Static and Dynamic EtherChannel Troubleshooting

What it is

An EtherChannel bundles several physical links into one logical port channel, and troubleshooting means finding why a bundle will not form or will not forward.

The bundle can be static, with the mode on, or dynamic, using LACP or PAgP. Spanning Tree treats the bundle as a single link, so no member is blocked. Most faults come from a mode mismatch or from member ports that are not alike.

How it works

The negotiation mode and the member port settings decide whether a bundle forms.

Modes and what forms

A bundle forms when at least one end asks. Active with active or passive forms a bundle. Desirable with desirable or auto forms one.

Passive with passive and auto with auto do not form a bundle. The mode on forms a bundle only with another on.

LACP and PAgP cannot be mixed at the two ends. A static bundle has no checks, so a mistake at one end can create a loop or black hole traffic.

Basic configuration

This is IOS XE syntax.

Switch(config)# interface range GigabitEthernet1/0/1 - 2
Switch(config-if-range)# channel-group 1 mode active
Switch(config-if-range)# exit
Switch(config)# interface Port-channel1
Switch(config-if)# switchport mode trunk

The channel-group command creates Port-channel1. Apply trunk and VLAN settings to the port channel interface, so that the members inherit them.

What members must have in common

Every member port must match in the following.

• Speed and duplex.
• Switchport mode, such as access or trunk.
• Access VLAN, or the native and allowed VLANs on a trunk.
• Layer 2 or Layer 3 status. You cannot mix switched and routed ports.

A member that differs is suspended or left out. After you correct it, you may need to shut and unshut the port, or the port channel, to bring it back.

Verification commands

Switch# show etherchannel summary
Switch# show etherchannel port-channel
Switch# show lacp neighbor
Switch# show pagp neighbor
Switch# show etherchannel load-balance

show etherchannel summary is the first command to use. It lists each port channel with its flags and its members.

Seeing SD on the port channel means it is Layer 2 and down. A member shown as I is not bundled, and one shown as s has been suspended, which often points to a setting mismatch.

show lacp neighbor and show pagp neighbor show what the far end is sending. If they are empty, the other end is not speaking that protocol, or the link is down.

Troubleshooting order

1. Check that the links are up and the speed and duplex match.
2. Compare the modes. Look for passive with passive, auto with auto, or a mix of protocols.
3. Compare the member settings on both switches.
4. Read the flags in show etherchannel summary.
5. Check that the trunk, native VLAN and allowed VLANs agree on both port channels.

Load balancing

A bundle does not send packets evenly. The switch hashes fields of each frame to choose a member, so one conversation always uses one link.

Switch(config)# port-channel load-balance src-dst-ip

The hash can use source and destination MAC, IP address or port numbers, depending on the platform. A bundle with few conversations may use its members unevenly, and a different hash can improve the spread.

LACP extras

LACP has a system priority and a port priority. The switch with the lower system priority makes the decisions about which ports are active when there are more members than the limit.

An LACP bundle can have up to eight active members, and extra members wait as hot standby. The command port-channel min-links sets how many links must be up for the bundle to stay up.