LPIC-2 Advanced Networking Configuration Practice Question
A Linux server with two NICs bonded in mode 1 (active-backup) was working correctly until a switch was replaced. Now, although both interfaces are up, the bond always shows only one active slave, and if that slave fails, traffic does not fail over. The bonding configuration uses miimon=100 and neither arp_interval nor arp_ip_target is set. You run 'cat /proc/net/bonding/bond0' and see that the MII status of both slaves is 'up' but the link failures count is 0 for the backup slave. What is the most likely cause, and which parameter should be adjusted?
⚠ Common exam trap
A common mix-up: candidates assume MII monitoring is sufficient for all failover scenarios, overlooking that MII only detects physical link loss, not upstream network failures that leave the NIC link up but break 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
✓
Configure arp_interval and arp_ip_target to enable ARP monitoring.
Without ARP monitoring (arp_interval and arp_ip_target), the bond relies solely on MII status to detect link failures. MII monitoring only detects physical link loss at the NIC level, not upstream switch failures or misconfigurations. In this scenario, the switch replacement likely caused a layer-2 or layer-3 issue that does not bring the NIC link down, so MII reports 'up' but traffic cannot pass. Enabling ARP monitoring forces the bond to verify reachability of a target IP, triggering failover when ARP replies are lost.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Change bonding mode to mode 4 (802.3ad) with LACP.
Why it's wrong here
Mode change will not solve link monitoring; it's a different aggregation method.
- ✗
Use ifenslave to manually reassign the backup slave.
Why it's wrong here
Manual reassignment defeats automatic failover.
- ✗
Decrease miimon to 50 for faster detection.
Why it's wrong here
MII cannot detect failures where link remains electrically up but data stops.
- ✓
Configure arp_interval and arp_ip_target to enable ARP monitoring.
Why this is correct
ARP monitoring can detect reachability of a gateway, catching switch-level issues.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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