mediumMultiple Choice
300-410 Practice Question: Is troubleshooting a BGP peering issue between…
A network engineer is troubleshooting a BGP peering issue between two directly connected routers, R1 and R2. R1 is configured with 'neighbor 10.1.1.2 remote-as 65002' and 'neighbor 10.1.1.2 update-source Loopback0', while R2 uses 'neighbor 10.1.1.1 remote-as 65001' and 'neighbor 10.1.1.1 update-source Loopback0'. The loopback interfaces are not advertised into any IGP, and there is no static route for the loopback addresses. The BGP session remains in Idle state. What is the most likely cause?
⚠ Common exam trap
The trap here is assuming that directly connected routers can always peer regardless of source address — candidates forget that update-source changes the source IP and therefore requires routing to the neighbor's configured address, not just link-level connectivity.
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 BGP session is stuck in Idle because the neighbor statements reference loopback interfaces that are not reachable.
BGP forms a TCP session between the addresses specified in the neighbor statements. Because update-source Loopback0 is configured, R1 sources its TCP packets from its loopback address and expects to reach R2's loopback (10.1.1.2) as the neighbor. Since the loopbacks are not advertised into any IGP and no static routes exist, neither router has a route to the other's loopback, so the TCP three-way handshake to port 179 never completes and the session stays in Idle.
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 BGP session is stuck in Idle because the neighbor statements reference loopback interfaces that are not reachable.
Why this is correct
BGP uses the update-source address as the TCP source, so each router attempts to reach the peer's loopback. With no IGP advertisement or static route for those loopbacks, the TCP connection cannot be established, leaving the session in Idle.
- ✗
The BGP session is stuck in Idle because the remote-as values are mismatched.
Why it's wrong here
Both routers use remote-as values matching the other's actual AS number, so the session is not failing on AS mismatch. It is tempting because remote-as mismatches commonly cause Idle, but here the loopbacks are unreachable, preventing TCP establishment.
- ✗
The BGP session is stuck in Idle because the update-source command is not allowed on directly connected interfaces.
Why it's wrong here
update-source is valid on any BGP neighbour, including directly connected ones, so it cannot cause the Idle state. The real fault is unreachable loopback addresses, since no IGP or static route advertises them. The option tempts engineers who assume update-source only applies to multihop or loopback-based peerings.
- ✗
The BGP session is stuck in Idle because the neighbor statements must use the directly connected interface IP addresses.
Why it's wrong here
BGP neighbour statements may reference any reachable address, not only directly connected interface IPs, so this is not the cause. The Idle state stems from the loopbacks being unreachable, as neither IGP nor static routes advertise them. It tempts engineers who conflate eBGP multihop restrictions with a general addressing rule.
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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