EtherChannels

EtherChannel provides fault-tolerant & high-speed links between switches, routers, and servers etc. You can use it to increase the bandwidth anywhere in the network where bottlenecks are likely to occur. EtherChannel provides automatic recovery if a link fails, EtherChannel redirects traffic from the failed link to the remaining links in the channel without intervention.

You can bundle up to 8 interfaces of same type & speed, bandwidth up to 800 Mbps (Fast EtherChannel) or 8 Gbps (Gigabit EtherChannel) between your switch and another switch or host and all must be configured as either Layer 2 or Layer 3 interfaces.

There are two protocols used for negotiating EtherChannel. We can configure EtherChannel in three ways in Cisco Switches.

•     Port Aggregation Protocol (PAgP) - Cisco Proprietary protocol
•     IEEE Link Aggregation Control Protocol (LACP) - Industry Standard
•     Manual Etherchannel Configuration - Without using any negotiation protocol listed above

The Port Aggregation Protocol (PAgP) and Link Aggregation Control Protocol (LACP) facilitate the automatic creation of EtherChannels by exchanging packets between Ethernet interfaces. PAgP is a Cisco-proprietary protocol that can be run only on Cisco switches and on those licensed to support PAgP. LACP is defined in IEEE 802.3AD.

With Layer 2 interfaces, the logical interface is dynamically created.

With Layer 3 interfaces, you manually create the logical interface by using the "interface port-channel" global configuration command.


Both the auto and desirable PAgP modes allow interfaces to negotiate with partner interfaces to determine if they can form an EtherChannel based on criteria such as interface speed and, for Layer 2 EtherChannels, trunking state and VLAN numbers.

PAgP	Desirable	Auto
Desirable	Yes	Yes
Auto	Yes	No

• An interface in the desirable mode can form an EtherChannel with another interface that is in the desirable or auto mode.

• An interface in the auto mode can form an EtherChannel with another interface in the desirable mode.

• An interface in the auto mode cannot form an EtherChannel with another interface that is also in the auto mode because neither interface starts PAgP negotiation.

• An interface in the on mode that is added to a port channel is forced to have the same characteristics as the already existing on mode interfaces in the channel. 

If your switch is connected to a partner that is PAgP-capable, you can configure the switch interface for nonsilent operation by using the non-silent keyword. If you do not specify non-silent with the auto or desirable mode, silent mode is assumed.

The silent mode is used when the switch is connected to a device that is not PAgP-capable. An example of a silent partner is a file server or a packet analyzer that is not generating traffic. In this case, running PAgP on a physical port connected to a silent partner prevents that switch port from ever becoming operational however, the silent setting allows PAgP to operate to attach the interface to a channel group and to use the interface for transmission.


The link aggregation control protocol (LACP) is a standards-based replacement for PAgP. If you want to dynamically negotiate etherchannels with non-Cisco gear (including some servers), LACP is the way to go.

 

LACP	Aactive	Passive
Aactive	Yes	Yes
Passive	Yes	No

• An interface in the active mode can form an EtherChannel with another interface that is in the active or passive mode.

• An interface in the active mode can form an EtherChannel with another interface in the passive mode.

• An interface in the passive mode cannot form an EtherChannel with another interface that is also in the passive mode because neither interface starts LACP negotiation.

• An interface in the on mode that is added to a port channel is forced to have the same characteristics as the already existing on mode interfaces in the channel. 

One big advantage of dynamically negotiating etherchannels is that the negotiation protocols will help prevent etherchannel mismatches. Setting the etherchannel to "on" can get you into trouble if the two channel members go to different switches, or go to a switch without etherchannel configured.

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