Category: IPv6

The brief display of the state of IPv6 interfaces in the router (show ipv6 interface brief) is significantly different from the well-known show ip interface brief display as the IPv6 address might not fit in the same line as all the other data. To filter the printout and display only the operational interfaces, you have to replace the include filter with the section filter, which displays all the lines matching the regular expression as well as associated follow-up lines.

One of the obscure facts about IPv6 OSPF (OSPFv3) is that it uses a 32-bit router ID like OSPFv2. It’s a reasonable choice; I have yet to see an OSPF network with over a billion routers. However, could you guess how this requirement is implemented in Cisco IOS? OSPFv3 searches for an IPv4 address (effectively the same algorithm used by OSPFv2) to get the router ID for the IPv6 routing process. Neat, isn’t it?

You might wonder what happens if you want to configure an IPv6-only router. OSPF won’t start unless you configure the router ID manually. And, no, you cannot enter a number (which would be the expected format, as the router ID is just a number in the IPv6 world); you have to enter an IPv4 address. Long live IPv4 :))

In response to my Never-Ending Story of IP Fragmentation, Stojanco Cavdarov made an interesting observation: routers are not allowed to fragment IPv6 packets, they have to respond back with ICMP unreachable (effectively, routers behave as if IPv6 packets would have an implicit don't fragment bit).

To make life easier for non-TCP IPv6 applications (TCP is supposed to use Path MTU Discovery), the minimum IPv6 packet size that has to be supported on all links was increased to 1280 bytes (which, incidentally, fits very nicely into GRE+IPSec envelope transported across links with 1500-byte MTU).