Today we’re going to take a look at how to configure an etherchannel between two Cisco Switches.
What is an etherchannel? It’s a way of taking multiple independent links and bundling them together, so that they appear as one logical connection between two devices. Etherchannels are commonly used between two switches, or between a switch and a host – which allows for both additional bandwidth and fault tolerance/redundancy. In the example today, we’ll be using an etherchannel protocol called Link Aggregation Control Protocol (LACP). LACP is an IEEE standard (802.3ad).
You might be thinking “Wait, wouldn’t multiple links cause a loop? Or trigger Spanning-tree to block ports?”. Not in this case! Etherchannel technologies work around those problems by creating a single logical interface for spanning-tree to worry about. The etherchannel protocol itself worries about loop prevention in between the two devices, so we get multiple ports of non-blocking bandwidth.
For everything we cover in this example, we’ll be using the following topology:
So we have two switches, which are connected together via Eth0/0 and Eth0/1. Each switch has three VLANs configured – 10, 20, and 30.
Configuring an Etherchannel
I’ll only be showing the configuration from the perspective of 0x2142-SW1 – but all configuration is replicated on 0x2142-SW2.
! We'll use the interface range command to apply the etherchannel configuration to ! both Eth0/0 and Eth0/1 at the same time: 0x2142-SW1(config)#int range Eth0/0 - 1 ! We specify which etherchannel protocol to use by configuring 'channel-protocol' ! PAgP is a Cisco Proprietary protocol, but we'll be using LACP for this example: 0x2142-SW1(config-if-range)#channel-protocol ? lacp Prepare interface for LACP protocol pagp Prepare interface for PAgP protocol 0x2142-SW1(config-if-range)#channel-protocol lacp ! Next we need to specify a channel-group and mode: 0x2142-SW1(config-if-range)#channel-group 1 mode ? active Enable LACP unconditionally auto Enable PAgP only if a PAgP device is detected desirable Enable PAgP unconditionally on Enable Etherchannel only passive Enable LACP only if a LACP device is detected 0x2142-SW1(config-if-range)#channel-group 1 mode active Creating a port-channel interface Port-channel 1 0x2142-SW1(config-if-range)# *Jan 26 01:03:04.532: %LINEPROTO-5-UPDOWN: Line protocol on Interface Port-channel1, changed state to up
Let’s talk through a few notes about the above configuration. In order to enable etherchannel, we only need to configure two commands: channel-protocol and channel-group. The channel-protocol command tells the switch which etherchannel protocol to use for negotiation (LACP in this case). The channel-group command provides two necessary components: the group number and mode. The group number is just a device-local identifier for which group to add these ports to. When we specified group 1, the switch adds both Eth0/0 and Eth0/1 into the new logical interface Port-Channel 1.
The etherchannel mode is also important. The two primary options we want to look at for LACP are active and passive. Active tells the switch to preemptively send out LACP negotiation packets. In this case, the switch really wants the ports to become a bundle and will ask it’s partner device for an etherchannel to be formed. Passive mode tells our switch to only negotiate if another device wants to. In this mode, our switch won’t send out any etherchannel negotiation packets unless its partner device does so first.
Generally speaking, the most common configuration is to set the mode on both devices to active. This ensures that both devices actively participate in trying to establish an etherchannel. Placing one device in active and one in passive will work as well. However, if both devices are placed into passive mode, an etherchannel will never form.
So how do we validate that the etherchannel has formed correctly? One way is using the show etherchannel summary command:
0x2142-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 N - not in use, no aggregation f - failed to allocate aggregator M - not in use, minimum links not met m - not in use, port not aggregated due to 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) LACP Et0/0(P) Et0/1(P)
From the output above, we see that there is one group configured with the group ID of 1. It shows that both Eth0/0 and Eth0/1 have been added into the Port-channel 1 interface. The (SU) next to the Port-channel interface indicate that the etherchannel is up (U) and configured for layer 2 (S).
I mentioned earlier that spanning-tree only worries about the port-channel interface, not the individual member ports. We can also check that out by using the show spanning-tree command:
0x2142-SW1#sh spanning-tree vlan 20 VLAN0020 Spanning tree enabled protocol rstp Root ID Priority 32788 Address aabb.cc00.1000 This bridge is the root Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32788 (priority 32768 sys-id-ext 20) Address aabb.cc00.1000 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 300 sec Interface Role Sts Cost Prio.Nbr Type ------------------- ---- --- --------- -------- -------------------------------- Et0/2 Desg FWD 100 128.3 Shr Et0/3 Desg FWD 100 128.4 Shr <-- Output omitted --> Po1 Desg FWD 56 128.65 Shr
Making Configuration Changes to an Etherchannel
Now that we have a working etherchannel – We have a few things that need special attention. The individual port configurations, Eth0/0 and Eth0/1 in this case, need to match at all times! Port configuration mis-matches are going to be an easy way to inadvertently bring down the port-channel. The good thing is that we now have a convenient Port-Channel interface which we can use for configuration. This logical port will replicate any configuration changes to all member ports.
! Let's jump into our Port-Channel 1 interface and configure a trunk for VLAN 20 0x2142-SW1(config)#int po1 0x2142-SW1(config-if)#switchport mode trunk 0x2142-SW1(config-if)#switchport trunk allowed vlan 20 ! Now we can check the individual port configs: 0x2142-SW1(config-if)#do sh run int e0/0 Building configuration... Current configuration : 176 bytes ! interface Ethernet0/0 switchport trunk allowed vlan 20 switchport mode trunk channel-protocol lacp channel-group 1 mode active end 0x2142-SW1(config-if)#do sh run int e0/1 Building configuration... Current configuration : 176 bytes ! interface Ethernet0/1 switchport trunk allowed vlan 20 switchport mode trunk channel-protocol lacp channel-group 1 mode active end
Easy enough, right? The configuration changes for the trunk are now on both Eth0/0 and Eth0/1.
There is always a possibility that something goes wrong – so let’s take a quick look at some common problems and how to fix them.
Remember how I said that the member port configurations had to match? Here’s what happens if we make a configuration change on only one of the two member ports:
0x2142-SW1(config)#int eth0/1 0x2142-SW1(config-if)#switchport trunk allowed vlan 30 0x2142-SW1(config-if)# *Jan 28 20:43:55.458: %EC-5-CANNOT_BUNDLE2: Et0/1 is not compatible with Et0/0 and will be suspended (vlan mask is different)
Eth0/1 immediately gets put into a suspended state, and is no longer active in the port-channel interface. In this case the switch gives us a good hint as to what’s wrong – vlan mask is different. Error messages will vary slightly, but a suspended port is easy to fix by comparing individual port configurations and fixing the mismatch.
Here’s another one:
*Jan 28 21:06:07.346: %EC-5-L3DONTBNDL2: Et0/0 suspended: LACP currently not enabled on the remote port. *Jan 28 21:06:08.009: %EC-5-L3DONTBNDL2: Et0/1 suspended: LACP currently not enabled on the remote port.
This error message can mean a few things – the common one being exactly what it states! Check both sides of the connection, and ensure that LACP is configured on each device. This error message can also occur on certain mismatches – like if one side is running as a Layer 2 etherchannel, but the other side is running as Layer 3.
Jan 28 20:83:55.458 %ETHPORT-5-IF_DOWN_PORT_CHANNEL_MEMBERS_DOWN: Interface port-channel1 is down (No operational members)
The above message is also somewhat self-explanatory. In this case, the switch is unable to bring up the port-channel interface, because none of the underlying member ports are coming online. Troubleshoot what might be wrong with those ports first, then the port-channel should come up.
Hope this was useful! In a later post, we’ll dig into more configuration and considerations – like packet hashing, layer 3 etherchannels, and how packets are weighted between interfaces.
Questions? Drop them in the comments below, or message me on Twitter!