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EtherChannel link aggregation

EtherChannel is Cisco’s implementation of Link Aggregation (LAG), bundling multiple physical links into one logical connection called a Port-Channel interface.

Purpose

Hardware limitations (ASIC)

EtherChannel protocols

Protocol Type Standard
PAgP Cisco proprietary Cisco only
LACP IEEE 802.3ad Open standard
ON No protocol Not recommended

⚠️ Cisco best practice: always use active on both sides with LACP.

Activation modes

Requirements for forming an EtherChannel (all MUST match)

  1. Interface type – all Layer 2 or all Layer 3
  2. Speed
  3. Duplex
  4. Switchport mode – access or trunk
  5. Access VLAN (if access)
  6. Allowed VLAN list (if trunk)
  7. Native VLAN (if trunk)
  8. Channel-group number
  9. EtherChannel protocol – LACP / PAgP / ON

PAgP negotiation table

Local \ Remote Desirable Auto On
Desirable ✅ Up (negotiates) ✅ Up (negotiates) ⚠️ Up (forced)
Auto ✅ Up (negotiates) ❌ Down ⚠️ Up (forced)
On ⚠️ Up (forced) ❌ Down ❌ Down

👉 Rule: always have desirable on at least one side – preferably on both.

LACP negotiation table

Local \ Remote Active Passive On
Active ✅ Up (negotiates) ✅ Up (negotiates) ⚠️ Up (forced)
Passive ✅ Up (negotiates) ❌ Down ⚠️ Up (forced)
On ⚠️ Up (forced) ❌ Down ❌ Down

Cisco memory rules (exam & operations)

Common faults

Fault Result
VLAN mismatch Interface suspended
Speed/duplex mismatch No bundle
Passive + Passive (LACP) No adjacency
ON on one side Loop risk
STP on the physical interface Ignored

Configuration

Create L2 EtherChannel (best practice – configure L2/L3 parameters before channel-group, otherwise members can be suspended):

conf t
interface range gi1/0/1-2
 switchport
 switchport mode trunk
 channel-group 1 mode active

Layer 3 EtherChannel (routed Port-Channel, typical in distribution/core):

conf t
interface range gi1/0/3-4
 no switchport
 channel-group 2 mode active

interface port-channel 2
 ip address 10.10.10.1 255.255.255.252

The Port-Channel is the master interface – all STP, QoS, ACL and VLAN config goes here; members inherit:

interface port-channel 1
 description Uplink_to_Core
 switchport mode trunk
 switchport trunk allowed vlan 10,20,30
 spanning-tree portfast trunk

Removing an EtherChannel safely (avoids STP flaps in production):

interface port-channel 1
 shutdown

interface range gi1/0/1-2
 no channel-group 1

LACP details (CCNP)

lacp system-priority 100        ! lower = higher priority; decides who controls active vs standby links

interface gi1/0/1
 lacp port-priority 100         ! used when >8 links are configured; lowest value becomes active

interface port-channel 1
 lacp max-bundle 4              ! CCNP classic: only 4 active links, the rest standby

Load balancing (advanced)

port-channel load-balance ?
show etherchannel load-balance

Typical methods: src-mac, dst-mac, src-dst-mac, src-ip, dst-ip, src-dst-ip, src-dst-port (platform dependent). Never per-packet – it would break TCP ordering.

Cisco’s recommendation: src-dst-ip – better distribution than MAC-only hashing, preserves flow consistency (same flow → same link), fits L3-dominated enterprise networks. src-dst-port gives the best distribution (L4) but is platform/ASIC dependent and mostly used in datacenter designs.

Method Cisco assessment
src-mac ❌ Limited
dst-mac ❌ Limited
src-dst-mac ⚠️ OK in L2
src-ip ⚠️ OK
dst-ip ⚠️ OK
src-dst-ip ✅ Recommended (default best practice)
src-dst-port 🚀 Best – but platform dependent

🧠 CCNP rule: if Cisco asks “which load-balancing method is recommended” → answer src-dst-ip.

Troubleshooting (CCNP)

show etherchannel summary
show lacp neighbor
show lacp internal
show interfaces port-channel 1
show interfaces gi1/0/1 etherchannel
Flag Meaning
P Bundled (active)
S Standby
I Individual (not bundled)
D Down
s Suspended
U Port-channel up
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