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EtherChannel Tutorial

January 17th, 2014 Go to comments

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): Switch_single_link.jpg 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: Switch_STP_block.jpg 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_Switch.jpg 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: EtherChannel_router.jpg 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. (Reference: 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.

Comments (29) Comments
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  1. avi
    December 10th, 2015

    thank you very much for this helpful tutorial… highly recommended!!

  2. maddy
    January 4th, 2016

    Wonderful explanation..many thanks for the post!!

  3. ghost
    January 7th, 2016

    Since PAgP is Cisco proprietary, don’t you have to use the command – channel-protocol lacp- under the interface before the command -channel-group 1-?

  4. Anonymous
    January 13th, 2016

    Thanks for the info! Although i noticed that the channel-group numbers have to be the same on both devices for it to work properly.

  5. siveeeeeeee
    February 3rd, 2016

    why looping don t present in etherchennal

  6. CHinedu
    February 4th, 2016

    God Bless every body on the site… I appreciate the tutorials i have been studying so far…. This is a big family… Thanks 9tut… Thanks all

    . ..Still studying

  7. Ccna
    February 7th, 2016

    The etherchannel numbers don’t have to be the same

  8. kgisl
    February 8th, 2016

    quite useful…

  9. Arvind
    February 28th, 2016

    Thanks

  10. Anonymous
    March 2nd, 2016

    thank you for this site management but how many maximum etherchannels capable to configure in cisco switch?

  11. JITJASWANT
    March 7th, 2016

    Really its very very help full to get knowledge ….

  12. javos
    April 2nd, 2016

    Helpful thanks

  13. Anonymous
    April 20th, 2016

    i’m so glad to read it sir…Thank’s

  14. Anonymous
    April 25th, 2016

    very Nice!! Thanks :)

  15. diallo
    May 21st, 2016

    tthanks it’s helpful

  16. Anonymous
    May 22nd, 2016

    @9tut team, Your work is highly appreciated.

  17. Omar Baloch
    May 22nd, 2016

    @9tut team, Your work is highly appreciated.

  18. dev
    June 11th, 2016

    It’s very useful for freshers.

  19. Vinnu
    June 18th, 2016

    Superbly presented. Highly appreciate your efforts.
    Thanks a ton :-)

  20. Ahmad
    June 21st, 2016

    asked permission to copy your article

  21. Ahmad
    June 21st, 2016

    tanks

  22. Mr Sho
    July 5th, 2016

    His ghost I think once you type in the mode it determines which of the 2 is being configured. If you type in “active” for instance, you can only have = LACP and if “desirable” then PAgP. Does that make sense?

  23. Tiyan Malaki
    July 15th, 2016

    Hiii.. Can someone send me the lastest dumps please. here’s my email {email not allowed}. Thank you so much!

  24. Anonymous
    August 3rd, 2016

    Awesome information Thanks a lot

  25. Raghuveer
    August 22nd, 2016

    Thank you so much…for clear the issue

  26. Aaryan
    August 30th, 2016

    Nice

  27. Elliot48
    November 1st, 2016

    9tut..doing a great job! Thanks!

  28. ipachauri
    November 3rd, 2016

    good one

  29. abdirahman badri
    December 13th, 2016

    EtherChannel is best thanks allot 9tut

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