<P> In the specific context of an MPLS - based virtual private network (VPN), LERs that function as ingress and / or egress routers to the VPN are often called PE (Provider Edge) routers . Devices that function only as transit routers are similarly called P (Provider) routers . The job of a P router is significantly easier than that of a PE router, so they can be less complex and may be more dependable because of this . </P> <P> Labels are distributed between LERs and LSRs using the Label Distribution Protocol (LDP). LSRs in an MPLS network regularly exchange label and reachability information with each other using standardized procedures in order to build a complete picture of the network they can then use to forward packets . </P> <P> Label - switched paths (LSPs) are established by the network operator for a variety of purposes, such as to create network - based IP virtual private networks or to route traffic along specified paths through the network . In many respects, LSPs are not different from permanent virtual circuits (PVCs) in ATM or Frame Relay networks, except that they are not dependent on a particular layer - 2 technology . </P> <P> When an unlabeled packet enters the ingress router and needs to be passed on to an MPLS tunnel, the router first determines the forwarding equivalence class (FEC) for the packet and then inserts one or more labels in the packet's newly created MPLS header . The packet is then passed on to the next hop router for this tunnel . </P>

Describe push swap pop functions in mpls and also explain penultimate hop popping (php)