Troubleshooting Etherchannel

Most of the issues with Etherchannels (Link Aggregation) are because of misconfiguration. Keep in mind that the configuration of all physical interfaces has to match. In this lesson we’ll take a look at some common issues with Etherchannels.

This is the topology I will use:

The idea is to bundle FastEthernet 0/13 and 0/14 into an Etherchannel but this is not working. Let’s look at the first issue…

Wrong Etherchannel Mode

Let’s look at our first issue, the Etherchannel is not working. First we’ll check if the interfaces are up and running:

SW1#show interfaces fa0/13 | include line protocol
FastEthernet0/13 is up, line protocol is up (connected)

SW1#show interfaces fa0/14 | include line protocol
FastEthernet0/14 is up, line protocol is up (connected)

SW2#show interfaces fa0/13 | include line protocol
FastEthernet0/13 is up, line protocol is up (connected)

SW2#show interfaces fa0/14 | include line protocol
FastEthernet0/14 is up, line protocol is up (connected)

All interfaces are operational. Let’s check if we have a port-channel interface:

SW1#show ip int brief | include Port
Port-channel1          unassigned      YES unset  down                  down

SW2#show ip int brief | include Port
Port-channel1          unassigned      YES unset  down                  down 

We can verify that a port-channel interface has been created but it is down. Let’s check its details:

SW1#show etherchannel summary | begin Group
Group  Port-channel  Protocol    Ports
------+-------------+-----------+-----------------------------------------------
1      Po1(SD)         LACP      Fa0/13(I)   Fa0/14(I)

SW2#show etherchannel summary | begin Group
Group  Port-channel  Protocol    Ports
------+-------------+-----------+-----------------------------------------------
1      Po1(SD)         PAgP      Fa0/13(I)   Fa0/14(I)

Here’s a nice command to verify your etherchannel. Use the show etherchannel summary to see your port-channels. We can see that SW1 has been configured for LACP and SW2 for PAgP, this is never going to work. Let’s take a closer look:

SW1#show etherchannel 1 detail 
Group state = L2 
Ports: 2   Maxports = 16
Port-channels: 1 Max Port-channels = 16
Protocol:   LACP
Minimum Links: 0
		Ports in the group:
		-------------------
Port: Fa0/13
------------

Port state    = Up Sngl-port-Bndl Mstr Not-in-Bndl 
Channel group = 1           Mode = Active          Gcchange = -
Port-channel  = null        GC   =   -             Pseudo port-channel = Po1
Port index    = 0           Load = 0x00            Protocol =   LACP

Flags:  S - Device is sending Slow LACPDUs   F - Device is sending fast LACPDUs.
        A - Device is in active mode.        P - Device is in passive mode.

Local information:
                            LACP port     Admin     Oper    Port        Port
Port      Flags   State     Priority      Key       Key     Number      State
Fa0/13    SA      indep     32768         0x1       0x1     0xF         0x7D  

Age of the port in the current state: 0d:00h:11m:59s
          
Port: Fa0/14
------------
          
Port state    = Up Sngl-port-Bndl Mstr Not-in-Bndl 
Channel group = 1           Mode = Active          Gcchange = -
Port-channel  = null        GC   =   -             Pseudo port-channel = Po1
Port index    = 0           Load = 0x00            Protocol =   LACP
          
Flags:  S - Device is sending Slow LACPDUs   F - Device is sending fast LACPDUs.
        A - Device is in active mode.        P - Device is in passive mode.
          
Local information:
                            LACP port     Admin     Oper    Port        Port
Port      Flags   State     Priority      Key       Key     Number      State
Fa0/14    SA      indep     32768         0x1       0x1     0x10        0x7D  
          
Age of the port in the current state: 0d:00h:12m:38s
          
                Port-channels in the group: 
                ---------------------------
          
Port-channel: Po1    (Primary Aggregator)
          
------------
          
Age of the Port-channel   = 0d:00h:12m:55s
Logical slot/port   = 2/1          Number of ports = 0
HotStandBy port = null 
Port state          = Port-channel Ag-Not-Inuse 
Protocol            =   LACP
Port security       = Disabled

The best command to use is show etherchannel detail. This gives you a lot of information but I’m particularly interested in seeing if LACP is configured for passive or active mode. Interfaces in active mode will “actively” try to form an etherchannel. Interfaces in passive mode will only respond to LACP requests. Let’s look at SW2:

SW2#show etherchannel 1 detail
Group state = L2 
Ports: 2   Maxports = 8
Port-channels: 1 Max Port-channels = 1
Protocol:   PAgP
Minimum Links: 0
		Ports in the group:
		-------------------
Port: Fa0/13
------------

Port state    = Up Sngl-port-Bndl Mstr Not-in-Bndl 
Channel group = 1           Mode = Desirable-Sl    Gcchange = 0
Port-channel  = null        GC   = 0x00010001      Pseudo port-channel = Po1
Port index    = 0           Load = 0x00            Protocol =   PAgP

Flags:  S - Device is sending Slow hello.  C - Device is in Consistent state.
        A - Device is in Auto mode.        P - Device learns on physical port.
        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.

Local information:
                                Hello    Partner  PAgP     Learning  Group
Port      Flags State   Timers  Interval Count   Priority   Method  Ifindex
Fa0/13    	U4/S4   H	30s	 0        128        Any      10013
          
Age of the port in the current state: 0d:00h:15m:25s
          
Port: Fa0/14
------------
          
Port state    = Up Sngl-port-Bndl Mstr Not-in-Bndl 
Channel group = 1           Mode = Desirable-Sl    Gcchange = 0
Port-channel  = null        GC   = 0x00010001      Pseudo port-channel = Po1
Port index    = 0           Load = 0x00            Protocol =   PAgP
          
Flags:  S - Device is sending Slow hello.  C - Device is in Consistent state.
        A - Device is in Auto mode.        P - Device learns on physical port.
        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.
          
Local information:
                                Hello    Partner  PAgP     Learning  Group
Port      Flags State   Timers  Interval Count   Priority   Method  Ifindex
Fa0/14    	U4/S4   H	30s	 0        128        Any      10014
          
Age of the port in the current state: 0d:00h:15m:28s
          
                Port-channels in the group: 
                ---------------------------
          
Port-channel: Po1
------------
          
Age of the Port-channel   = 0d:00h:15m:51s
Logical slot/port   = 2/1          Number of ports = 0
GC                  = 0x00000000      HotStandBy port = null
Port state          = Port-channel Ag-Not-Inuse 
Protocol            =   PAgP
Port security       = Disabled

Here’s the output of show etherchannel detail of SW2. We can see that it has been configured for PAgP and the interfaces are configured for desirable mode. If they would have been configured for auto mode we would see the A flag. Let’s re-configure SW2 so that is uses LACP:

SW2(config)#no interface po1
SW2(config)#interface fa0/13
SW2(config-if)#channel-group 1 mode passive
SW2(config-if)#exit
SW2(config)#interface fa0/14
SW2(config-if)#channel-group 1 mode passive

Let’s get rid of the port-channel interface first, if we don’t do this you’ll see an error when you try to change the channel-group mode on the interfaces. After a few seconds you’ll see the port-channel interface appear:

SW1# %LINK-3-UPDOWN: Interface Port-channel1, changed state to up

SW2# %LINK-3-UPDOWN: Interface Port-channel1, changed state to up

After changing the configuration we see the port-channel1 going up. Problem solved!

Lesson learned: Make sure you use the same EtherChannel mode (PaGP or LACP) on both sides.

PaGP / LACP Negotiation not working

Same topology, different problem:

Once again the Etherchannel is down:

SW1#show ip interface brief | include Port
Port-channel1          unassigned      YES unset  down                  down

SW2#show ip interface brief | include Port
Port-channel1          unassigned      YES unset  down                  down

We can verify that the port-channel interface exists but it’s down on both sides. Let’s check the Etherchannel configuration: