If you’re upgrading your Service Provider network from ATM- or SDH-based core to Carrier Ethernet core, you could be tempted to keep the unnumbered point-to-point links. The practice of using unnumbered P2P links is debatable, but if you want to, you can configure them on VLAN interfaces in recent IOS releases.
Ivan Pepelnjak, CCIE#1354, is the chief technology advisor for NIL Data Communications. He has been designing and implementing large-scale data communications networks as well as teaching and writing books about advanced technologies since 1990. See his full profile, contact him or follow @ioshints on Twitter.