3.1.b Troubleshoot static and dynamic EtherChannels

What this objective tests

The objective says "troubleshoot". The exam shows you show etherchannel summary output. You must explain why a member port is not bundled.

EtherChannel combines up to eight physical links into one logical link. Spanning tree sees the bundle as one port. No member link is blocked.

The member links must match. Speed, duplex, trunk or access mode, allowed VLANs, and native VLAN must be identical. A mismatched member cannot join the bundle.

Channel protocols and modes

Pairs that never form a channel:

• LACP passive on both sides. Neither side starts.
• PAgP auto on both sides. Neither side starts.
• LACP on one side and PAgP or on on the other side.
• on on one side and a negotiating mode on the other side.

Note: Use Link Aggregation Control Protocol (LACP) active on both ends. LACP is the open standard and it detects miswiring.

Correct LACP configuration

SW1 and SW2 connect with two links. The bundle is a trunk for VLANs 10 and 20.

interface range GigabitEthernet1/0/1 - 2
 description LACP member to SW2
 switchport mode trunk
 switchport trunk native vlan 999
 switchport trunk allowed vlan 10,20,999
 channel-group 1 mode active
 no shutdown
!
interface Port-channel1
 description Trunk bundle to SW2
 switchport mode trunk
 switchport trunk native vlan 999
 switchport trunk allowed vlan 10,20,999

Configure the physical members and the port-channel with the same intent. Spanning tree and trunks refer to Port-channel1, not the members.

Example: LACP on one side, static on the other

SW1 uses channel-group 1 mode active. SW2 uses channel-group 1 mode on.

SW1 sends LACP packets. SW2 never answers. SW1 cannot bundle its ports. SW1 marks its members suspended:

SW1# show etherchannel summary
Flags:  D - down        P - bundled in port-channel
        I - stand-alone s - suspended
        H - Hot-standby (LACP only)
        R - Layer3      S - Layer2
        U - in use      f - failed to allocate aggregator
        M - not in use, minimum links not met
        u - unsuitable for bundling
        w - waiting to be aggregated
        d - default port

Group  Port-channel  Protocol    Ports
------+-------------+-----------+-----------------------------------------------
1      Po1(SD)         LACP      Gi1/0/1(s)    Gi1/0/2(s)

SW2 bundles its ports anyway because on does not negotiate:

SW2# show etherchannel summary

Group  Port-channel  Protocol    Ports
------+-------------+-----------+-----------------------------------------------
1      Po1(SU)          -         Gi1/0/1(P)    Gi1/0/2(P)

This is the dangerous case. SW2 forwards on a bundle that SW1 refuses to use. Traffic can loop or drop.

Warning: Static mode on can create loops when the other side does not bundle. Use LACP on production links.

Fix SW2:

interface range GigabitEthernet1/0/1 - 2
 no channel-group 1
 channel-group 1 mode active

Example: member config mismatch

SW2 Gi1/0/2 has a different allowed VLAN list than Gi1/0/1. LACP keeps the mismatched member out of the bundle:

SW2# show etherchannel summary

Group  Port-channel  Protocol    Ports
------+-------------+-----------+-----------------------------------------------
1      Po1(SU)         LACP      Gi1/0/1(P)    Gi1/0/2(I)

The member shows I (standalone) or s (suspended), depending on the platform. Compare the member configs:

show run interface gi1/0/1
show run interface gi1/0/2

Make the members identical. Then both members show P.

Reading show etherchannel summary

Lab: build, break, and repair a bundle

Topology:

SW1 Gi1/0/1 ==== Gi1/0/1 SW2
SW1 Gi1/0/2 ==== Gi1/0/2 SW2

Do these steps:

1. Create VLANs 10, 20, and 999 on both switches.
2. Configure both links as LACP active trunks in channel-group 1 on both switches. Use the config above.
3. Verify with show etherchannel summary. Both members must show P and Po1 must show SU.
4. Verify the trunk with show interfaces trunk. Po1 must appear in the list, not the members.
5. Break SW2: set Gi1/0/2 to switchport trunk allowed vlan 10.
6. Check show etherchannel summary on both switches. Find the member that left the bundle.
7. Repair the member with the full allowed list. Verify both members show P again.
8. Change SW2 to channel-group 1 mode on. Observe SW1 suspend its members.
9. Restore LACP active on SW2. Verify the final state.

Exam traps

• LACP passive/passive and PAgP auto/auto never form a channel.
• Static on does not detect a mismatch. It can cause loops.
• A member with different VLANs, speed, or duplex cannot bundle.
• Configure the Port-channel interface too. The members alone are not enough.
• EtherChannel load balancing is per flow, not per packet.
• P is the only healthy member flag in a working bundle.

Pass check

You are ready when you can do these things:

• Read the flags in show etherchannel summary and name the fault.
• Choose compatible modes for LACP, PAgP, and static bundles.
• Explain why one side on and one side active is dangerous.
• Repair a suspended member and prove the fix.