A mpls domain router should have connectivity to its neighbor, if your pe routers are under mpls domain then they should have the ldp adjacency with his neighbors directly, not via cross any other router/ LFIB lookup.
thanks for replying but in the my case will there be any problem with label switching for PE4 router. Because i have another case where PE1 / PE2 /PE4 are same location and only PE1 and PE2 are directly connected to P router and PE4 will be connecting to PE1 and PE2 due to no fiber pairs. Will there be any problem for Label switching.
Can you explain why we require P router in MPLS backbone ?
It is not a mandatory rule that MPLS cloud should be designed with P routers. Normally PE routers are edge LSRs which are connected to customer site (to which VPN service is required to be provided), and P routers are core routers where customers are not connected.
But still u can design MPLS cloud with just 2 PE devices back to back connected or with a mesh of PE devices connected among themself without any designated P router in between. (Though it might not be a good design).
But the basic requirement is that the core facing interface should be capable of handling/transmitting labelled packet. This raises the requirement that LDP or any label signaling protocol should be enabled on the core facing interface.
In your case interface between PE4 and PE2 should be enabled with LDP (or other labe lsignaling protocol like RSVP-TE).By doing this when PE4 send VPN traffic to customer site connected to PE2 will have VPN label as only label or as bottom label with top label as explicit-null label. In eitehr case, PE2's dataplane will be programmed to deliver the same to respective VPN cusotmer site. And if PE4 is sending any traffic destinated to VPN site connected to otehr PE like PE3 will be treated as transit traffic by PE2.
The below training may give you some idea about MPLS VPN,
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