Hello Smart,

>> What will happen to packets that have unknown labels in cisco routers?

>> Do the packets get dropped?

yes they should because the router doesn't know what to do with them they are not in the mpls forwarding table.

Notice that usually entries can be thought as made of (input interface, label) providing (output interface, label) (one or more out pairs).

>> Can we provide something like default route to forward all unknowns to upstream router?

the MPLS label is locally significant in the link between two LSRs: there is no sense on forwarding it unchanged on another link to somebody else.

Think of FR DLCIs or ATM VPI/VCI they change at each switch hop : if a frame with an undefined DLCI is sent to a switch it will drop it.

An MPLS label switched path can be associated to an IPv4 default route but that is a different matter.

Hope to help

Giuseppe