EtherChannel Tutorial
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.
There are 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.
(http://www.cisco.com/en/US/tech/tk389/tk213/technologies_tech_note09186a0080094714.shtml)
Next we will learn more about the three EtherChannel mechanisms above.
Port Aggregation Protocol (PAgP)
PAgP dynamically negotiates the formation of a channel. 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. 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:
+ 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.
In the next part we will learn how to configure EtherChannel on switch/router interfaces.
hello, is there any question about etherchannel? I’m taking my ccna exam this week and I didn’t find questions in this topic on 9tut!!!!!! HELP
@Asma: There is only 2 EtherChannel questions that were reported and you can read here: http://www.9tut.com/new-ccna-new-questions-2
Thanks a lot :) I didn’t notice them!
I’m taking my exam tomorrow, I wish myself success
Hi!! Where can I download questions for 200-120??
SwA(config-if)#switchport trunk encapsulation dot1q
This command seems not able to type in on 2960 or older switches. It can be put in on 3560 switch.
1st enable trunk and then try for switchport trunk encapsulation dot1q
switchport mode trunk
switchport trunk encapsulation dot1q
9tut is wat u need to go over but you must have understanding about how it all work . Labs were eigrp and acl . Theres alot of play on th wording so b carefull all the best
Hi 9tut, longtime no see \;0
About Link Aggregation Protocol (LACP), I search and found it is Link Aggression Control Protocol and I think it is matcher, would you please confirm it.
Thank you!
@Wind: It is Link Aggregation Control Protocol (LACP) in fact.
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gd content to understand
Simply the best to understand. :)
Hi,
I configured static etherchannel using two links f1/7 and f1/8 between two switches. My switch showing it doesnt have either active or auto modes. I dont know why.
I can reach one switch to other switch even if I shutdown f1/7, but when I shutdown f1/8 then they are not able to communicate. Could you please guide me?
Thank you
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Nice info.
Keep writing :)
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its very usefull.Thank you
hi,had a question about etherchannel ip addressing.where do you configure the ip address on an etherchannel?
Q. Network admin creates a layer 3 Etherchannel, bounding 4 interfaces into channel group 1. On what interface is the IP address configured?
A. the port-channel 1 interface
In above EtherChannel configuration. I think both side should be same group, but you named a side channel-group 1 and other side channel-group 2 ???
Am I right?
@Mason: You can use different channel-group on two sides. There is no problem with that config.
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All Q came fom 9tut and bain dumps, Watson.Except Following this Multiple Q asked based given on LAB scenario.1 fom Eigrp and Ospf…Practice all troubleshooting commands and concepts,.All the best
a pass the exam today.it has a new question about EtherChannel .
what does not need to mach on en EtherChannel? (something like this) and the options was: a)speed b)duplex c)Trunking Encapsulation d)DTP trunking —the good answer has d
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