Cisco EtherChannel Notes

Here are a couple of notes I gathered around the subject of Cisco Etherchannel. We start by some concepts then learn how to configure L3 Etherchannels.

Etherchannel concepts

EtherChannel is a technology that allows a set of ports to be bundled into one logical interface. There is the logical port (the bundle) and one or more physical ports that constitute it. The physical ports are also called member ports.

When one member port fails, traffic that was flowing through it gets redirected over the remaining operational member ports, transparently to the end user and without the network administrator intervention.

A physical port can be a member of only one Etherchannel.

We can configure an Etherchannel between a network device and a router, a switch, a server,…

An EtherChannel can be:

  • a Fast EtherChannel: an EtherChannel formed from Fast Ethernet interfaces
  • a Giga EtherChannel: an EtherChannel formed from Gigabit Ethernet interfaces
  • a port Channel: a layer 3 EtherChannel

A port channel can be of two types: L2 port channel or L3 port channel.

  • L2 port channel: formed of L2 interfaces.
  • L3 port channel: formed of L3 interfaces. We get a L3 interface by issuing no switchport interface command.

Spanning Tree Protocol sees the port channel as a single port

channel-group x mode on : neither PAgP, nor LACP is used. To specify that we want LACP or PAgP, we must choose one of these modes: desirable, auto, active, passive.

The channel group number must be the same on both ends of the bundle.

Etherchannel Misconfig Guard

  • When one Etherchannel port on one side detects that the other side is not an Etherchannel, it puts the local EC member ports in Err-disabled state.
  • The system can detect the non-bundling side of EC, because it expects to see different BPDUs coming from different interfaces, with the same Port ID, but not different Port IDs.

L3 Cisco Etherchannel

I am going to discuss L3 EtherChannels on a Cisco ISR only (such as the famous Cisco 2811 router).

A layer 3 Etherchannel is also called port channel. For inter-VLAN trunking, it only supports IEEE 802.1Q.

single-switch-Etherchannel-deployment-keyboardbanger
Figure: Single switch EtherChannel deployment © Cisco.com

We can use a layer 3 EtherChannel to connect a router to a switch or a stack of switches. When you connect the ISR router to a stack of switches, you can have two deployment options:

  • concentrate all member ports on one switch, or
  • build a cross-stack EtherChannel, where we take one port from each switch and connect it to the router

On an ISR, we can have up to two layer 3 EtherChannels. Each layer 3 EtherChannel can have up to four member ports. Also, the member ports must be of the same speed, duplex, type and trunk mode. What I mean by “same type” is that member ports can not a mixture of network module interfaces. Trying, for example, to bundle a NM-1GE port with a HWIC-1GE-SFP port in an EtherChannel is not supported. However, member ports can be on different slots on the router.

A Cisco ISR does not support layer 2 EtherChannels. But, if it is equipped with an EtherSwitch service module, this one supports both layer 2 and layer 3 EtherChannels.

cross-stack-Etherchannel-deployment-keyboardbanger
Figure: Cross-stack Etherchannel deployment © Cisco.com

When you add a configuration to an EtherChannel, the settings will propagate to the member ports. However, making a change in the configuration of the member ports does not propagate to the EtherChannel.

Beware that LACP and PAgP do not work on an ISR if you want to negotiate port aggregation. Simply configure the EtherChannel manually on both ends of the bundled link (even if the other device supports these protocols).

The layer 3 EtherChannel can be configured with an IP address. We can also create layer 3 EtherChannel subinterfaces and assign them IP addresses.

Configuring a Layer 3 EtherChannel

    • first, remove L2 capability on bundle member interfaces, by issuing the command no switchport
    • on the port channel, remove L2 capability and give it an ip address.
    • Do the same on the other end of the bundle.
    • Now the portchannel is a router port and can ping port channel of the other end !

When we make an interface a L3 one, and it’s a part of an etherchannel bundle, then it disassociates from it. A show etherchannel summary shows this:

show-etherchannel-summ

After we make the interface as L3, we should re-put it into the etherchannel bundle with channel-group x mode … command.

Then, we make the port channel as L3 with no switchport

Now, if we do this on both sides of the etherchannel, we get two L3 interfaces. That means they behave like normal router ports! Let’s give them IP addresses and ping each other:

etherchannel_show run int po44

to see load balancing on the port channel: show etherchannel load-balance

show etherchannel load-balancing

to change the load balancing default method: port-channel load-balance {method} on global configuration mode:

port-channel-load-balance-dst-mac

Load Balancing in a layer 3 EtherChannel

Load balancing in a layer 3 EtherChannel on a ISR router is based on both the source and destination IP addresses. It can not be done based solely on either source IP address or destination IP address.

Also, VLAN ID has no impact on load balancing.

Bandwidth aggregation in an EtherChannel

Bundling a set of interfaces into an EtherChannel allows to have an aggregation of the interfaces bandwidth, in Catalyst switches. However, in ISR routers, there is no guarantee that this holds true. In fact, don’t make EtherChannels on routers if your goal is to increase the bandwidth between the router and the other device. Cisco does not guarantee that your EtherChannel will operate at an aggregate wire rate speed. Your only purpose of doing EtherChannel between an ISR router and another device is to have load balancing and to increase redundancy.

Configuring load balancing

If we change the default method, we must

    • make the change,
    • unbundle the ports,
    • rebundle them
    • do the same at the other end

PAgP

    • auto + desirable = portchannel created
    • if something changes on one member port, PAgP applies same changes on the other member ports.
    • “no-silent” submode: port channel requests that the other end sends PAgP packets

Troubleshooting Etherchannel

    • show etherchannel summary : see portchannels and their member ports
    • show interfaces
    • show vlan brief

Conclusion

We learned in this article what etherchannel are, the difference between layer 2 and layer 3 etherchannels. We also learned how to configure a port channel and we saw some commands to display the etherchannel configuration.

How much do you see Etherchannel configured in your production network?

Sources

Understanding Layer 3 EtherChannel Support on Cisco Integrated Services Router, Cisco.com

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