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EtherChannel- High Speed Data Transmission

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EtherChannel- High Speed Data Transmission

EtherChannel is a port link aggregation technology or port-channel architecture used primarily on Cisco switches. It allows grouping of several physical Ethernet links to create one logical Ethernet link for the purpose of providing fault-tolerance and high-speed links between switches, routers and servers. An EtherChannel can be created from between two and eight active Fast, Gigabit or 10-Gigabit Ethernet ports, with an additional one to eight inactive (failover) ports which become active as the other active ports fail. EtherChannel is primarily used in the backbone network, but can also be used to connect end user machines.

EtherChannel between a Switch and a Server
EtherChannel between a Switch and a Server

EtherChannel is the technology which is used to combine several physical links between switches or routers into one logical connection and treat them as a single link. Let’s take an example to see the benefits of this technology:

Suppose your company has two switches connecting with each other via a FastEthernet link (100Mbps)

Your company is growing and you need to transfer more than 100 Mbps between these switches. If you only connect other links between the two switches it will not work because Spanning-tree protocol (STP) will block redundant links to prevent a loop:

To extend the capacity of the link you have two ways:

  • Buy two 1000Mbps (1Gbps) interfaces.
  • Use EtherChannel technology to bundle them into a bigger link.

The first solution is expensive with the new hardware installed on the two switches. By using EtherChannel you only need some more unused ports on your switches:

EtherChannel bundles the physical links into one logical link with the combined bandwidth and it is awesome! STP sees this link as a single link so STP will not block any links! EtherChannel also does load balancing among the links in the channel automatically. If a link within the EtherChannel bundle fails, traffic previously carried over the failed link is carried over the remaining links within the EtherChannel. If one of the links in the channel fails but at least one of the links is up, the logical link (EtherChannel link) remains up.

EtherChannel also works well for router connections:

When an EtherChannel is created, a logical interface will be created on the switches or routers representing for that EtherChannel. You can configure this logical interface in the way you want. For example, assign access/trunk mode on switches or assign IP address for the logical interface on routers.

(Note: A maximum of 8 Fast Ethernet or 8 Gigabit Ethernet ports can be grouped together when forming an EtherChannel.)

The main advantages of EtherChannel technology is that it allows load sharing of traffic among the links in the channel as well as redundancy in the event that one or more links in the EtherChannel fail.

EtherChannel is a Cisco Copyrighted term and the term which industry adopted is “Link Aggregation”.

EtherChannel links can be either access or trunk links, but if they are trunked (usually the case), they require the following the be the same on all connected interfaces:

  1. VLANs
  2. Trunking Mode
  3. Native VLAN
  4. Speed
  5. Duplex

History

EtherChannel technology was invented by Kalpana in the early 1990s. They were later acquired by Cisco Systems in 1994. In 2000 the IEEE passed 802.3ad which is an open standard version of EtherChannel.

Three mechanisms you can choose to configure EtherChannel:

  1. Port Aggregation Protocol (PAgP)
  2. Link Aggregation Control Protocol (LACP)
  3. Static (“On”)

LACP is the IEEE Standard (IEEE 802.3ad) and is the most common dynamic ether-channel protocol, whereas PAgP is a Cisco proprietary protocol and works only between supported vendors and Cisco devices. All ports in an EtherChannel must use the same protocol; you cannot run two protocols on two ends. In other words, PAgP and LACP are not compatible so both ends of a channel must use the same protocol.

The Static Persistence (or “on” mode) bundles the links unconditionally and no negotiation protocol is used. In this mode, neither PAgP nor LACP packets are sent or received.

Port Aggregation Protocol (PAgP)

  1. PAgP dynamically negotiates the formation of a channel.
  2. Cisco proprietary.
  3. Forms EtherChannel only if ports are configured for identical static VLANs or trunking.
  4. Will automatically modify interface parameters on all ports of the bundle if the EtherChannel interface is changed.
  5. STP sends packets over only one physical link in a PAgP bundle. Because STP’s algorithm uses the lowest port priority (priority + port ID), if defaults are set, STP will always use the lowest number port for BPDUs.

There are two PAgP modes:

Auto Responds to PAgP messages but does not aggressively negotiate a PAgP EtherChannel. A channel is formed only if the port on the other end is set to Desirable. This is the default mode.
Desirable Port actively negotiates channeling status with the interface on the other end of the link. A channel is formed if the other side is Auto or Desirable.

The table below lists if an EtherChannel will be formed or not for PAgP:

PAgP Desirable Auto
Desirable Yes Yes
Auto Yes No

Link Aggregation Protocol (LACP)

  1. LACP also dynamically negotiates the formation of a channel.
  2. An open standard to PAgP.
  3. IEEE 802.3ad
  4. Uses priority system for end switches.
  5. Switch with the lowest system priority (2 byte value followed by MAC – lowest wins).
  6. Determines which ports are active in the EtherChannel at any given time.
  7. Uses port priority to determine which ports to place in standby mode if hardware.
  8. Limitations do not allow all ports to participate in the EtherChannel.
  9. Most implementations leave the system and port priority to defaults.

There are two LACP modes:

Passive Responds to LACP messages but does not aggressively negotiate a LACP EtherChannel. A channel is forms only if the other end is set to Active.
Active Port actively negotiates channeling with the interface on the other end of the link. A channel is formed if the other side is Passive or Active.

The table below lists if an EtherChannel will be formed or not for LACP:

LACP Active Passive
Active Yes Yes
Passive Yes No

In general, Auto mode in PAgP is the same as Passive mode in LACP and Desirable mode is same as Active mode.

  1. Auto = Passive
  2. Desirable = Active

Static (“On”)

In this mode, no negotiation is needed. The interfaces become members of the EtherChannel immediately. When using this mode make sure the other end must use this mode too because they will not check if port parameters match. Otherwise the EtherChannel would not come up and may cause some troubles (like loop…).

(Note: All interfaces in an EtherChannel must be configured identically to form an EtherChannel.)

Specific settings that must be identical include:

  1. Speed settings
  2. Duplex settings
  3. STP settings
  4. VLAN membership (for access ports)
  5. Native VLAN (for trunk ports)
  6. Allowed VLANs (for trunk ports)
  7. Trunking Encapsulation (ISL or 802.1Q, for trunk ports)

(Note: EtherChannels will not form if either dynamic VLANs or port security are enabled on the participating EtherChannel interfaces.)

Benefits of using EtherChannel

  1. Increased Bandwidth: Use EtherChannel and combine two or four links into one logical link. It will double or quadruple your bandwidth. For example, four 100Mb Fast Ethernet connections bonded into one could provide you up to 800Mb/second, full duplex.
  2. Provides Redundancy: Since there are many Ethernet links combined into one logical channel, it automatically allows more available links in case one or more links go down.
  3. Load Balance Traffic: EtherChannel balances the traffic load across the links, thereby increasing efficiency on your networks.

EtherChannel vs. 802.3ad

EtherChannel and IEEE 802.3ad standards are very similar and accomplish the same goal. There are a few differences between the two, other than the fact that EtherChannel is Cisco proprietary and 802.3ad is an open standard, listed below:

EtherChannel IEEE 802.3ad
Requires switch configuration. Little, if any, configuration of switch required to form aggregation. Some initial setup of the switch may be required.
Supports different packet distribution modes. Supports only standard distribution mode.

Both technologies are capable of automatically configuring this logical link. EtherChannel supports both LACP and Cisco’s PAgP, whereas 802.3ad uses LACP.

EtherChannel Configuration

EtherChannel between Switch Topology
EtherChannel between Switch Topology

Configure EtherChannel using Port Aggregation Protocol (PAgP)

SW1>enable
SW1#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
SW1(config)#interface range gigabitEthernet 0/1 - 2
SW1(config-if-range)#channel-group 1 mode desirable
SW1(config-if-range)#channel-protocol pagp
SW1(config-if-range)#exit
SW1(config)#exit

SW2>enable
SW2#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
SW2(config)#interface range gigabitEthernet 0/1 - 2
SW2(config-if-range)#channel-group 1 mode desirable
SW2(config-if-range)#channel-protocol pagp
SW2(config-if-range)#exit
SW2(config)#exit

Configure EtherChannel using Link Aggregation Control Protocol (LACP)

SW1>enable
SW1#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
SW1(config)#interface range gigabitEthernet 0/1 - 2
SW1(config-if-range)#channel-group 1 mode active
SW1(config-if-range)#channel-protocol lacp
SW1(config-if-range)#exit
SW1(config)#exit

SW2>enable
SW2#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
SW2(config)#interface range gigabitEthernet 0/1 - 2
SW2(config-if-range)#channel-group 1 mode active
SW2(config-if-range)#channel-protocol lacp
SW2(config-if-range)#exit
SW2(config)#exit

Configure EtherChannel manually using “on”

SW1>enable
SW1#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
SW1(config)#interface range gigabitEthernet 0/1 - 2
SW1(config-if-range)#channel-group 1 mode on
SW1(config-if-range)#exit
SW1(config)#exit

SW2>enable
SW2#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
SW2(config)#interface range gigabitEthernet 0/1 - 2
SW2(config-if-range)#channel-group 1 mode on
SW2(config-if-range)#exit
SW2(config)#exit

EtherChannel Load-Balancing

EtherChannel load-balances traffic among port members of the same channel. Load balancing between member interface is based on:

  • Source MAC address
  • Destination MAC address
  • Source IP Address
  • Destination IP Address
  • Combinations of the four

(Note: Some old switch/router flatforms do not support all the load-balancing methods above.)

To configure the EtherChannel load balancing type globally on the switch

Switch(config)# port-channel load-balance method

Methods

  • src-ip source IP
  • dst-ip destination IP
  • src-dst-ip source and destination IP (XOR) **DEFAULT METHOD**
  • src-mac source MAC
  • dst-mac destination MAC
  • src-dst-mac source and destination MAC (XOR)
  • src-port source port
  • dst-port destination port
  • src-dst-port source and destination port (XOR)

Troubleshooting an EtherChannel

Remember that there should be consistent configurations on both ends of the bundle.
If using mode “on”, make sure both ends are set to it.

If one end is set to desirable (PAgP) or active (LACP), the other side must be set to either desirable or auto.

Auto (PAgP) passive (LACP) modes require the far end to request for participation.

PAgP auto and desirable modes default to silent submode – which will establish an EtherChannel without hearing from the far end. If set to non-silent submode, packets must be received from the far end before a channel will form.

To verify the EtherChannel Status

Switch# show etherchannel summary

To verify an individual port’s configuration

Switch# sh run interface xx/xx

To check for EtherChannel errors on an interface

Switch# sh run interface xx/xx etherchannel

To verify the EtherChannel load balancing on a switch

Switch# sh etherchannel load-balance

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