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CCNA practice test / CCNA EtherChannel: Practice Questions

CCNA EtherChannel: Practice Questions

10 original questions, each with an explanation for every option.

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I failed my first CCNA attempt with 58%, and I work as a network engineer today. EtherChannel is a topic many people think they have covered because they can recite the mode table: active, passive, desirable, auto, on. The exam does not ask for the table. It shows you a show etherchannel summary output with one port marked (s) and asks what to change. If you have never looked at the flags closely, you guess.

The questions here are built the way the exam builds them. You get output from a switch, or a short description of a bundle that will not come up, and you have to say which mode combination fails, which member setting is mismatched, why one link carries all the traffic, or why a Layer 3 port-channel refuses an IP address. Every question was written from scratch with its own switch names, interfaces and addresses, and every choice has its own explanation, including the wrong ones, because knowing why an option is wrong is what saves you when the exam rewords the question.

Read the exhibits slowly. The flag letters in show etherchannel summary tell you almost everything: whether the bundle is Layer 2 or Layer 3, whether it is in use, and whether a member is bundled, suspended, stand-alone or down. Once you can read that line without the legend, the rest of the topic is small.

Adrian, network engineer and founder of CertAce.

How to work these questions

  1. Check the physical layer first: show interfaces status on both switches. Every member must be up, at the same speed and duplex. A member at a different speed is suspended and never joins.
  2. Run show etherchannel summary on both ends and read the flags. Po in (SU) or (RU) is working; (SD) means nothing is bundled yet. Members marked (I) are running as individual ports, usually a negotiation problem; members marked (s) are suspended, usually a configuration mismatch.
  3. Compare the modes: show etherchannel port-channel or the running configuration. LACP needs at least one active side, PAgP needs at least one desirable side, and on must be on at both ends. Mixing LACP with PAgP, or on with a negotiating mode, never bundles.
  4. Compare the member port configurations with show running-config interface for each member: switchport mode, native VLAN, allowed VLAN list, speed and duplex. Everything a member sets must match the other members in the same channel group.
  5. For a Layer 3 bundle, confirm no switchport is present on the members and on the port-channel before you assign the IP address. An IP address on a Layer 2 port-channel is rejected.
  6. After each fix, shut and no shut the member ports or the port-channel and re-check the summary. Verify traffic is actually spread with show etherchannel load-balance and the interface counters of each member.

Practice questions

Pick an answer to see right away whether it is correct and why every option is right or wrong.

  1. Question 1 of 10Exhibit

    Refer to the output from SW-A. Which statement about port-channel 1 is accurate?

    SW-A# show etherchannel summary
Flags:  D - down        P - bundled in port-channel
        I - stand-alone s - suspended
        H - Hot-standby (LACP only)
        R - Layer3      S - Layer2
        U - in use      f - failed to allocate aggregator

        M - not in use, minimum links not met
        u - unsuitable for bundling
        w - waiting to be aggregated
        d - default port

Number of channel-groups in use: 1
Number of aggregators:           1

Group  Port-channel  Protocol    Ports
------+-------------+-----------+-----------------------------------------------
1      Po1(SU)         LACP      Gi1/0/1(P)    Gi1/0/2(P)    Gi1/0/3(s)
    1. Po1 is operational at Layer 3 and Gi1/0/3 is in hot-standby because the bundle already holds its maximum number of active links.
    2. Po1 is down because one of its three member ports is not bundled, so no traffic is crossing the link to the neighbor switch.
    3. Po1 is operational and Gi1/0/3 is waiting for the LACP neighbor to finish negotiating before it is added to the bundle.
    4. Po1 is operational at Layer 2 and Gi1/0/3 was removed from the bundle because its settings do not match the other members.
    Show the answer

    Answer: D

    Read the flags in pairs. Po1(SU) is a Layer 2 port-channel that is in use. Gi1/0/1 and Gi1/0/2 carry the P flag and are bundled. Gi1/0/3 carries s, which means suspended: it is up, but a mismatch such as speed, duplex, native VLAN or allowed VLANs keeps it out of the bundle.

    • A. The S flag means Layer 2, not Layer 3, and hot-standby would be shown as H. A bundle of three links is far below the limit of eight active links.
    • B. SU means the port-channel is in use and forwarding. A suspended member does not bring the whole bundle down; the two P ports keep carrying traffic.
    • C. A port waiting for aggregation would carry the w flag. The lowercase s means suspended, which is a configuration mismatch, not an ongoing negotiation.
    • D. S is Layer 2, U is in use, P marks the two bundled members, and the lowercase s marks a member that the switch suspended because its configuration differs from the others.

  2. Question 2 of 10Scenario

    SW1 and SW2 are connected by two links that should form one EtherChannel. Which combination of channel-group modes on the two switches produces a working bundle?

    1. SW1 ports set to desirable and SW2 ports set to active
    2. SW1 ports set to auto and SW2 ports set to auto
    3. SW1 ports set to passive and SW2 ports set to active
    4. SW1 ports set to on and SW2 ports set to passive
    Show the answer

    Answer: C

    LACP forms with active+active or active+passive, never passive+passive. PAgP forms with desirable+desirable or desirable+auto, never auto+auto. Mode on works only with on at both ends. LACP and PAgP never mix, and on never mixes with a negotiating mode.

    • A. Desirable is PAgP and active is LACP. The two protocols do not talk to each other, so the ports never bundle.
    • B. Both sides in PAgP auto only respond, neither one initiates. Each switch waits for the other and the links stay as individual ports.
    • C. Active initiates LACP and passive answers it. One active side is enough for an LACP bundle to form.
    • D. Mode on sends no negotiation frames at all. The passive side on SW2 never hears LACP and refuses to bundle, while SW1 bundles blindly.

  3. Question 3 of 10Config

    You need a Layer 3 EtherChannel between SW-R and the core switch using Gi1/0/23 and Gi1/0/24, negotiated with LACP, numbered 10, with the address 10.40.12.1/30 on the SW-R side. Which configuration on SW-R is correct?

    1. interface range gi1/0/23-24 switchport mode trunk channel-group 10 mode active interface port-channel 10 switchport mode trunk ip address 10.40.12.1 255.255.255.252
    2. interface range gi1/0/23-24 no switchport channel-group 10 mode active interface port-channel 10 no switchport ip address 10.40.12.1 255.255.255.252
    3. interface range gi1/0/23-24 no switchport channel-group 10 mode desirable interface port-channel 10 no switchport ip address 10.40.12.1 255.255.255.252
    4. interface port-channel 10 no switchport ip address 10.40.12.1 255.255.255.252 interface range gi1/0/23-24 switchport mode trunk channel-group 10 mode on
    Show the answer

    Answer: B

    A Layer 3 EtherChannel needs no switchport on the member ports and on the port-channel, the IP address on the port-channel interface, and an LACP mode (active or passive) when LACP is required. Desirable and auto are PAgP, and on does not negotiate.

    • A. The members and the port-channel are trunks, so Po10 is a Layer 2 interface and the ip address command is rejected on it.
    • B. no switchport turns the members and the port-channel into routed ports, mode active negotiates with LACP, and the address goes on the port-channel, not on the members.
    • C. The Layer 3 part is right, but desirable is PAgP, not LACP. If the core is set to active the bundle never forms.
    • D. Mode on does not negotiate with LACP, and the members are still Layer 2 trunks while Po10 is routed, so the switch refuses to add them to the group.

  4. Question 4 of 10Exhibit

    SW-B is connected to SW-C by Gi1/0/7 and Gi1/0/8. Both interfaces are up/up and the cabling has been verified. The port-channel does not come up. Based on the exhibit, what is the most likely cause?

    SW-B# show running-config interface gi1/0/7
interface GigabitEthernet1/0/7
 switchport trunk allowed vlan 10,20,30
 switchport mode trunk
 channel-group 2 mode passive
!
SW-B# show running-config interface gi1/0/8
interface GigabitEthernet1/0/8
 switchport trunk allowed vlan 10,20,30
 switchport mode trunk
 channel-group 2 mode passive
!
SW-B# show etherchannel summary | begin Group
Group  Port-channel  Protocol    Ports
------+-------------+-----------+-----------------------------------------------
2      Po2(SD)         LACP      Gi1/0/7(I)    Gi1/0/8(I)
    1. SW-C is also configured with mode passive, so neither switch sends an LACP request and no bundle forms.
    2. SW-C is configured with mode active, which cannot be combined with passive on the SW-B side of the link.
    3. SW-C uses channel-group 7 instead of 2, so the two switches cannot agree on the same channel group.
    4. The port-channel interface was not created by hand before channel-group was applied on the members.
    Show the answer

    Answer: A

    Members with the I flag are running as individual ports because LACP never negotiated. Both SW-B ports are passive; if SW-C is passive too, nobody initiates. Set at least one side to active and the ports move from I to P and Po2 changes from SD to SU.

    • A. Both SW-B ports are passive and show as stand-alone (I), which is what happens when no LACP partner initiates. Passive on both ends is the classic cause.
    • B. Active with passive is a valid LACP pair. If SW-C were active, the SW-B ports would answer and show P, not I.
    • C. The channel-group number is local to each switch. SW-C can use any number; it has no effect on whether LACP negotiates.
    • D. The channel-group command creates the port-channel interface automatically. Po2 already exists in the output, so this is not the issue.

  5. Question 5 of 10Scenario

    You are adding Gi1/0/3 and Gi1/0/4 on SW-P to an existing Layer 2 trunk EtherChannel, Po2, which already contains Gi1/0/1 and Gi1/0/2. Which two settings must match the existing members for the new ports to be bundled? (Choose two.)

    1. Speed and duplex settings
    2. The channel-group number used on the neighbor switch
    3. Allowed VLAN list and native VLAN
    4. LACP port priority on every member
    5. The physical port numbers used on the neighbor switch
    Show the answer

    Answer: A and C

    Member ports in one channel group must agree on speed, duplex, switchport mode, native VLAN and allowed VLANs. The channel-group number and the port numbers only matter locally; the neighbor switch picks its own.

    • A. A member running at a different speed or duplex from the rest of the group is suspended and never bundled.
    • B. The group number only has to be the same among the local members. SW-P can use channel-group 2 while the neighbor uses channel-group 7 for the same bundle.
    • C. On a trunk EtherChannel every member must carry the same allowed VLANs and the same native VLAN; a difference on one port suspends that port.
    • D. LACP port priority may differ between members. It decides which ports go active when there are more than eight, not whether a port qualifies for the bundle.
    • E. The neighbor can use any physical ports, as long as each one is cabled to a member on SW-P and configured consistently on its own switch.

  6. Question 6 of 10Exhibit

    Po3 on SW-D is a PAgP bundle to the distribution switch. Traffic in VLAN 30 is slower than expected and the summary shows only one member forwarding. Based on the exhibit, which command entered on Gi1/0/12 allows it to join the bundle?

    SW-D# show running-config interface gi1/0/11
interface GigabitEthernet1/0/11
 switchport trunk native vlan 99
 switchport trunk allowed vlan 10,20,30,99
 switchport mode trunk
 channel-group 3 mode desirable
!
SW-D# show running-config interface gi1/0/12
interface GigabitEthernet1/0/12
 switchport trunk native vlan 99
 switchport trunk allowed vlan 10,20,99
 switchport mode trunk
 channel-group 3 mode desirable
!
SW-D# show etherchannel summary | begin Group
Group  Port-channel  Protocol    Ports
------+-------------+-----------+-----------------------------------------------
3      Po3(SU)         PAgP      Gi1/0/11(P)   Gi1/0/12(s)
    1. switchport trunk allowed vlan add 30
    2. switchport trunk native vlan 30
    3. channel-group 3 mode desirable non-silent
    4. switchport trunk allowed vlan 10,20,30
    Show the answer

    Answer: A

    Compare the two member configurations line by line. The only difference is VLAN 30 missing from the allowed list on Gi1/0/12, which is enough for the switch to suspend it (flag s). Add the VLAN with the add keyword so the rest of the list stays intact.

    • A. Adding VLAN 30 makes the allowed list identical to Gi1/0/11, which clears the mismatch and lets PAgP bundle the port.
    • B. The native VLAN is already 99 on both members, matching Gi1/0/11. Moving it to 30 on one port creates a native VLAN mismatch on top of the existing one.
    • C. The PAgP mode is not the problem; desirable already negotiates, and non-silent only changes how the port treats a quiet neighbor. The VLAN mismatch stays and the port stays suspended.
    • D. Without the add keyword the list is replaced, so VLAN 99 is dropped from Gi1/0/12. The allowed lists still differ and the port remains suspended.

  7. Question 7 of 10Scenario

    A backup server on SW-H copies a 400 GB image to a single storage array on SW-J. The two switches are connected by Po7, four 1 Gbps links that all show the P flag. During the copy one member runs near 100% utilization while the other three stay almost idle. What explains this?

    1. LACP keeps only one link active at a time and holds the other three in hot-standby for redundancy.
    2. The switch hashes each flow to one member link, and a single copy between two hosts is one flow.
    3. Spanning tree is blocking three members because it treats each physical link as a separate path.
    4. The bundle is using mode on, which forwards on the first member only and uses the rest as backup.
    Show the answer

    Answer: B

    EtherChannel spreads flows, not frames. The hash on source and destination addresses maps each conversation to one member, so a single transfer between two hosts uses one link and the aggregate bandwidth only shows with many flows. The reward is that frames of one flow never arrive out of order.

    • A. LACP activates up to eight links at once. Hot-standby only appears when more than eight ports are in the group, and these ports show P, not H.
    • B. The load-balancing hash takes addresses from each frame and always picks the same member for the same source and destination pair, so one conversation cannot exceed one link.
    • C. STP sees Po7 as a single logical link and does not block individual members. Blocked ports would not show as bundled either.
    • D. Mode on only skips negotiation; it uses the same per-flow hashing as LACP and PAgP across all bundled members.

  8. Question 8 of 10Config

    SW-E has Po4 to the server access switch and is load balancing on source MAC address only. Most traffic on the uplink comes from a single router, so one member of Po4 carries nearly everything. You want the switch to hash on both the source and destination IP address instead. Which command does this?

    1. SW-E(config-if)# channel-group 4 load-balance src-dst-ip
    2. SW-E(config)# etherchannel load-balance src-dst-ip
    3. SW-E(config)# port-channel load-balance src-dst-ip
    4. SW-E(config-if)# lacp load-balance src-dst-ip
    Show the answer

    Answer: C

    The load-balancing method is set once, in global configuration, with port-channel load-balance, and applies to all bundles on the switch. When one address is the same for most traffic, pick a method that includes the varying address, such as src-dst-ip or src-dst-mac.

    • A. The channel-group command only assigns a port to a group and sets the mode; it has no load-balance option.
    • B. There is no etherchannel load-balance command in IOS. The keyword is port-channel, even though the feature is called EtherChannel.
    • C. port-channel load-balance is a global command that sets the hashing method for every EtherChannel on the switch, and src-dst-ip uses both IP addresses of each flow.
    • D. LACP has no load-balance command. The hashing method is a switch-wide setting, independent of which protocol built the bundle.

  9. Question 9 of 10Scenario

    SW-K and SW-L are connected by four parallel 1 Gbps links carrying VLAN 20, not bundled. Spanning tree shows three of the four links in a blocking state. The engineer places all four links into channel-group 1 mode active on both switches and the bundle forms. How does spanning tree treat the links now?

    1. STP still blocks three of the four physical links until PortFast is enabled on each member.
    2. STP places two members in forwarding and two in blocking to split the load between them.
    3. STP blocks the whole bundle because parallel links between two switches always create a loop.
    4. STP sees one logical link, Po1, and forwards on it, so all four members can carry traffic.
    Show the answer

    Answer: D

    Spanning tree treats an EtherChannel as a single interface. Instead of four parallel paths with three blocked, there is one path, Po1, in forwarding. That is the main reason to bundle links between switches: you keep the bandwidth that STP would otherwise block.

    • A. PortFast is for edge ports facing hosts. STP stops seeing the four physical links once they are bundled, so there is nothing left for it to block.
    • B. STP does not split a bundle. Load sharing across members is done by the EtherChannel hash, not by spanning tree states.
    • C. A single logical link between two switches is not a loop. STP only blocks when there is more than one path, and the bundle counts as one.
    • D. STP runs on the port-channel interface, not on the members. One logical link with no parallel path is placed in forwarding, and the hash spreads flows over all four members.

  10. Question 10 of 10Scenario

    SW-M and SW-N are connected by ten 1 Gbps links. The engineer adds all ten to channel-group 5 mode active on both switches. After the bundle forms, what is the expected state?

    1. All ten links are bundled and forward traffic, because LACP places no limit on the number of members.
    2. The channel-group command is rejected on the ninth and tenth ports, because a group accepts at most eight.
    3. Eight links are bundled and forward traffic; the other two are hot-standby and replace an active link if it fails.
    4. The port-channel stays down until the engineer removes two of the ten links from the channel group.
    Show the answer

    Answer: C

    An LACP channel group can hold up to sixteen ports, but only eight forward at a time; the rest are hot-standby (flag H) and are promoted when an active link fails. PAgP and mode on stop at eight ports in total.

    • A. LACP allows at most eight active links in one bundle. The extra ports are accepted into the group but not into the active set.
    • B. LACP accepts up to sixteen ports in a channel group; the eight-port limit applies to active links, not to the group configuration.
    • C. LACP activates eight links and marks the remaining two with the H flag. When an active member goes down, a standby link takes its place without manual work.
    • D. The bundle comes up with eight active members. Nothing has to be removed; the extra links simply wait in standby.

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Common mistakes

Questions people ask

What does the (s) flag next to a member port mean in show etherchannel summary?

The port is suspended. It is physically up but the switch removed it from the bundle because its settings do not match the other members, for example a different allowed VLAN list, native VLAN, speed or duplex. Fix the mismatch on that member and it rejoins.

Why should I avoid mode on when the neighbor supports LACP?

Mode on bundles blindly without checking that the other side is a port-channel too. If the neighbor is misconfigured or the cables are crossed between two groups, the switch forwards on the bundle anyway and you can get a loop or black-holed traffic. LACP checks the partner before bundling.

How many links can an LACP EtherChannel have?

Up to 16 ports can be assigned to one LACP channel group, but at most 8 are active at a time. The remaining ports are hot-standby and take over when an active link fails. PAgP and mode on allow up to 8 ports in total.

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