Network Access // Lesson 05

EtherChannel.

Several physical links can work as one logical connection. EtherChannel adds bandwidth and resilience while giving spanning tree a single logical port to manage.

LACPPort-channelLayer 2Layer 3Troubleshooting
01 // Why it exists

More links, one logical interface.

Connecting parallel switch links without bundling them creates a Layer 2 loop. Spanning tree normally blocks redundant paths, so the extra bandwidth sits idle. An EtherChannel groups compatible physical interfaces into one logical port-channel.

SW1Po1
Gi0/1–2
SW2Po1
Key idea: forwarding and spanning tree treat the bundle as one logical link, while traffic is distributed across its active member links.
02 // LACP

Negotiate the bundle.

ACTIVE

Initiates

Sends LACP messages and will form a channel with an active or passive peer.

PASSIVE

Listens

Responds to LACP but does not initiate. Passive plus passive will not form.

ON

Static

Forces a channel without negotiation. Both ends must be configured consistently.

Exam shortcut: active + active works; active + passive works; passive + passive does not.
03 // Requirements

Member links must agree.

Interfaces in the same channel need compatible settings. Check speed and duplex, and for Layer 2 bundles check access or trunk mode, allowed VLANs and native VLAN. For Layer 3 bundles, member ports must be routed ports.

Bundle typeMember configurationLogical configuration
Layer 2 accessSwitchports with matching access VLANAccess settings on Port-channel
Layer 2 trunkSwitchports with compatible trunk settingsTrunk settings on Port-channel
Layer 3no switchportIP address on Port-channel
04 // Configure Layer 2

Build an LACP trunk.

SW1(config)# interface range gigabitEthernet 0/1 - 2
SW1(config-if-range)# channel-group 1 mode active
SW1(config-if-range)# exit
SW1(config)# interface port-channel 1
SW1(config-if)# switchport mode trunk
SW1(config-if)# switchport trunk allowed vlan 10,20,99

Use a matching channel group on the peer. The local group number does not have to match the remote number, but using the same number makes the topology easier to read.

05 // Configure Layer 3

Route across a port-channel.

R1(config)# interface range gigabitEthernet 0/1 - 2
R1(config-if-range)# no switchport
R1(config-if-range)# channel-group 10 mode active
R1(config)# interface port-channel 10
R1(config-if)# ip address 10.0.12.1 255.255.255.252
R1(config-if)# no shutdown
Do not put the IP address on each physical member. The routed address belongs to the logical port-channel interface.
06 // Verify and troubleshoot

Read the summary first.

SW1# show etherchannel summary

Group  Port-channel  Protocol  Ports
------+-------------+---------+-------------------------
1      Po1(SU)       LACP      Gi0/1(P) Gi0/2(P)

S means Layer 2, U means the port-channel is in use, and P means the member is bundled. Also use show interfaces port-channel 1 and show lacp neighbor.

Fast fault checklist

  • Are both sides using compatible LACP modes?
  • Do speed, duplex and switchport settings match?
  • Do trunk allowed and native VLAN settings agree?
  • Was configuration applied consistently to the physical members and logical port-channel?
07 // Check yourself

EtherChannel quiz.

1. Which LACP pairing forms an EtherChannel?

2. How does spanning tree view an EtherChannel?

3. Where is an IP address configured for a Layer 3 EtherChannel?

4. Which command gives the quickest bundle status overview?

5. What does the P flag beside a member port indicate?

Score: 0 / 5