1. What are the drawbacks of using unnumbered links?
Individual WAN interfaces are no longer reachable by ping or telnet if you
use unnumbered links.
2. Where should you use unnumbered links in the MPLS backbone?
Unnumbered links are recommended in the ATM parts of the MPLS
3. Where would you use unnumbered links between PE and CE routers?
Using unnumbered links between PE and CE routers is highly
discouraged. There are, however, applications like dial-up access that
benefit from unnumbered links.
4. Why would you use private address space in your IP backbone?
IP backbones usually only use private address space if there is no public
address space available.