There are three different methods to create an EtherChannel. One of them is Cisco’s own proprietary protocol, PAgP, Port Aggregation Protocol. It is similar to LACP which is the industry standard. A PAgP link is usually used between Cisco switches.
PAgP will aggregate physical port links into one logical link to provide redundancy and increased bandwidth. Within a PAgP link there can be up to eight active ports. PDUs are sent on the lowest numbered VLAN on the trunk link. Each port joining the PAgP link must have the same configuration.
There are two modes used in PAgP:
Auto mode will passively negotiate PAgP. If the remote end of the link is also configured with Auto mode, a PAgP EtherChannel will not form.
Desirable mode will actively negotiate PAgP. The remote end must be configured in Auto or Desirable mode for a PAgP EtherChannel to form.
To configure PAgP, identify the physical ports which will join to the same logical bundle.
SW1#config t SW1(config)#interface range f0/1 - 2 SW1(config-if)#channel-group 5 mode desirable SW1(config-if)#end
SW2#config t SW2(config)#interface range f0/1 - 2 SW2(config-if)#channel-group 5 mode desirable SW2(config-if)#end
Let’s break down the command that creates the PAgP bundle.
channel-group 5 is how we identify that we want to create an EtherChannel. 5 is the identifying value we want to assign to this EtherChannel. Depending on your switch/IOS version you will have a specific range of numbers available to use.
To find out which EtherChannel numbers already in use issue the command
show etherchannel summary.
mode desirable is how we configure these interfaces to actively negotiate PAgP. If we wanted the interfaces to passively negotiate a PAgP link then we would configure it with mode auto.
The remote end of the link is configured similarly with the difference being that the channel-group numbers do not have to match.
After configuring the interfaces to be part of the same channel-group, a port-channel will automatically be configured. This is the logical interface of the bundle. The port-channel number will be the same as your channel-group number. In the example above, port-channel 5 was automatically created.
You can view this port-channel interface with the following commands:
- show interface status
- show interfaces po5