The Real Reason
PE sends back ICMP Time Exceeded via appropriate Routing Table
P router has no routing information!
- Only information he has is receiving LSP, but LSPs are unidirectional
- So P router sends ICMP Time Exceeded via receiving LSP
- ICMP packet travels through the MPLS backbone to the other PE, which then has the routing information to send the ICMP packet back through another LSP to the sender
- So each traceroute probe travels through the whole MPLS backbone twice
- TTL is high!
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Customer can accept this because his productive traffic is not affected