350-501 Networking Practice Question
An SP engineer is designing a BGP-based MPLS L3VPN service. The PE routers are fully meshed via iBGP for VPNv4 routes, and an RR is deployed to reduce sessions. The engineer notices that all PE routers are receiving duplicate routes from different PEs, causing suboptimal path selection. Which BGP feature should be enabled to ensure proper load balancing across multiple equal-cost paths?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Enable BGP multipath on the PE routers
BGP multipath on PE routers enables the router to install multiple equal-cost paths into the routing table, allowing load balancing across them. In an MPLS L3VPN iBGP full mesh with an RR, duplicate routes are received from different PEs; BGP multipath selects and uses up to the configured number of equal-cost paths. Option A (BGP additional paths) allows advertising multiple paths but does not directly install them. Option B (second RR) reduces sessions but does not affect duplicate routes. Option C (ignoring interior cost) modifies best-path selection but does not enable multipath.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Enable BGP additional paths on the RR
Why it's wrong here
Enabling BGP additional paths on the RR allows the RR to advertise multiple paths per prefix, but it does not cause the PE routers to install multiple paths in the routing table. It is not the correct solution for load balancing.
- ✗
Deploy a second RR to reduce the number of iBGP sessions
Why it's wrong here
Deploying a second RR would increase the number of iBGP sessions and potentially lead to even more duplicate routes, worsening the problem. It is not a solution for load balancing.
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Configure the BGP best path selection algorithm to ignore interior cost
Why it's wrong here
Configuring the BGP best path selection algorithm to ignore interior cost is not a standard feature in BGP. Even if it were, it would not enable the installation of multiple equal-cost paths for load balancing.
- ✓
Enable BGP multipath on the PE routers
Why this is correct
Enabling BGP multipath on the PE routers allows them to install up to a configured number of equal-cost paths in the routing table. This enables load balancing across the multiple paths, which is the required outcome.
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