Cisco switch lacp rate fast config and limitation

lacp rate fast controls how often LACP PDUs (keepalive/control packets) are sent.

slow (default) = every 30 seconds

fast = every 1 second

Example config:

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interface GigabitEthernet1/0/1
 channel-group 1 mode active
 lacp rate fast
What lacp rate fast really does

how to check the current mode

show lacp internal

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Cisco-C3560#   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 12
                            LACP port     Admin     Oper    Port        Port
Port      Flags   State     Priority      Key       Key     Number      State
Gi1/2     SA      down      32768         0xC       0x0     0x133       0x45  

Channel group 14
                            LACP port     Admin     Oper    Port        Port
Port      Flags   State     Priority      Key       Key     Number      State
Gi0/14    SA      down      32768         0xE       0x0     0x10F       0x45  
H07-41U-DS-HA-ASW-C3560#

Notice

It does NOT make EtherChannel bandwidth faster.

It only improves:

failure detection time

member link timeout/convergence

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With fast mode:

timeout ≈ 3 seconds

With slow mode:

timeout ≈ 90 seconds

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(802.3ad uses 3 missed packets before timeout)

Important limitation

Many older Cisco Catalyst switches do not support:

lacp rate fast

especially:

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Cisco Catalyst 3560
Cisco Catalyst 3750

on older IOS releases.

Your example (3560 / 3750)
Older 3560/3750

These support:

LACP

EtherChannel

BUT:

only slow timer
no lacp rate fast command

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Typical CLI:

Switch(config-if)#lacp ?
  port-priority

No rate option appears.

Cisco community confirms:

classic 3560/3750 only support slow mode

Exception: 3750-X / 3560-X

Later models:

Cisco Catalyst 3750-X

Cisco Catalyst 3560-X

WITH newer IOS:

15.2(4)E and later

added support for:

lacp rate fast

min-links

additional LACP enhancements

Cisco release notes/community mention this explicitly.

Platform summary

Platform Fast LACP Support

3560 No

3750 No

3560-X Yes (newer IOS)

3750-X Yes (15.2(4)E+)

3850 Yes

4500 Yes

6500 Yes

Nexus series Yes

Operational notes

Link-down detection is already fast

If fiber/cable physically disconnects:

switch detects immediately via PHY/link state

LACP timer usually irrelevant

Fast LACP mainly helps:

unidirectional failures

upstream device software hangs

MC-LAG/vPC edge cases

firewall/server bonding failures

Fast mode increases control traffic

Usually negligible.

Example:

slow = 1 packet/30s

fast = 1 packet/s

Even with many links, overhead is tiny.

Mixed fast/slow peers

Allowed.

If one side:

lacp rate fast

and the other side only supports slow:

channel still forms

but behavior becomes asymmetric

one side detects failures faster

Verification commands

Check LACP state:

show lacp neighbor

show etherchannel summary

show lacp internal

On newer switches:

show run interface gi1/0/1

shows:

lacp rate fast