EtherChannel is a port link aggregation technology that allows multiple physical Ethernet links to be combined into one logical channel. This provides increased bandwidth, redundancy, and load balancing between switches, routers, or servers. Cisco supports both Layer 2 and Layer 3 EtherChannel conf…EtherChannel is a port link aggregation technology that allows multiple physical Ethernet links to be combined into one logical channel. This provides increased bandwidth, redundancy, and load balancing between switches, routers, or servers. Cisco supports both Layer 2 and Layer 3 EtherChannel configurations.
Layer 2 EtherChannel operates at the data link layer and is used for switch-to-switch connections where the bundled ports function as a single trunk or access port. To configure Layer 2 EtherChannel, you first select the interfaces you want to bundle using the interface range command. Then apply the channel-group command with a group number and specify the negotiation protocol - either PAgP (Port Aggregation Protocol, Cisco proprietary) using keywords 'desirable' or 'auto', or LACP (Link Aggregation Control Protocol, IEEE 802.3ad standard) using keywords 'active' or 'passive'. You can also use 'on' mode which forces the channel formation with no negotiation.
Layer 3 EtherChannel assigns an IP address to the port-channel interface itself, allowing it to function as a routed interface. Configuration involves creating the port-channel interface, assigning it to the desired channel group number, applying the 'no switchport' command to make it a routed port, and then configuring the IP address.
Verification commands include 'show etherchannel summary' which displays the status of all EtherChannels, their protocol, and member ports. The 'show etherchannel port-channel' command provides detailed information about the port-channel. Use 'show interfaces port-channel' followed by the number to view interface statistics. The 'show etherchannel load-balance' command reveals the current load-balancing algorithm being used.
Key requirements for successful EtherChannel formation include matching speed, duplex settings, VLAN configuration, trunk mode, and native VLAN across all member ports. Mismatched configurations will prevent the channel from forming properly and may cause network connectivity issues.
Configure and Verify Layer 2/Layer 3 EtherChannel
Why EtherChannel is Important
EtherChannel is a critical technology in modern network infrastructure that allows network administrators to bundle multiple physical links into a single logical link. This provides several key benefits:
• Increased Bandwidth: Combines the bandwidth of multiple links (e.g., four 1Gbps links = 4Gbps logical link) • Redundancy: If one physical link fails, traffic continues through remaining links • Load Balancing: Traffic is distributed across all active links • Cost Efficiency: Utilize existing ports rather than purchasing higher-speed interfaces • Spanning Tree Optimization: STP sees the bundle as one link, preventing blocked ports
What is EtherChannel?
EtherChannel (also called Port Channel or Link Aggregation) is a port link aggregation technology that groups multiple physical Ethernet links into one logical link. It can be configured as:
• Layer 2 EtherChannel: Acts as a single switch port for switching traffic • Layer 3 EtherChannel: Acts as a routed interface with an IP address
EtherChannel Protocols
PAgP (Port Aggregation Protocol): • Cisco proprietary protocol • Modes: Desirable (actively negotiates) and Auto (passively waits) • Both sides set to Auto will NOT form a channel
LACP (Link Aggregation Control Protocol): • IEEE 802.3ad standard (industry standard) • Modes: Active (actively negotiates) and Passive (passively waits) • Both sides set to Passive will NOT form a channel
Static (On Mode): • No negotiation protocol used • Forces the channel without PAgP or LACP • Both sides must be set to On
How EtherChannel Works
1. Physical ports are grouped into a port-channel interface 2. All member ports must have identical configurations: - Same speed and duplex - Same VLAN configuration (access or trunk) - Same allowed VLANs (if trunk) - Same native VLAN (if trunk) - Same STP settings
3. Load balancing distributes traffic using various methods: - src-mac, dst-mac, src-dst-mac - src-ip, dst-ip, src-dst-ip - src-port, dst-port, src-dst-port
Configuration Commands
Layer 2 EtherChannel with LACP: interface range GigabitEthernet0/1 - 2 channel-group 1 mode active interface port-channel 1 switchport mode trunk
Layer 2 EtherChannel with PAgP: interface range GigabitEthernet0/1 - 2 channel-group 1 mode desirable
Layer 3 EtherChannel: interface range GigabitEthernet0/1 - 2 no switchport channel-group 1 mode active interface port-channel 1 ip address 192.168.1.1 255.255.255.0
Verification Commands
• show etherchannel summary - Quick overview of all EtherChannels • show etherchannel port-channel - Detailed port-channel information • show interfaces port-channel 1 - Interface statistics • show etherchannel load-balance - Current load-balancing method • show spanning-tree - Verify STP sees port-channel as single link
EtherChannel Status Flags
In show etherchannel summary output: • P - Bundled in port-channel • I - Stand-alone (not bundled) • s - Suspended • D - Down • U - In use • S - Layer 2 • R - Layer 3
Exam Tips: Answering Questions on Configure and Verify Layer 2/Layer 3 EtherChannel
1. Memorize Protocol Mode Combinations: - LACP: Active-Active ✓, Active-Passive ✓, Passive-Passive ✗ - PAgP: Desirable-Desirable ✓, Desirable-Auto ✓, Auto-Auto ✗ - On-On ✓ (no negotiation) - Never mix On with LACP/PAgP modes
2. Remember LACP is the Standard: When questions mention IEEE 802.3ad or interoperability with non-Cisco devices, LACP is the answer
3. Configuration Consistency is Critical: If asked why an EtherChannel is not forming, look for mismatched settings between member ports
4. Layer 2 vs Layer 3 Distinction: - Layer 2: Uses switchport commands, no IP on port-channel - Layer 3: Uses no switchport, IP address assigned to port-channel interface
5. Know the Verification Commands:show etherchannel summary is the most commonly tested command
6. Load Balancing Questions: Remember that the method is configured globally, not per-channel
7. Maximum Links: Up to 8 active links per EtherChannel (LACP allows 8 active + 8 standby)
8. Common Troubleshooting Scenarios: - Mismatched modes on each end - Inconsistent port configurations - Speed/duplex mismatches - Native VLAN mismatches on trunks