Port Channel / Etherchannel

By default STP will block all but one of the links between two switches. See the the links between Cat1-Cat2 and Cat3-Cat4.

Using port-channeling between the pair of switches allows the combined links to be treated as one link from an STP perspective, so that all the parallel physical segments are used. The port channel can configured manually  or using a dynamic protocol which allows the switch to work out which ports are connected and should be part of the portchannel.

Those protocols are the Cisco-proprietary Port Aggregation Protocol (PAgP) and the IEEE 802.1AD Link Aggregation Control Protocol (LACP).

PagP – Port Aggregation Protocol

Normally you get a huge table (like the one below) which gives you the combinations that allow a port channel to be formed. This is quite hard to memorise so use this smaller table below which what each mode does (TX or RX or both)

The rules are then simple:

1) At least one end has to send a packet (ie one end has to be desirable)
2) The other has to listen (ie auto / desirable )
3) Anything else will not form a port channel using PagP. (On / On does not pass any PagP packets)

Mode Sends Packet Acts on Packet
Auto No Yes
Desirable Yes Yes
On No No
Off No No

So the only modes that exchange PagP packets are auto and desirable. The table below shows what settings will allow a PagP port channel to form.

Either side of the link settings PagP Port Channel formed?
Desirable / Desirable Yes
Desirable / Auto Yes
Desirable / On No
Desirable / Off No
Auto / Auto No
Auto / On No
Auto / Off No
On / On Yes (no PagP packets passed)
On / Off No
Off / Off No

802.1AD LACP – Link Aggregation Control Protocol

Similar to PagP you only need to remember this table:

Mode Sends Packet Acts on Packet
Active Yes Yes
Passive No Yes
On No No
Off No No

  • Aggregrates up to 8 ports with the rest in standby
  • Interoperates with other vendors as its IEEE
  • Supports active / back portchannels aggegrate links.

Either side of the link settings LACP Port Channel formed?
Active /Active Yes
Active / Passive Yes
Active / On No
Active / Off No
Passive / Passive No
Passive / On No
Passive / Off No
On / On Yes (no LACP packets passed)
On / Off No
Off / Off No
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