LPIC-1 System Architecture Practice Question
A data center server with two NICs (eth0 and eth1) is configured for network bonding in mode 1 (active-backup). The admin notices that after a cable pull on eth0, the bond interface fails over to eth1 as expected. However, when the cable is reconnected to eth0, the bond remains on eth1 indefinitely. The admin checks /proc/net/bonding/bond0 and sees that eth0 is marked as 'up' but not as 'active'. Which parameter is most likely missing from the bond configuration? Options: A) 'miimon=100' to enable link monitoring, B) 'downdelay=0', C) 'updelay=0', D) 'primary=eth0' to prefer eth0 as the active slave.
⚠ Common exam trap
Test-takers frequently assume 'miimon' alone handles both failover and failback, but in active-backup mode, failback to a preferred slave requires the explicit 'primary' parameter.
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
✓
primary=eth0
The bond is in active-backup mode (mode 1) and eth0 is marked as 'up' but not 'active' after reconnection. Without the 'primary=eth0' parameter, the bond does not automatically switch back to the preferred slave (eth0) once it becomes available; it only fails over to eth1 when eth0 goes down. Setting 'primary=eth0' ensures that eth0 is always preferred as the active slave when it is in 'up' state, triggering a failback.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
updelay=0
Why it's wrong here
Controls delay before using a newly up link; but without primary, it won't preempt.
- ✗
downdelay=0
Why it's wrong here
Controls delay after link goes down; not related to failback.
- ✗
miimon=100
Why it's wrong here
Already present as failover works; missing miimon would prevent any detection.
- ✓
primary=eth0
Why this is correct
Designates eth0 as preferred; the bond will switch back when it becomes available.
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