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300-410 Practice Question: Configures iBGP between DMVPN hub and spokes…
A network engineer configures iBGP between DMVPN hub and spokes using the hub as a route reflector. On the hub, the BGP configuration includes 'neighbor <spoke-ip> next-hop-self'. Unexpectedly, spokes receive routes from other spokes with the next-hop set to the hub's tunnel IP, but the spokes cannot reach that next-hop because it is not in their routing table. Which is the most likely explanation?
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
✓
The hub's 'next-hop-self' command is configured under the BGP neighbor statement for the spoke, but the route reflector behavior overrides it, causing the hub to not modify the next-hop for routes reflected between spokes.
In a DMVPN Phase 2 or 3 network, the hub typically sets the next-hop to itself using 'next-hop-self' for routes advertised to spokes. However, if the hub is a route reflector, it does not change the next-hop for routes received from one spoke and advertised to another spoke, unless 'next-hop-self' is explicitly configured. The corner case is that 'next-hop-self' must be applied under the address-family or neighbor configuration, and if it is misapplied or missing for the route-reflector client sessions, the spoke-to-spoke routes retain the original next-hop (the other spoke's tunnel IP), which may not be reachable if NHRP redirect or shortcuts are not enabled.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The hub's 'next-hop-self' command is configured under the BGP neighbor statement for the spoke, but the route reflector behavior overrides it, causing the hub to not modify the next-hop for routes reflected between spokes.
Why this is correct
In a route reflector setup, 'next-hop-self' must be configured under the address-family for the neighbor; otherwise, the reflector does not change the next-hop for reflected routes.
- ✗
The spokes are not configured as route-reflector clients, so the hub does not reflect routes between them, and the next-hop remains unchanged.
Why it's wrong here
If the spokes are not clients, the hub does not reflect routes; but the symptom is that spokes receive routes from other spokes, indicating reflection is happening.
- ✗
The iBGP session between hub and spokes is using loopback interfaces, and the next-hop is set to the loopback IP, which is not reachable via the tunnel.
Why it's wrong here
If loopbacks are used, the next-hop would be the loopback IP, but the issue here is that the next-hop is the hub's tunnel IP, not a loopback.
- ✗
The 'next-hop-self' command is only applicable for eBGP sessions, not iBGP, so it has no effect on the reflected routes.
Why it's wrong here
'next-hop-self' works for both eBGP and iBGP; it is commonly used in iBGP to ensure reachability.
Go deeper
Related to this question
Learn chapter
Introduction to ENARSI Exam and Network Fundamentals
Key term
DMVPN Phase 2
DMVPN Phase 2 is an advanced Cisco routing technology that allows spoke routers to communicate directly with one another without sending traffic through a central hub, using dynamic routing protocols and multipoint GRE tunnels.
Key term
BGP Route Reflector
A BGP Route Reflector is a networking device that reduces the number of BGP peer connections needed in a large network by allowing routers to share routing information without needing to connect to every other router directly.
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