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Fix the bundle 52 — desirable against on
Enterprise Advanced 4 devices 3 checked tasks 100 points Plan lab
Scenario — the two cables between SW-A and SW-B in Meerut were meant to be one bundle, Po4, but the ports show as suspended and only one cable carries traffic. SW-A runs "channel-group 4 mode desirable" and SW-B runs "mode on". Read both ends, decide which has to change and make the bundle come up with both ports in it. You build it in a browser, type real commands at each device, and every task below is
checked against the running configuration — so you find out it is wrong before anybody else does.
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The scenario
the two cables between SW-A and SW-B in Meerut were meant to be one bundle, Po4, but the ports show as suspended and only one cable carries traffic. SW-A runs "channel-group 4 mode desirable" and SW-B runs "mode on". Read both ends, decide which has to change and make the bundle come up with both ports in it.
Your role: a network engineer on the enterprise team. Level: Advanced Done when: all 3 goals show as passed (100 points).
Network status
Devices: 2 switchs, 2 PCs, cabled as drawn. No Layer 1 or Layer 2 faults: every link is connected and up. SW-A starts with and EtherChannel already configured. SW-B starts with and EtherChannel already configured. PC-A is a host with 10.71.0.11/24. PC-B is a host with 10.71.0.12/24. Known problems
The network worked and now does not: the scenario above says what users report. The cause is a configuration fault, not a cable. Find it with show commands (interfaces, routes, neighbours, running-config) before you change anything; fix only what is wrong. Goals
SW-A has Po4 with both member ports bundled (40 pts) SW-B shows Po4 in use (SU) (35 pts) The PCs reach each other across the bundle (25 pts) The activity, in order
SW-A has Po4 with both member ports bundled Why: EtherChannel joins links into one logical link, so spanning tree does not block the extras. Verify with: show etherchannel summary SW-B shows Po4 in use (SU) Why: Reading a show command is the core skill of troubleshooting: the output is the truth, not your configuration. Verify with: show etherchannel summary The PCs reach each other across the bundle Why: A ping needs a route in BOTH directions. This is the end-to-end proof that everything before it worked. Verify with: ping 10.71.0.12 Tips
Work in the order given: later goals depend on the earlier ones. After every change, run the verify command before moving on; do not guess. Stuck? Read the verify output first. It names the interface, neighbour or route that is wrong. Common mistake: Changing both ends and creating a new mismatch Common mistake: Mixing LACP with PAgP Common mistake: Two passive ends — nobody initiates Why this matters
LACP, PAgP and static "on" do not interoperate, and two passive or two auto ends never start the negotiation. The bundle only forms when the modes are a valid pair — the fix is to look at both ends and change the one that is out of step.
What you will be able to do
Read show etherchannel summary and the port flags
Recognise an invalid mode pair
Rebuild a channel group safely
The network
Catalyst 2960-24TT, PC — 4 devices in all, already cabled.
Gi0/1 Gi0/1 Gi0/2 Gi0/2 Fa0/1 Fa0 Fa0/1 Fa0 SW-A SW-B PC-A 10.71.0.11/24 PC-B 10.71.0.12/24 Drawn from this lab: devices, cables, ports and the addresses already set. Device Model Address already set
SW-A Catalyst 2960-24TT configure it SW-B Catalyst 2960-24TT configure it PC-A PC 10.71.0.11 / 255.255.255.0 PC-B PC 10.71.0.12 / 255.255.255.0
Cabling
SW-A Gi0/1 ↔ SW-B Gi0/1 SW-A Gi0/2 ↔ SW-B Gi0/2 SW-A Fa0/1 ↔ PC-A Fa0 SW-B Fa0/1 ↔ PC-B Fa0
What is configured at the start
These are already configured when the lab opens — including, where this is a troubleshooting lab, the fault itself:
SW-A interface range g0/1 - 2
channel-group 4 mode desirable
exit SW-B interface range g0/1 - 2
channel-group 4 mode on
exit Tasks, and how each one is checked
Every task is decided by reading the live network — a neighbour in FULL, a ping that succeeds, a line in the running configuration — not by looking for a keyword you typed. The same command shows you what the lab sees.
# Task Pts How to verify it yourself
1 SW-A has Po4 with both member ports bundled 40 On SW-A: show etherchannel summary — the port-channel has 2 bundled members. 2 SW-B shows Po4 in use (SU) 35 On SW-B: show etherchannel summary output matches Po4\(SU\). 3 The PCs reach each other across the bundle 25 On PC-A: ping 10.71.0.12 must succeed.
Stuck? Ask the coach
Tell the coach what you see or what you tried. It gives you a hint, not the answer, the way a senior engineer would. Included with a free trial or any plan.
Ask the coach
Common mistakes
Changing both ends and creating a new mismatch
Mixing LACP with PAgP
Two passive ends — nobody initiates
Reference solution
Try it yourself first — the lab checks your work either way. This is one configuration that makes every task pass, device by device:
SW-B interface range g0/1 - 2
no channel-group
channel-group 4 mode desirable
exit Where this is examined
CCNA 2.0 EtherChannel troubleshooting; CCNP ENCOR Layer 2.
This lab practises these topics in the zNetLaB certification courses:
EtherChannel (LACP/PAgP) — CCNA · Network Access Trunking, EtherChannel, RSTP and MST — CCNP Enterprise · ENCOR · Infrastructure Switched campus: VTPv3, MST, EtherChannel, UDLD, loop guard — CCIE Enterprise Infrastructure · Network Infrastructure VLANs, trunking, STP, EtherChannel, VTP — CompTIA Network+ · Network Implementation EtherChannel — Switching and VLAN Mastery · Loop prevention and bundling
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