LACP timer and what it means

LACP (IEEE 802.3ad)is protocol used to bundle several physical interfaces to form single logical channel. It has a timer which defines how often devices inter-connected via this bundle exchange LACP PDUs or control messages. Currently, this timer can be set to either "rate fast" - 1 second, or "rate normal" - 30 seconds. What is not always clear is that when you configure "lacp rate " on Cisco or "set interfaces ae1 aggregated-ether-options lacp periodic fast" on Juniper, you do not configure how often this switch will send LACP PDUs. This command means that switch where this command is applied will expect to receive LACP PDUs with this frequency from the partner on the other side of logical channel.
Here is quick test. I have Nexus5500 connected to Cat6500. Let's configure port-channel between them with one physical member interface.

Cat6500#show run interface TenGigabitEthernet 1/5
!
interface TenGigabitEthernet1/5
 switchport
 switchport trunk encapsulation dot1q
 switchport mode trunk
 lacp rate fast
 channel-group 5 mode active

Nexus5500# show running-config interface Ethernet 1/1
!
interface Ethernet1/1
  switchport mode trunk
  channel-group 1 mode active

"lacp rate normal" is default setting on Nexus, so this command does not show up in the output, but we can confirm:

Nexus5500# show running-config interface Ethernet 1/1 all | include lacp
  lacp port-priority 32768
  lacp rate normal


Cat6500 is configured with rate fast and Nexus5500 - with rate normal. Let's see what's going on behind the scene.
On Catalyst:
Cat6500#show lacp internal
Flags:  S - Device is requesting Slow LACPDUs
           F - Device is requesting Fast LACPDUs
           A - Device is in Active mode       P - Device is in Passive mode    

Channel group 3
                            LACP port     Admin     Oper    Port        Port
Port      Flags   State     Priority      Key       Key     Number      State
Te1/5     FA      bndl      32768         0x3       0x3     0x106       0x3F

F flags says that Cat6500 requesting fast LACP PDUs from its partner.

On Nexus it's a little bit backwards, the "show" command tells you partner status, not its own.

Nexus5500# show lacp neighbor interface port-channel 1
Flags:  S - Device is sending Slow LACPDUs F - Device is sending Fast LACPDUs
          A - Device is in Active mode       P - Device is in Passive mode
port-channel1 neighbors
Partner's information
               Partner                           Partner                            Partner
Port        System ID                      Port Number     Age         Flags
Eth1/1     20,0-13-5f-20-63-80    0x106               910         SA

            LACP Partner           Partner                     Partner
            Port Priority              Oper Key                 Port State
            32768                      0x3                         0x3f

Nexus5500 says, that its partner - Cat6500 - is sending LACP PDUs every 30 seconds.