In this topic, I will go through EtherChannel configuration and a few of the common issues that make EtherChannel down.

The idea behind EtherChannel is to have multiple physical links between switches combined into a single logical link with built-in mechanisms to prevent packet loops. The benefits of aggregating switch links are:

  1. Increased perceived redundancy level; a single or double link failure between two switches can be immediately overcome.
  2. Expansion of bandwidth between switches; added bandwidth requirements can be satisfied by bundling interconnection links.

Prerequisites of Enabling EtherChannel

  1. EtherChannel bundle links must be of the same type and speed. For example, if we intend to combine Fast Ethernet links (100Mbps) then all bundle links MUST be Fast Ethernet links of 100Mbps speed.
  2. All links intended to be bundled MUST NOT exceed the number of eight physical links. Up to eight physical links can be combined into one logical link.
  3. All links MUST belong to the same VLAN if used as access links. They MUST carry the same VLANS if used as trunk links and MUST be configured with identical STP settings.

EtherChannel Negotiation Protocols

  1. Port Aggregation Protocol (PAgP): Cisco Proprietary protocol; EtherChannel negotiation is carried out on EtherChannel capable ports in order to achieve bundling of physical links.
  2. Link Aggregation Control Protocol (LACP): Standardized protocol defined in IEEE 802.3ad; Similar to PAgP. Nevertheless, LACP has the following major differences compared to PAgP:
  • LACP assigns roles to interconnected switches so that the master switch is allowed to make decisions on which ports are participating in the EtherChannel at any given time.
  • Up to sixteen switch links can be part of the bundle group, though, eight of them can be active at the same time. Activation of a standby link is performed only when an active link goes down.

EtherChannel Modes

Mode Protocol Description
Auto PAgP Sets the interface to respond to PAgP negotiation packets, but
the interface will start negotiations on its own.
Desirable PAgP Sets the interface to actively attempt to negotiate a PAgP
connection.
On EtherChannel Forces the connection to bring all links up without using a
protocol to negotiate connections. This mode can only connect to
another device that is also set to on. When using this mode, the
switch does not negotiate the link using either PAgP or LACP.
Active LACP Sets the interface to actively attempt to negotiate connections
with other LACP devices.
Passive LACP Sets the interface to respond to LACP data if it receives
negotiation requests from other systems.

EtherChannel Load Balancing Methods

  1. Src-dst-ip: Traffic is distributed to bundled links according to source and destination IP address combination. Therefore, in networks where traffic originates from many different sources destined for the same or different destinations, is distributed in a balanced way across aggregated links.
  2. Src-dst-mac: Traffic distribution is based upon source and destination mac-address combination. This method accomplishes fair load distribution only in cases that only Layer 2 switching takes place, and traffic route patterns do not cross router interfaces.
  3. Src-dst-port: In case that we have most of the traffic traversing constant end-devices distributing traffic based on source and destination ports might be the better solution to achieve better load balancing between aggregated links.

Example of switch configuration :

Switch 1:

INBSW1> enable

INBSW1# configure terminal

INBSW1 (config)# interface range fasttethernet0/11 -12

INBSW1 (config-if-range)# switchport mode access

INBSW1 (config-if-range)# switchport access vlan 10

INBSW1 (config-if-range)# channel-group 5 mode desirable

INBSW1 (config-if-range)# end

Switch 2:

INBSW2> enable

INBSW2# configure terminal

INBSW2 (config)# interface range fasttethernet0/1 -2

INBSW2 (config-if-range)# switchport mode access

INBSW2 (config-if-range)# switchport access vlan 10

INBSW2 (config-if-range)# channel-group 2 mode auto

INBSW2 (config-if-range)# end

In this configuration, this pair of ports allows you to send data only for VLAN 10 over the EtherChannel link. To pass traffic for all VLANs, you must configure the switchport as a trunk because Access ports will only send traffic for one VLAN.

Troubleshooting:

The main problem that we face in Etherchannel is the mismatch of configuration on both sides.

1.Use of different protocols LACP or PAgP

2.Mismatch of Mode inside Protocols. (see above section EtherChannel Modes to check details on modes)

Basic Commands to verify Parameters on EtherChannel to troubleshoot:

Show EtherChannel summary

Use the above command to verify the Protocol on switch for EtherChannel.

Show EtherChannel 1 detail

Use the above command to verify modes of Protocol.