The real reason
PE router gets a packet with TTL=0
- sends back ICMP Time Exceeded via appropriate Routing Table
P router gets a packet with TTL=0
- wants to send back a ICMP Time Exceeded packet
- How to route?
- P router has no routing information
- Only thing he can do is to send the packet via a LSP
- LSPs are unidirectional
- P router does not know which LSP goes back to the sender
- So P router sends packet via receiving LSP
- So ICMP packet travels through the MPLS backbone to the other PE, which then has the routing information to send ICMP packet back through another LSP to the sender
- So each traceroute probe travels through the whole MPLS backbone twice
- TTL is high!
Customer can accept this because his productive traffic is not affected