EtherChannel bundles multiple physical links into a single logical port-channel. The CCNA tests LACP vs PAgP vs static EtherChannel, the mode combinations that form a channel, and common configuration errors. These appear as configuration tasks and troubleshooting scenarios.
Start Scenario PracticeA network engineer runs the following command on Switch SW1: SW1# 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 N - not in use, no aggregation 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 Gi0/1(P) Gi0/2(P) Gi0/3(D) Based on this output, what can be concluded?
Explanation: The output shows that Gi0/1 and Gi0/2 have a flag of 'P' (bundled in port-channel), while Gi0/3 has a flag of 'D' (down). Therefore, only two ports are actively bundled, making option D correct. The 'SU' flags on Po1 indicate the port-channel is Layer 2 (S) and in use (U), not Layer 3.
Consider this configuration: interface Port-channel1 switchport mode trunk switchport trunk native vlan 999 ! interface GigabitEthernet0/1 switchport mode trunk switchport trunk native vlan 999 channel-group 1 mode active ! interface GigabitEthernet0/2 switchport mode trunk switchport trunk native vlan 999 channel-group 1 mode active What is the effect of the 'switchport trunk native vlan 999' command on the EtherChannel?
Explanation: The 'switchport trunk native vlan 999' command on each physical interface and on the port-channel interface ensures that the EtherChannel uses VLAN 999 as the native VLAN. Untagged frames received on any member link are associated with VLAN 999, and frames tagged with VLAN 999 are sent untagged. Since the configuration is consistent across all interfaces, the EtherChannel forms correctly and operates with VLAN 999 as the native VLAN.
A network engineer is troubleshooting an EtherChannel between two Cisco switches. The show etherchannel 1 port-channel command shows the port-channel is up, but traffic is not load-balanced evenly. The engineer notices that all traffic is using only one link. The physical ports are all configured identically. What is the most likely cause?
Explanation: When the load-balancing method is set to src-dst-ip, the hash is computed from the source and destination IP addresses. If all traffic flows between the same two IP addresses, the hash result will always be identical, causing all frames to be forwarded over the same physical link. This explains why the port-channel is up but only one link carries all traffic.
A network engineer runs the following command on Switch SW7: SW7# show interfaces port-channel 1 Port-channel1 is up, line protocol is up (connected) Hardware is EtherChannel, address is aaaa.bbbb.cccc (bia aaaa.bbbb.cccc) Description: Link to Core Internet address is 192.168.1.1/30 MTU 1500 bytes, BW 2000000 Kbit/sec, DLY 10 usec, reliability 255/255, txload 1/255, rxload 1/255 Encapsulation ARPA, loopback not set Keepalive set (10 sec) Full-duplex, 1000Mb/s, link type is auto, media type is unknown input flow-control is off, output flow-control is unsupported ARP type: ARPA, ARP Timeout 04:00:00 Last input 00:00:00, output 00:00:00, output hang never Last clearing of "show interface" counters never Input queue: 0/2000/0/0 (size/max/drops/flushes); Total output drops: 0 Queueing strategy: fifo Output queue: 0/40 (size/max) 5 minute input rate 1000 bits/sec, 2 packets/sec 5 minute output rate 500 bits/sec, 1 packets/sec 12345 packets input, 1234567 bytes, 0 no buffer Received 0 broadcasts (0 IP multicasts) 0 runts, 0 giants, 0 throttles 0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored 0 watchdog, 0 multicast, 0 pause input 0 input packets with dribble condition detected 67890 packets output, 9876543 bytes, 0 underruns 0 output errors, 0 collisions, 2 interface resets 0 unknown protocol drops 0 babbles, 0 late collision, 0 deferred 0 lost carrier, 0 no carrier, 0 pause output 0 output buffer failures, 0 output buffers swapped out Based on this output, what can be concluded?
Explanation: The output shows a bandwidth of 2000000 Kbit/sec, which equals 2 Gbps. Since each individual link in the EtherChannel is operating at 1000 Mb/s (1 Gbps) and full-duplex, the total bandwidth of 2 Gbps indicates that two 1 Gbps links have been successfully aggregated into the Port-channel 1. This is a direct conclusion from the 'BW 2000000 Kbit/sec' field in the show interfaces port-channel output.
Which LACP mode must be configured on at least one side of an EtherChannel for the channel to establish?
Explanation: LACP (Link Aggregation Control Protocol) uses two modes: Active and Passive. For an EtherChannel to form, at least one side must be configured as Active, which actively sends LACP packets to negotiate the link. If both sides are Passive, neither will initiate negotiation, and the channel will not establish.
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Practice all EtherChannel and LACP ScenariosEtherChannel bundles multiple physical links into a single logical port-channel. The CCNA tests LACP vs PAgP vs static EtherChannel, the mode combinations that form a channel, and common configuration errors. These appear as configuration tasks and troubleshooting scenarios. These appear throughout the 350-401 and require you to apply your knowledge, not just recall facts.
Cisco doesn't publish an exact breakdown, but scenario-based questions (especially exhibit and command-output formats) make up a significant portion of the 350-401. Practicing each scenario type ensures you're ready for any format.
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