next-hop-self
Explanation:
To determine the correct command, we need to analyze BGP behavior in a Hub-Spoke topology and how routes are advertised in MPLS VPN environments.
1. Understanding the Hub-Spoke BGP/MPLS VPN Topology
* The network consists of Hub-PE and Spoke-PE routers, which are MPLS VPN PEs.
* Hub-PE is responsible for distributing routes between Spoke-PE1 and Spoke-PE2.
* Spoke routers do not have direct IBGP peering with each other; instead, they rely on the Hub-PE to relay routes.
* BGP next-hop handling is critical for proper route advertisement.
2. Why is the "next-hop-self" Command Required?
* By default, when BGP advertises a route, the next-hop remains unchanged (i.e., the original next- hop is retained).
* In Hub-Spoke topology, Spoke routers would receive routes with a next-hop IP that belongs to another Spoke-PE, which is not reachable directly.
* To solve this issue, we use the next-hop-self command on the Hub-PE, so that:
* Hub-PE updates the next-hop to its own address before advertising routes to the Spoke-PEs.
* This ensures that Spoke-PE routers use the Hub-PE as the next-hop, making routes reachable and preventing blackholing.
3. Correct Command Usage
The administrator must run the following command under the BGP IPv4 address family of the BGP- VPN_out instance on Hub-PE:
shell
CopyEdit
peer 10.1.2.1 next-hop-self
This ensures that the next-hop for advertised routes is changed to the Hub-PE's IP, allowing proper route transmission in the Hub-Spoke MPLS VPN environment.

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.