EtherChannel (Link Aggregation)

EtherChannel bundles multiple physical Ethernet links between two switches into a single logical link. This provides increased bandwidth and redundancy without triggering STP to block redundant links.

Why EtherChannel?

Without EtherChannel, if you connect two switches with 4 Gigabit links, STP will block 3 of them to prevent loops. With EtherChannel, all 4 links are active and appear as a single logical link to STP.

EtherChannel Negotiation Protocols

LACP (Link Aggregation Control Protocol)

LACP is the IEEE 802.3ad open standard for EtherChannel negotiation. It is the recommended protocol for multi-vendor environments.

Successful LACP combinations:

PAgP (Port Aggregation Protocol)

PAgP is Cisco-proprietary. Use it only in all-Cisco environments.

Successful PAgP combinations:

Static (on mode)

Configuring EtherChannel with on mode forces the channel without negotiation. Both sides must be set to on. No protocol packets are exchanged.

Warning: Static EtherChannel is risky because there is no negotiation to detect misconfigurations. LACP is preferred.

EtherChannel Configuration

Layer 2 EtherChannel (LACP)

Switch(config)# interface range GigabitEthernet0/1 - 4
Switch(config-if-range)# channel-group 1 mode active        ! LACP active
Switch(config-if-range)# exit

Switch(config)# interface Port-channel1
Switch(config-if)# switchport mode trunk
Switch(config-if)# switchport trunk allowed vlan 10,20,30

Layer 3 EtherChannel

Switch(config)# interface range GigabitEthernet0/1 - 4
Switch(config-if-range)# no switchport                       ! Make Layer 3
Switch(config-if-range)# channel-group 1 mode active

Switch(config)# interface Port-channel1
Switch(config-if)# no switchport
Switch(config-if)# ip address 10.0.0.1 255.255.255.252

Member Port Requirements

All ports in an EtherChannel must match:

• Same speed (all 1 Gbps or all 10 Gbps)
• Same duplex (all full-duplex)
• Same VLAN configuration (access VLAN or trunk allowed VLANs)
• Same trunk mode (all access or all trunk)
• Same native VLAN (for trunk ports)
• Same STP cost settings

If any port has different settings, the EtherChannel will not form (or will be suspended).

Load Balancing

EtherChannel distributes traffic across member links using a hash algorithm. The switch can use different criteria:

Switch(config)# port-channel load-balance src-dst-ip   ! Configure load balance method
Switch# show etherchannel load-balance                  ! Verify current method

Verification Commands

Switch# show etherchannel summary              ! Quick status of all channels
Switch# show etherchannel port-channel          ! Port-channel details
Switch# show etherchannel detail                ! Detailed information
Switch# show interfaces port-channel 1          ! Interface status

Sample "show etherchannel summary" Output

Group  Port-channel  Protocol    Ports
------+-------------+-----------+----------------------------------------------
1      Po1(SU)       LACP        Gi0/1(P)    Gi0/2(P)    Gi0/3(P)    Gi0/4(P)

Status flags: SU = Layer 2 in use, RU = Layer 3 in use, P = bundled in port-channel, s = suspended, D = down

On the Exam: Know that LACP active-active and active-passive form a channel, but passive-passive does not. This mirrors PAgP where desirable-auto works but auto-auto does not. Also know that all member ports must have matching configurations.