--- tags: - networking - l1 - flashcard-deck - lacp --- [Portal](../../../../library/portal/index.md) | **Level:** [L1: Foundations](../../../../library/portal/levels.md) | **Topics:** [LACP / Link Aggregation](../../../../library/portal/topics.md) | **Domain:** Networking id category difficulty tags question answer source_path lacp/c520b3ab6547 lacp easy lacp, bonding, basics What is link aggregation and what two benefits does it provide? 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).\n\nRemember: LACP bundles physical links. Benefits: more bandwidth + automatic failover. training/library/topics/lacp/primer.md lacp/d6fc0606ece5 lacp easy lacp, 802.3ad, modes What is the difference between LACP active and passive modes? 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.\n\nRemember: LACP = Link Aggregation Control Protocol (802.3ad). Bundles links for BW+redundancy.\n\nFun fact: LACP PDUs exchanged every 1s (fast) or 30s (slow) to detect failures. training/library/topics/lacp/primer.md lacp/90101bd9aa31 lacp easy lacp, bonding, linux What Linux bonding mode provides simple failover without requiring switch configuration? 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.\n\nRemember: Active=initiates, Passive=responds. Both passive=nothing. "At least one Active."\n\nGotcha: Mismatched modes (static vs LACP) = no aggregation. training/library/topics/lacp/primer.md lacp/970e6020a612 lacp medium lacp, bonding-modes, switch Which Linux bonding modes require switch-side LAG configuration? 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.\n\nRemember: Active=initiates, Passive=responds. Both passive=nothing. "At least one Active."\n\nGotcha: Mismatched modes (static vs LACP) = no aggregation. training/library/topics/lacp/primer.md lacp/a889e5c85054 lacp medium lacp, hash-policy, traffic What does the xmit_hash_policy control and which is best for IP traffic? 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.\n\nUnder the hood: LACP hashes (MAC/IP/port) to pick link per flow. Single flow = one link.\n\nGotcha: Single TCP connection stays on one link. Aggregate BW needs multiple flows. training/library/topics/lacp/primer.md lacp/eb3c7418c5d2 lacp medium lacp, monitoring, miimon How does link monitoring work in Linux bonding? 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.\n\nRemember: Linux bonding mode 4 = 802.3ad (LACP). Modes 0-6: rr,ab,xor,bcast,802.3ad,tlb,alb. training/library/topics/lacp/primer.md lacp/1587d4900626 lacp medium lacp, status, troubleshooting How do you check the status of a Linux bond and its member interfaces? 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.\n\nRemember: Linux bonding mode 4 = 802.3ad (LACP). Modes 0-6: rr,ab,xor,bcast,802.3ad,tlb,alb. training/library/topics/lacp/primer.md lacp/adc9d9a42bbb lacp hard lacp, asymmetric, debugging Why might a bonded link show asymmetric traffic distribution and how do you fix it? 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.\n\nRemember: Linux bonding mode 4 = 802.3ad (LACP). Modes 0-6: rr,ab,xor,bcast,802.3ad,tlb,alb. training/library/topics/lacp/primer.md lacp/cc4746654bb1 lacp hard lacp, failover, detection What happens when LACP PDU exchange fails and how quickly is a link removed? 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.\n\nRemember: LACP = Link Aggregation Control Protocol (802.3ad). Bundles links for BW+redundancy.\n\nFun fact: LACP PDUs exchanged every 1s (fast) or 30s (slow) to detect failures. training/library/topics/lacp/primer.md lacp/926eccaab027 lacp hard lacp, nmcli, configuration How do you create an 802.3ad LACP bond using nmcli with optimal settings? 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.\n\nRemember: Linux bonding mode 4 = 802.3ad (LACP). Modes 0-6: rr,ab,xor,bcast,802.3ad,tlb,alb. training/library/topics/lacp/primer.md --- ## Wiki Navigation ### Related Content - [Case Study: Bonding Failover Not Working](../../../../library/case-studies/datacenter_ops/bonding-failover-not-working/README.md) (Case Study, L1) — LACP / Link Aggregation - [Case Study: LACP Mismatch One Link Hot](../../../../library/case-studies/networking/lacp-mismatch-one-link-hot/README.md) (Case Study, L2) — LACP / Link Aggregation - [Cisco Fundamentals for DevOps](../../../../library/topics/cisco-fundamentals-for-devops/index.md) (Topic Pack, L1) — LACP / Link Aggregation - [LACP](../../../../library/topics/lacp/index.md) (Topic Pack, L1) — LACP / Link Aggregation - [Networking Deep Dive](../../../../library/topics/networking/index.md) (Topic Pack, L1) — LACP / Link Aggregation - [Scenario: NIC Flapping / LACP Mismatch](../../../../library/scenarios/datacenter/nic-flapping.md) (Scenario, L2) — LACP / Link Aggregation