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 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:
- VLANs
- Trunking Mode
- Native VLAN
- Speed
- 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
- Port Aggregation Protocol (PAgP)
- Link Aggregation Control Protocol (LACP)
- 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)
- PAgP dynamically negotiates the formation of a channel.
- Cisco proprietary
- Forms EtherChannel only if ports are configured for identical static VLANs or trunking
- Will automatically modify interface parameters on all ports of the bundle if the EtherChannel interface is changed
- 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)
- LACP also dynamically negotiates the formation of a channel.
- An open standard to PAgP
- IEEE 802.3ad
- Uses priority system for end switches
- Switch with the lowest system priority (2 byte value followed by MAC – lowest wins)
- Determines which ports are active in the EtherChannel at any given time
- Uses port priority to determine which ports to place in standby mode if hardware
- Limitations do not allow all ports to participate in the EtherChannel
- 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.
- Auto = Passive
- 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:
Specific settings that must be identical include:
- Speed settings
- Duplex settings
- STP settings
- VLAN membership (for access ports)
- Native VLAN (for trunk ports)
- Allowed VLANs (for trunk ports)
- 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
- 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.
- 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.
- 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
- 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
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