The Inner Workings of EtherChannel
EtherChannel is a very important technology for modern networks. As bandwidth demands continue to increase with more and more traffic forms converging onto the single network infrastructure, EtherChannels importance is magnified even more.
Link Aggregation Protocols
As mentioned earlier, EtherChannel actually aggregates individual Ethernet links into a single logical link that provides bandwidth up to 1600 Mbps, in the case of Fast Ethernet, or 16 Gbps when Gigabit interfaces are used to create the logical link. The restriction, however, is that all the bundled interfaces must be configured with matching speed and duplex settings, and both ends of each link must be configured as either a Layer 2 or Layer 3 interface.
If an individual link within an EtherChannel bundle fails, traffic previously carried over the failed link is carried over the remaining links within the EtherChannel.
An EtherChannel can be configured in one of these two dynamic modes:
• Port Aggregation Protocol (PAgP) is a Cisco proprietary protocol used to automate the logical aggregation of Ethernet switch ports. This means PAgP can only be used between Cisco switches.
• Link Aggregation Control Protocol (LACP) is an industry standard protocol designed to automate the logical aggregation ethernet ports. Also know by its IEEE designation 802.3ad LACP is not tied to any specific vendor.
Port Aggregation Protocol (PAgP)
As we have already discussed, PAgP packets are sent between EtherChannel capable ports to facilitate the negotiation needed for the successful creation of a channel. When PAgP sees matched Ethernet links, it will group the links into an EtherChannel.
PAgP uses three modes of operation:
• Auto—places an interface into a passive negotiating state, meaning that the interface will respond to PAgP packets it receives but it will not initiate PAgP packet negotiation. This setting minimizes the transmission of PAgP packets and is the default on devices like Catalyst 3560.
• Desirable—places an interface into an active negotiating state, meaning that the interface will start negotiations with other interfaces by sending PAgP packets.
• On—forces the interface to channel without PAgP. With the on mode, a usable EtherChannel exists only when an interface group in the on mode is connected to another interface group in the on mode. This is referred to as static aggregation.
Link Aggregation Control Protocol (LACP)
LACP performs the exact same function as the Cisco proprietary PAgP but it does it by sending LACP packets to its peer. Because LACP is an IEEE standard, it can be used to facilitate EtherChannels in mixed vendor environments.
LACP, like PAgP, has three modes of operation:
• Passive— The switch does not initiate the channel, but does respond to incoming LACP packets. When a peer initiates negotiation (by sending out an LACP packet) which we receive and reply to, eventually forming the aggregation channel with the peer. This is similar to the auto mode in PAgP.
• Active—We are willing to form an aggregate link and will actively seek to start the negotiation. The link aggregate will be formed if the other end is running in LACP active or passive mode. This is similar to the desirable mode of PAgP.
• On—A link aggregation is forced to be formed without any LACP negotiation. In other words, the switch will neither send the LACP packet nor process any incoming LACP packet. This is similar to the on state for PAgP. Again, this is referred to as static aggregation.
Configuration
We only need a single command line to configure a group of ports to operate as an EtherChannel:
SW1(config)# interface range f0/23 -24
SW1(config-if-range)# channel-group 23 mode active
Creating a port-channel interface Port-channel 23
As expected, we have successfully created the logical interface Port-channel23. Note that any switchport configurations applied to this virtual interface will be replicated to the physical member interfaces. We can verify the configuration and the status of an EtherChannel by using the 'show EtherChannel summary' command:
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 f–failed to allocate aggregator
------+-------------+-----------+-------------------------------
1 Po23(SD) LACP Fa0/23(D) Fa0/24(D)
