Unanswered Question
Jan 12th, 2009

hi all, can anyone explain briefly how lacp works, am i right in saying lacp is a dynamic way of teaming links? what would be classed as fixed type ?

I have this problem too.
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Roberto Salazar Mon, 01/12/2009 - 14:53

LACP is the industry standard on port-channel protocol. Like PaGP is also support dynamic

creation of Etherchannels.

If you mean by fixed type is no negotiation, This would mean there is no etherchannel protocol but just forcing to "on". "on" mode would be no PAGP nor LACP since no negotiation protocol is sent out by an "on" channel.

So, PaGP = Cisco, LaCP = industry standard.

LACP supports the automatic creation of EtherChannels by exchanging LACP packets between LAN ports. LACP packets are exchanged only between ports in passive and active modes.

The protocol learns the capabilities of LAN port groups dynamically and informs the other LAN ports. Once LACP identifies correctly matched Ethernet links, it facilitates grouping the links into an EtherChannel. The EtherChannel is then added to the spanning tree as a single bridge port.

Both the passive and active modes allow LACP to negotiate between LAN ports to determine if they can form an EtherChannel, based on criteria such as port speed and trunking state. Layer 2 EtherChannels also use VLAN numbers.

carl_townshend Mon, 01/12/2009 - 15:05

can you please expain the active and passive modes?, does lacp send out advertisements to the neighbour switch ?

Roberto Salazar Mon, 01/12/2009 - 15:06

Passive - LACP mode that places a port into a passive negotiating state, in which the port responds to LACP packets it receives but does not initiate LACP negotiation. (Default)

Active - LACP mode that places a port into an active negotiating state, in which the port initiates negotiations with other ports by sending LACP packets.

Costin Vilcu Thu, 01/03/2013 - 07:51


how is the actual link in the LACP bundle, selected for passsing user traffic? by mac address? IP or session? if i monitor all the links participating in the etherchannel, will the traffic from one host to a server (let's say) go through the same "cable" or it will be splitted among all the physical links? what about returning traffic? I am planning to "sniff" the traffic on a LACP link, and i wanted to make sure i can capture all the traffic in the right order, from a layer 7 point of view.


Peter Paluch Thu, 01/03/2013 - 09:06

Hi Costin,

In an EtherChannel, a single frame is always carried over a single link in the bundle. The choice of a particular link in the bundle is not related to the LACP or PAgP. These protocols only negotiate the creation of an EtherChannel, but once the EtherChannel is established, the choice of a particular link is performed by the switch alone. This choice is commonly based on addressing information in the frame: source/destination MAC, IP, L4 ports or even higher-level protocol, or combination thereof. Usually, you can configure your switch to use a particular set of these addressing fields to choose a particular link. Each device on either end of an EtherChannel bundle can use a different load-balancing mechanism.

On a Catalyst 3560, you can choose from these fields:

SW-Dist1(config)#port-channel load-balance ?

  dst-ip       Dst IP Addr

  dst-mac      Dst Mac Addr

  src-dst-ip   Src XOR Dst IP Addr

  src-dst-mac  Src XOR Dst Mac Addr

  src-ip       Src IP Addr

  src-mac      Src Mac Addr

It is important to understand that a single flow (i.e. the same source/destination MACs, IPs, L4 protocol and ports) will be carried over a single link. Therefore, a single flow (e.g. a download from a server) will not experience any performance improvement. Only an aggregate flow of many flows will get distributed over different links in the EtherChannel bundle and achieve an increase in aggregated throughput.

Best regards,



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