Lacp

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21 cards — 🟢 3 easy | 🟡 4 medium | 🔴 3 hard

🟢 Easy (3)

1. What is link aggregation and what two benefits does it provide?

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Link aggregation combines multiple physical links into one logical channel, providing bandwidth aggregation (N links = up to Nx bandwidth for concurrent flows) and redundancy (traffic shifts to surviving links if one fails).

Remember: LACP bundles physical links. Benefits: more bandwidth + automatic failover.

2. What is the difference between LACP active and passive modes?

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Active mode sends LACP PDUs and actively initiates bond formation. Passive mode only responds to LACP PDUs from the partner. At least one side must be active; both sides passive means no bond forms.

Remember: LACP = Link Aggregation Control Protocol (802.3ad). Bundles links for BW+redundancy.

Fun fact: LACP PDUs exchanged every 1s (fast) or 30s (slow) to detect failures.

3. What Linux bonding mode provides simple failover without requiring switch configuration?

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Mode 1 (active-backup). One interface is active, the rest are standby. If the active fails, a standby takes over. No special switch configuration is needed because only one link is active at a time.

Remember: Active=initiates, Passive=responds. Both passive=nothing. "At least one Active."

Gotcha: Mismatched modes (static vs LACP) = no aggregation.

🟡 Medium (4)

1. Which Linux bonding modes require switch-side LAG configuration?

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Modes 0 (balance-rr), 2 (balance-xor), 3 (broadcast), and 4 (802.3ad/LACP) require the switch to be configured for link aggregation. Modes 1 (active-backup), 5 (balance-tlb), and 6 (balance-alb) do not.

Remember: Active=initiates, Passive=responds. Both passive=nothing. "At least one Active."

Gotcha: Mismatched modes (static vs LACP) = no aggregation.

2. What does the xmit_hash_policy control and which is best for IP traffic?

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xmit_hash_policy determines how outbound traffic is distributed across bonded links. layer3+4 (hashes on source/destination IP and port) is best for IP traffic as it distributes individual TCP flows across links. A single flow still uses one link.

Under the hood: LACP hashes (MAC/IP/port) to pick link per flow. Single flow = one link.

Gotcha: Single TCP connection stays on one link. Aggregate BW needs multiple flows.

3. How does link monitoring work in Linux bonding?

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miimon polls link status at a configurable interval (e.g., 100ms). If a link goes down, traffic shifts to remaining links. Alternative: ARP monitoring (arp_interval + arp_ip_target) verifies end-to-end reachability by sending ARP probes to a gateway.

Remember: Linux bonding mode 4 = 802.3ad (LACP). Modes 0-6: rr,ab,xor,bcast,802.3ad,tlb,alb.

4. How do you check the status of a Linux bond and its member interfaces?

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Read /proc/net/bonding/bond0 for full details including LACP partner info, member interface status, and bond parameters. Quick check: ip link show bond0. Member interfaces show "master bond0" in their ip link output.

Remember: Linux bonding mode 4 = 802.3ad (LACP). Modes 0-6: rr,ab,xor,bcast,802.3ad,tlb,alb.

🔴 Hard (3)

1. Why might a bonded link show asymmetric traffic distribution and how do you fix it?

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Hash-based distribution means a single TCP flow uses one link. With few concurrent flows, most traffic goes through one link. This is normal, not a fault. Fix by switching to layer3+4 hash policy for better distribution. True round-robin (mode 0) distributes evenly but can cause out-of-order packets.

Remember: Linux bonding mode 4 = 802.3ad (LACP). Modes 0-6: rr,ab,xor,bcast,802.3ad,tlb,alb.

2. What happens when LACP PDU exchange fails and how quickly is a link removed?

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LACP partners exchange PDUs at fast (1s) or slow (30s) intervals. If 3 consecutive PDUs are missed, the link is removed from the aggregate. With fast rate, detection takes ~3 seconds. With slow rate, ~90 seconds. Both sides should use the same rate to avoid mismatches.

Remember: LACP = Link Aggregation Control Protocol (802.3ad). Bundles links for BW+redundancy.

Fun fact: LACP PDUs exchanged every 1s (fast) or 30s (slow) to detect failures.

3. How do you create an 802.3ad LACP bond using nmcli with optimal settings?

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nmcli con add type bond con-name bond0 ifname bond0 bond.options "mode=802.3ad,lacp_rate=fast,xmit_hash_policy=layer3+4,miimon=100"; then add members: nmcli con add type ethernet con-name bond0-eth0 ifname eth0 master bond0. Fast LACP rate gives 3-second failover, layer3+4 hashing distributes IP flows.

Remember: Linux bonding mode 4 = 802.3ad (LACP). Modes 0-6: rr,ab,xor,bcast,802.3ad,tlb,alb.