EtherChannel Protocols- PAgP and LACP

EtherChannel is used to bundle physical links into a single logical link. We can bundle maximum of 8 physical links into one logical link. When physical links bundled into a single logical link, the STP only sees a single logical link and is not able to block anything. There are two types of the protocol used for EtherChannel, PAgP and LACP. EtherChannel protocols remove any loops within the physical links.

The Etherchannel protocols also maintain a record of each physical link. In case of any physical link failure or restoration, the protocols manage the deletion and addition of the link without informing the STP about the change.  Cisco switch uses the IEEE standard Link Aggregation Control Protocol (LACP)  and Cisco proprietary PAgP. Each EtherChannel is called a channel group. We can add a physical port into the group using the “channel-group group-number mode on” command in the interface configuration mode. We can also create and configure the EtherChannel without the use of PAgP or LACP. This type of EtherChannel is called a static or Unconditional EtherChannel.

PAgP

Port Aggregation Protocol is a Cisco-proprietary protocol that can only work on Cisco switches or on switches licensed by vendors to support Port Aggregation Protocol. The protocol helps the automatic creation of Etherchannel using the exchange of PAGP packets. Port Aggregation Protocol packets are exchanged between EtherChannel-capable ports to negotiate the establishment of a channel. Port Aggregation Protocol also checks for configuration stability and manages link additions and failures between two switches. It ensures that when an EtherChannel is created, all ports have the same type of configuration.

PAGP packets contain all the information of the neighbour switch. The receiving switch learns the neighbour switch identity capability of supporting PAGP and then dynamically groups similarly configured ports into a single logical link.  When PAgP is enabled, the PAgP packets are sent after every 30 seconds. The Port-Aggregation Protocol (PAgP) uses the layer 2 multicast address 01-00-0C-CC-CC-CC.

For establishing EtherChannel, all ports must have the same data speed, duplex setting, and VLAN information. Any modification on the port configuration can cause changes on all other ports of the channel. The figure shows the modes for Port Aggregation Protocol.

PAgP

  • On– Interfaces configured with this mode do not exchange PAgP packets. On mode force the interface to channel without PAgP or LACP. Port with “on” mode, will be created EtherChannel only when another interface group in EtherChannel “on” mode.
  • PAgP desirable– Interface with PAgP desirable mode remain in an active negotiating state in which the interface initiates negotiations with other interfaces by sending PAgP packets after every 30 seconds.
  • PAgP auto– Interface with PAgP auto mode places the interface in a passive negotiating state in which the interface reply to the PAgP packets received, but does not initiate any Port Aggregation Protocolnegotiation.

For establishing EtherChannel the modes compatibility on both side is important. For example, if one side is configured to be in auto mode, waiting for the other side to initiate the EtherChannel negotiation. If the other side is also set to auto, the PAgP packet will never exchange and the EtherChannel does not form. If all modes are disabled, or if no mode is configured, then the EtherChannel is disabled. The figure below illustrates the mode of the Port Aggregation Protocol for EtherChannel establishment.

The on mode manually set the interface in an EtherChannel, without any negotiation. If one side is set to on the other side must be set to for establishing EtherChannel. If the other side is set to negotiate parameters through Port Aggregation Protocol, the EtherChannel is not possible, because the side that is set to on mode does not negotiate. The Etherchannel configuration for the above topology is as under:

 

Note:- PAgP Modes are: On, Desirable, Auto

Switch1

Switch1(config)#interface range e0/0-3
Switch1(config-if-range)#channel-group 1 mode auto
Switch1(config-if-range)#interface port-channel 1
Switch1(config-if)#switchport mode trunk

Switch2

Switch2(config)#interface range e0/0-3
Switch2(config-if-range)#channel-group 1 mode desirable
Switch2(config-if-range)#interface port-channel 1
Switch2(config-if)#switchport mode trunk

The Port Aggregation Protocol does a configuration check on participating interfaces and confirms that the neighbouring interfaces are also using Port Aggregation Protocol. In the Port Aggregation Protocol interfaces that don’t have similar configurations will not participate, and we won’t get an accidental switching loop.

LACP

LACP is an open protocol, published by IEEE under the 802.3ad specification. The IEEE also release a new definition of the LACP in IEEE 802.1AX standard for local and metropolitan area networks. LACP, similarly allows several physical ports to be bundled to establish a single logical channel. It allows a switch to negotiate an automatic bundle using the LACP packets. The function of the LACP is similar to PAgP with Cisco EtherChannel. The difference is that LACP is an IEEE standard and PAgP is Cisco Propiaritry. The LACP is used to establish EtherChannels in multivendor environments. On Cisco devices, we can use both protocols. The LACP uses multicast address 01-80-c2-00-00-02.

LACP is the same in the functioning and proved the same negotiation benefits as PAgP. It helps establish the EtherChannel link by detecting the configuration of each side and check the compatibility. The figure shows the different modes for LACP.

LACP

 

  • On– similarly the on mode ensure the interface to channel without LACP. Interfaces with this mode do not exchange LACP packets.
  • LACP active– The active mode places a port in an active negotiating state. The port starts negotiations with other ports by sending LACP packets.
  • LACP passive– The passive mode places a port in a passive negotiating state. In the passive mode, the port responds to the LACP packets that it receives, however the passive port does not initiate LACP packet negotiation.

Similar to PAgP, modes must be compatible on both sides for establishing EtherChannel. The on mode is repeated because it creates the unconditional EtherChannel configuration without PAgP or LACP dynamic negotiation. The simple configuration of LACP for the above topology is as under:

Note:- LACP Modes are: On, Active, Passive

Switch1

Switch1(config)#interface range e0/0-1
Switch1(config-if-range)#channel-group 1 mode active
Switch1(config-if-range)#interface port-channel 1
Switch1(config-if)#switchport mode trunk

Switch2

Switch2(config)#interface range e0/0-1
Switch2(config-if-range)#channel-group 1 mode passive
Switch2(config-if-range)#interface port-channel 1
Switch2(config-if)#switchport mode trunk

The configuration of LACP is almost same to PAgP configuration. The difference is only the use of the keyword. The keywords used by LACP is active and passive. The active keyword shows that the interface uses LACP protocol. The passive keyword indicates the use of LACP, however, it can only respond to requests but cannot send any LACP packet.