Etherchannel: PAgP and LACP

Port Aggregation Protocol (PAgP) and Link Aggregation Control Protocol (LACP) are aggregate link control protocols that dynamically negotiate a port channel with a neighbor switches.

PAgP allows a max of eight links in a port channel, there are no configurable parameters for PAgP except for the frequency at which PAgP messages are transmitted.  The frequency is configurable on a per port basis and defaults to 30 seconds but can be configured down to 1 second if needed.

For PAgP to dynamically form a port channel you would use the channel-group command and choose what mode it will operate in, either auto or desirable.

LACP allows for a maximum of 16 links to be placed into a port channel and is the IEEE standard used for multi vendor port channels, however only 8 links at most will ever be active in  the bundle, the rest are put into a standby state.  switches will choose the port with the lowest port id to become active should an active port fail with multiple standby ports available.  Priority on these ports can also be manually configured.

Here are the two modes for PAgP / LACP respectively and their actions:

  • Auto/passive – considered passive and waits for an initiating connection from the neighbor.
  • Desirable/Active – auto initiates port channel negotiation

When PAgP or LACP begins to negotiate a port channel messages include the exchange of specific information that allows detecting whether all links being bundled are on a single port channel and are connected to the same neighbor and if the neighbor is willing to negotiate a bundle as well.

The values included in  these messages include:

  • System IDs
  • identifiers of physical ports
  • aggregation groups

PAgP and LACP do not check to see if the ports are configured identically…they only check to ensure consistent connections are in place on both neighboring switches.

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