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Links and neighbors

LAGs and Link Validation

Build a two-member routed PortChannel and separate carrier, LACP selection, and logical-interface health.

Lesson 12 of 18Prerequisite: core lessons 01–09 or equivalent operator knowledge
By the end, you should be able to:
  • Place addresses on a logical LAG rather than its members.
  • Read member selection alongside operational state.
  • Test a bounded member failure without assuming throughput.
Scope: community SONiC 202505 source baseline; single ASIC/default namespace unless stated. Commands, outputs, faults, and recovery are illustrative only. Confirm image, management mode, and platform support before a separate lab. No device commands, builds, containers, or packet tests were executed for this course.
Two physical links, one routed PortChannel
Two physical links, one routed PortChannelS1 PortChannel1 192.0.2.1/30 and S2 PortChannel1 192.0.2.2/30 each contain Ethernet0 and Ethernet4. One cable connects matching Ethernet0 ports and a second connects matching Ethernet4 ports. IP addresses belong to the logical interfaces only.S1 · PortChannel1192.0.2.1/30S2 · PortChannel1192.0.2.2/30Ethernet0 ↔ Ethernet0Ethernet4 ↔ Ethernet4Two LACP member links; no IP addresses on the physical members.
Original physical topology. E0/E4 mean Ethernet0/Ethernet4; the bundle has two paired cables and no separate bridge path.

Scenario and baseline

Both switches start with Ethernet0/Ethernet4 admin-down and free of IP addresses, VLAN membership, and other LAGs. PortChannel1 is absent. Cabling pairs the matching ports. Both images support LACP/teamd and matching speed/MTU; management is independent. The /30 addresses belong on PortChannel1 only.

The lab selects min-links=1: one selected member may preserve the bundle. A policy requiring two members would produce a different outcome. LACP selection and a physical up indication are not interchangeable.

Configure each logical interface

# On BOTH switches first
sudo config portchannel add PortChannel1 --min-links 1
sudo config portchannel member add PortChannel1 Ethernet0
sudo config portchannel member add PortChannel1 Ethernet4
sudo config interface startup Ethernet0
sudo config interface startup Ethernet4
# S1 only
sudo config interface ip add PortChannel1 192.0.2.1/30
# S2 only
sudo config interface ip add PortChannel1 192.0.2.2/30

The pinned add command creates an administratively up PortChannel. Do not add the same /30 to the physical members. Wait for the intended member selection before checking the routed adjacency.

Inspect the bundle and demand

# On each switch
show interfaces portchannel
show interfaces status
show ip interfaces
show arp
# S1 only
ping -c 3 192.0.2.2
PortChannel1  LACP(A)(Up)  Ethernet0(S) Ethernet4(S)

Illustrative shortened output, not a device observation.

The invented excerpt indicates selected members and an up logical bundle. Check the legend on the actual image. A bounded local-peer ping and member counters support adjacency, not transit scale. One flow may hash to one member; this is not a promise to split each packet or double one flow’s rate.

One member down, then restore and remove

# S1 only: bounded fault
sudo config interface shutdown Ethernet4
show interfaces portchannel
# Repair
sudo config interface startup Ethernet4

With min-links1 and a healthy remaining selected member, the logical interface may remain up after convergence. Check S2 as well: miswiring, LACP partner mismatch, or no selected survivor changes the result. A timer or hashing effect can affect packet loss; do not invent a zero-loss guarantee.

# S1
sudo config interface ip remove PortChannel1 192.0.2.1/30
# S2
sudo config interface ip remove PortChannel1 192.0.2.2/30
# On BOTH, after their address is removed
sudo config portchannel member del PortChannel1 Ethernet0
sudo config portchannel member del PortChannel1 Ethernet4
sudo config portchannel del PortChannel1
sudo config interface shutdown Ethernet0
sudo config interface shutdown Ethernet4

Recheck the absent-bundle/admin-down baseline. Preserve unrelated configuration and do not save the temporary lab.

Key takeaways

  • LAG members and logical routed interfaces have different roles.
  • Carrier, LACP selection, and min-links jointly affect health.
  • One surviving member is sufficient only under the stated policy.
  • A single ping cannot establish throughput or zero-loss convergence.

Check your understanding

Six questions, four options each. Check each answer for an explanation, review your score, and retry.

Sources & scope

Reviewed 18 October 2026. Community release-source pins: utilities 7511bf2f8ea3582c2b67cf5016f8e8237e62882b; buildimage ff3c0c9f686a1d0dbddbe813b233b3b7a51bd8e2. FRR 10.2 and RFC references explain semantics; they do not identify an installed image’s exact daemon version. Windows reference code is separately pinned: SWSS b8a9b65494c234ac33b6cf0a2b7f13fb291cee7f, buildimage 128f6802b6e1, utilities cc4fbb3e8313, SAI Redis dd925f4cf6fa. These are source-inspection baselines, not deployed-binary or hardware validation.

Exact inspected files and actual website checks are recorded in SOURCE-REVIEW.txt. No vendor CLI extension, production support level, scale, or timing guarantee is implied.

Lesson 12 · Knowledge check