• A broken EtherChannel rarely announces itself.
    Most of the time the bundle quietly fails to form, and your traffic falls back on a single link.

    You are the engineer on duty, and four tickets are waiting in your queue.
    For each one, you will read the symptoms, find the cause, apply the fix, and verify it.

    Cisco EtherChannel lab topology with SW1 and SW2 connected by two links G0/0 and G0/1

    Figure 1 – EtherChannel Lab Topology

    Every ticket starts from this topology.

    Reading the Flags

    Ticket 1: users report poor performance between two access switches.
    Both uplinks are cabled and up, yet bandwidth feels like a single link.

    Open the console of SW1 and start with the summary.

    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
        A - formed by Auto LAG

Number of channel-groups in use: 1
Number of aggregators:           1

Group  Port-channel  Protocol    Ports
------+-------------+-----------+-----------------------------------------------
1      Po1(SD)         PAgP      Gi0/0(I)    Gi0/1(I)

    • Po1 (SD): the port-channel is Layer 2 and down.

    • Gi0/0 (I) and Gi0/1 (I): each member gave up on the negotiation and now runs as a plain stand-alone interface.

    That matches the ticket: a stand-alone port still forwards traffic like an ordinary link, STP blocks the redundant one, and the network keeps working.
    Nothing is down, you only lost the aggregation.

    Run the same check from SW2.

    SW2# 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
        A - formed by Auto LAG

Number of channel-groups in use: 1
Number of aggregators:           1

Group  Port-channel  Protocol    Ports
------+-------------+-----------+-----------------------------------------------
1      Po1(SD)         PAgP      Gi0/0(I)    Gi0/1(I)

    Identical state on both sides: the failure is symmetrical.

    Port-channel Po1 down with both EtherChannel members in stand-alone I mode

    Figure 2 – Po1 down, both members stand-alone

    You now know what you are chasing: the links are healthy, the negotiation is not.

    Answer the question below

    Which flag marks a PAgP port that failed to bundle?

    Finding the Mismatch

    Compare the channel-group line on both sides of the same link.

    SW1# show running-config interface g0/0
Building configuration...

Current configuration : 154 bytes
!
interface GigabitEthernet0/0
 switchport trunk encapsulation dot1q
 switchport mode trunk
 channel-group 1 mode auto
end

    Same command on SW2.

    SW2# show running-config interface g0/0
Building configuration...

Current configuration : 154 bytes
!
interface GigabitEthernet0/0
 switchport trunk encapsulation dot1q
 switchport mode trunk
 channel-group 1 mode auto
end

    Both sides sit in auto.
    Straight from the mode-compatibility table in the previous lesson: auto plus auto is the dead cell.
    Each side waits for the other to speak first, and nobody ever does.

    PAgP mode auto on both switches, no PAgP packet sent, the EtherChannel never forms

    Figure 3 – Auto on both sides, nobody initiates

    To fix it, one side has to initiate.

    Answer the question below

    Which PAgP mode waits and never initiates?

    Fixing the Mode

    Move SW1 to desirable.
    The change stays inside PAgP, so you can type the new mode directly over the old one.

    SW1# configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
SW1(config)# interface range g0/0-1
SW1(config-if-range)# channel-group 1 mode desirable
SW1(config-if-range)# end
%LINK-3-UPDOWN: Interface Port-channel1, changed state to up
%LINEPROTO-5-UPDOWN: Line protocol on Interface Port-channel1, changed state to up

    The syslog already announces the result.
    Confirm it with the summary.

    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
        A - formed by Auto LAG

Number of channel-groups in use: 1
Number of aggregators:           1

Group  Port-channel  Protocol    Ports
------+-------------+-----------+-----------------------------------------------
1      Po1(SU)         PAgP      Gi0/0(P)    Gi0/1(P)

    Confirm that SW2 agrees before you close anything.

    SW2# 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
        A - formed by Auto LAG

Number of channel-groups in use: 1
Number of aggregators:           1

Group  Port-channel  Protocol    Ports
------+-------------+-----------+-----------------------------------------------
1      Po1(SU)         PAgP      Gi0/0(P)    Gi0/1(P)

    SU on the port-channel, P on both members: the healthy signature from the previous lesson is back.
    Desirable plus auto is one of that table's green cells.

    Answer the question below

    Does a stand-alone (I) port still forward traffic?