interface information any time you happen to have asymmetrical routing or multiple equal-cost paths. Without any additional configuration, in most implementations the source IP address in the ICMP TTL exceeded reply is the IP address of the outgoing interface – the interface the router would use to reach the traceroute prober – and not the IP address of the interface through which the probe was received. Is there a box out there that’s smarter than this? I would love to hear about it!

2016-01-12: RFC 5837 specifies ICMP extensions that would make ICMP replies to traceroute probes way more useful (and accurate). However, it looks like nobody implemented it in almost five years since it was published.

Are there any other reasons to avoid unnumbered interfaces (vendor X doesn’t implement them on interface type Y doesn’t count)? Please write a comment!