Comprehensive and Detailed In-Depth Explanation:
1. Understanding MPLS TTL Processing
* TTL (Time to Live) is an IP field that prevents routing loops by decrementing at each hop.
* In an MPLS network, the TTL field can be processed in two modes:
* Uniform Mode (Default Mode)
* Pipe Mode (TTL is preserved in the MPLS domain)
2. TTL Processing in Pipe Mode
Pipe mode is used to prevent the MPLS backbone from affecting the TTL of an IP packet.
* When an IP packet enters the MPLS domain, the original TTL is copied into the MPLS header.
* Inside the MPLS network:
* MPLS labels use their own TTL values, and the IP TTL is not decremented.
* Each P router (transit router) decrements only the MPLS TTL, not the IP TTL.
* When the packet leaves the MPLS domain (at PE2):
* The original IP TTL is restored but decremented by 1 as it exits to CE2.
3. Analyzing the TTL Behavior in the Given Topology
* Initial IP TTL from CE1 = 255
* At PE1:
* IP TTL remains 255 as it is copied into the MPLS label.
* MPLS TTL starts at 255 (same as IP TTL).
* At P Router:
* MPLS TTL is decremented (255 # 254).
* IP TTL remains unchanged (255) because of Pipe mode.
* At PE2 (Egress Router):
* IP TTL is restored and decremented by 1 (255 # 254).
Thus, the correct sequence is:# IP TTL: 255 # 255 # 254
4. Evaluating the Answer Choices
Option
TTL Values
Correct?
Reasoning
A
255 # 254 # 252
# Incorrect
IP TTL should remain 255 inside MPLS
B
254 # 253 # 252
# Incorrect
IP TTL starts at 255, not 254
C
255 # 255 # 254
# Correct
IP TTL is preserved in pipe mode inside MPLS and decremented only at egress.
D
254 # 254 # 253
# Incorrect
Initial IP TTL should be 255, not 254
# Correct answer: C. 255 # 255 # 254
Final Conclusion:
* Pipe mode ensures that the MPLS domain does not affect the IP TTL while the packet is inside the MPLS network.
* Only the egress PE (PE2) decrements the IP TTL before forwarding the packet to CE2.
* Thus, the correct TTL sequence is 255 # 255 # 254.