Sanjib
So R2 and R4 are PE devices and R3 is a P device ? And you are not doing MPLS VPNs, just normal MPLS ?
If so R4 must have a route to 5.5.5.5. Did you mean R3 does not have a route ?
With MPLS any P device only knows how to get to other P devices and PE devices. It does not know about any networks outside the MPLS network. The PE devices have routes to any external networks connected to them and know how to get to P and other PE devices.
Note that in terms of PE/P reachability the routing/LDP information has already been exchanged and an MPLS forwarding table built. So for reachability between PE/P routers (ie. not outside the MPLS network) route lookup do not need to be performed, it is simply a matter of removing and adding labels.
So assuming you are not using MPLS VPNs and that MPLS labels have been exchanged between all P and PE routers -
R1 pings 5.5.5.5
1) R1 does a route lookup for 5.5.5.5 and sends it to R2.
2) R2 is a PE device. It does a route lookup for 5.5.5.5 and sees R4 as the next hop (this route has been exchanged via MP-BGP).
3) R2 then need to send the packet to R4. To do this it looks in it's MPLS forwarding table to see how to get to R4. The MPLS forwarding table tells it which label to attach to the packet. It adds this label and sends the packet out of it's interface to R3.
3) R3 is a P device. It does not need to do a route lookup for 5.5.5.5. It simply looks at the incoming MPLS label and consults it's MPLS forwarding table. It will have an entry in it's table saying that an incoming packet with an MPLS label of x should be sent out with an MPLS label of y. It changes the label and sends it out to R4.
4) R4 receives the packet, removes the label, does an IP lookup for 5.5.5.5 and sends it to R5.
Note that actually R3 might not add a label as there is something called Penultimate Hop Popping where if the net hop from the P router is a PE router it sends the packet without a label to save extra processing on the PE router. If you had multiple P routers only the last P router before the PE router would do PHP.
The key thing to understand is that the P devices don't do route lookups when forwarding data. They simply switch labels. Whereas the PE devices do need to do route lookups for destinations outside the MPLS network.
Jon
