Go CCIE Go!! - A Cisco Certified Internetwork Expert

Gday,
I think I’m missing something quite fundamental here but I can’t seem to find the reason why…
I have the following setup:
R1 (CE) - R2 (PE) - R3 (P) - R4 (P) - R5 (PE) - R6 (CE)
R2 is matching ICMP inbound from R1 and marking it MPLS EXP 4 R3 is matching MPLS EXP 4 from R2 and remarking it to MPLS EXP1
I see that is happening when I debug on R4 (the COS=4 on the XMIT is the VPN label) which is what I expect as R5 is currently sending implicit null to R4. If I configure R5 to advertise explicit null in LDP, the debug output changes correctly and the xmit CoS = 1. Anyhow that’s not my issue - just background info.
R4 *Mar 1 02:40:51.859: MPLS: Et0/0.34: recvd: CoS=1, TTL=253, Label(s)=19/21 *Mar 1 02:40:51.863: MPLS: Et0/0.45: xmit: CoS=4, TTL=252, Label(s)=21
The bit that is eluding my understanding is the output on R3.
R3 *Mar 1 00:10:59.075: MPLS: Et0/0.23: recvd: CoS=4, TTL=254, Label(s)=17/21 *Mar 1 00:10:59.079: MPLS: Et0/0.34: xmit: CoS=4, TTL=253, Label(s)=19/21
I can’t work out why the xmit is showing the CoS=4 - even though R4 does receive it as CoS=1 ? I expected on R3 to see CoS=1 on the XMIT.
This is the policy map in have inbound on R3’s connection to R2.
class-map match-all EXP4 match mpls experimental topmost 4 ! policy-map IN class EXP4 set mpls experimental topmost 1
The policy-map output is incrementing counters correctly. So whilst the re-marking is working - I don’t quite understand why the debug on R3 isn’t consistent with what I subsequently see on R4.
Any insight would be great. Thanks.
Con.
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