Comprehensive and Detailed In-Depth Explanation:
1. Understanding the Hub-Spoke Network Model in MPLS VPN
* Hub-Spoke (or Route Reflector) topology is used in MPLS Layer 3 VPNs where multiple spoke sites communicate through a central hub.
* The Hub-PE acts as an intermediate BGP speaker and redistributes routes between spokes.
* The challenge is preventing routing loops when exchanging routes between spokes.
2. Understanding the Role of Site of Origin (SoO) in BGP/MPLS VPNs
* BGP Site of Origin (SoO) is a route attribute used to prevent routing loops in scenarios where a hub redistributes routes between spokes.
* If a route originating from a spoke is advertised back to the same site, SoO prevents it from being re-learned.
* The SoO value (e.g., 200:1) ensures that a site does not accept its own previously advertised prefixes.
3. Analyzing the Answer Choices
# A. [Hub-PE-bgp-VPN_out] peer x.x.x.x allow-as-loop (Incorrect)
* The allow-as-loop command allows BGP to accept routes with its own AS number in the AS path.
* This is NOT needed in a Hub-Spoke VPN because BGP routes are not being looped through the same AS.
# This is unnecessary for preventing loops in a Hub-Spoke VPN.
# B. [Hub-PE-bgp-VPN_in] peer x.x.x.x allow-as-loop (Incorrect)
* Again, allow-as-loop is used when a router needs to accept its own AS number in the BGP AS path.
* This is NOT the mechanism used for preventing loops in Hub-Spoke VPNs; SoO is used instead.
# This is not required in this scenario.
# C. [Hub-PE-bgp-VPN_in] peer x.x.x.x soo 200:1 (Correct Answer)
* The correct way to prevent spoke routes from looping through the hub is to apply the Site of Origin (SoO) attribute.
* This command ensures that if a route with SoO 200:1 is received, it is not advertised back to the same spoke.
# This is the correct answer.
# D. [Hub-PE-bgp-VPN_out] peer x.x.x.x soo 200:1 (Incorrect)
* Applying SoO on outgoing routes is NOT effective because the hub should mark received routes and prevent loops when re-advertising them.
* SoO should be applied on the inbound (received) routes at the Hub-PE, not outbound.
# This is not the correct implementation of SoO.
Final Conclusion:
# Correct answer: C. [Hub-PE-bgp-VPN_in] peer x.x.x.x soo 200:1# A. allow-as-loop is unnecessary in a Hub-Spoke MPLS VPN.# B. allow-as-loop is not relevant here.# D. SoO should be applied inbound, not outbound.
Thus, the correct answer is: C. [Hub-PE-bgp-VPN_in] peer x.x.x.x soo 200:1.