EtherChannel Diagnostic Information Gathering

To diagnose EtherChannel problems on a Cisco network, you first must be able to collect information about your switch and its configuration. To perform this task, you, as always, rely on the show and debug commands. First, a brief overview of what you get from the show command.

Switch1> enable
Switch1# configure terminal
Switch2#show EtherChannel ?
  <1-6>         Channel group number
  detail        Detail information
  load-balance  Load-balance/frame-distribution scheme among ports in
                port-channel
  port          Port information
  port-channel  Port-channel information
  protocol      protocol enabled
  summary       One-line summary per channel-group
  |             Output modifiers
  <cr>

Following the typical Cisco command standard, detail gives you more information than you probably want, whereas summary gives you little more than the basics. You should check with summary first, and if the information you need is there, then you are done; otherwise, load up the screen with information from the detail option.

The other options for the show EtherChannel command give you more information about your ports, port-channels, and protocols (PAgP or LACP) if in your troubleshooting you feel that you need to drill down a little deeper. Here is the summary information for the current connection, which reveals the ports that make up the EtherChannel.

Switch1> enable
Switch1# configure terminal
Switch2#show etherchannel summary
Flags:  D - down        P - in port-channel
        I - stand-alone s - suspended
        H - Hot-standby (LACP only)
        R - Layer3      S - Layer2
        u - unsuitable for bundling
        U - in use      f - failed to allocate aggregator
        d - default port
Number of channel-groups in use: 1
Number of aggregators:           1
Group  Port-channel  Protocol    Ports
------+-------------+-----------+----------------------------------------------
2      Po2(SD)         PAgP      Fa0/1(D)    Fa0/2(D)

In addition to the EtherChannel information, both PAgP and LACP offer further information via the show command. The information can be selected by channel group number and includes counters related to the data that has gone through the links, information about the links that is internal to the switch, and information about the devices on the other side of the EtherChannel links.

Switch1> enable
Switch1# configure terminal
Switch2#show PAgP ?
  <1-6>     Channel group number
  counters  Traffic information
  internal  Internal information
  neighbor  Neighbor information
Switch1> enable
Switch1# configure terminal
Switch2#show LACP ?
  <1-6>     Channel group number
  counters  Traffic information
  internal  Internal information
  neighbor  Neighbor information
  sys-id    LACP System ID

Here is an example of one of those commands, it is retrieving information about PAgP, using the internal option. You can see in the output that even though you configured the switch, one of the cables is not attached (flag is d), and as such, only half of the EtherChannel is up. With the second cable attached, you see both ports with the SC flags and the H timer running.

Switch2> enable
Switch2# show pagp internal
Flags:  S - Device is sending Slow hello.  C - Device is in Consistent state.
        A - Device is in Auto mode.        d - PAgP is down
Timers: H - Hello timer is running.        Q - Quit timer is running.
        S - Switching timer is running.    I - Interface timer is running.
Channel group 2
                                Hello    Partner  PAgP     Learning  Group
Port      Flags State   Timers  Interval Count   Priority   Method  Ifindex
Fa0/1     SC    U6/S7   H       30s      1        128        Any      15
Fa0/2     d     U1/S1           1s       0        128        Any      0

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