Cisco LACP Link Aggregation Control Protocol Configuration

Link Aggregation Control Protocol (LACP) mode has additional tuning configurations not available with Port Aggregation Protocol (PAgP). These advanced Cisco LACP configurations further optimize the links.

 

Cisco LACP Fast

In the original standards, LACP packets are sent out every 30 seconds. If an LACP packet is not received after three intervals, that’s a total of 90 seconds, then the link will be considered unusable and the interface will be removed as a member of a port channel.

The amended standards significantly reduced the 90 seconds waiting time to 3 seconds, an interval of 1 second. So if there’s no LACP packet received within 3 seconds, then the link will be removed as a member of a port channel. This feature is called LACP fast and it is enabled on the physical interface/s using the ‘lacp rate fast’ command. The command must be entered on both devices for LACP fast to work.

Switch1(config)# interface range Gig0/0-1
Switch1(config-if-range)# lacp rate fast
Switch2(config)# interface range Gig0/0-1
Switch2(config-if-range)# lacp rate fast

 

If you want to disable LACP fast and go back to using 30 seconds LACP packets interval, then you can use the ‘lacp rate slow’ command. To check the LACP state, the command ‘show lacp internal’ can be used.

Switch1#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 1
LACP port Admin Oper Port Port
Port  Flags State Priority Key Key Number State
Gi0/0  FA   bndl   32768   0x1 0x1  0x1   0x3F
Gi0/1  FA   bndl   32768   0x1 0x1  0x2   0x3F

 

Cisco LACP System Priority

LACP identifies a port channel master Cisco switch for the port channels. The switch with the lowest system priority will be preferred as the port channel master switch. However, the LACP priority value can be changed using the ‘lacp system-priority <number>’ command.

To check the initial priority value, we can use the ‘show lacp sys-id’ command.

Switch1#show lacp sys-id
32768, 5254.0003.8000

 

We’ll configure Switch1 to have a system priority of 1. Therefore, it will become the master switch.

SW1(config)# lacp system-priority 1

Switch1#show lacp sys-id
1, 5254.0003.8000

 

Cisco LACP Interface Priority

If there are more port channel interfaces than the configured maximum number of member interfaces, then the master switch will choose the interface with the lowest port priority. If the port priority values are the same, the lowest interface number will be preferred. We can also change the interface priority using the interface configuration ‘lacp port-priority <number>’ command.

For example, if the maximum number of port channel member interfaces is set to 1, then G0/0 will be the active interface because it has a lower interface number. Let’s set G0/1 on Switch1 as the most preferred interface by configuring it with a port priority of 1.

Switch1(config)# interface g0/1
Switch1(config-if)# lacp port-priority 1

 

If we do a ‘show etherchannel summary’ command, you can see that G0/1 interface is active and G0/0 is now in Hot-standby status.

Switch1#show etherchannel summary
Flags: D - down P - bundled in port-channel
I - stand-alone s - suspended
H - Hot-standby (LACP only)
R - Layer3 S - Layer2
U - in use N - not in use, no aggregation
f - failed to allocate aggregator
M - not in use, minimum links not met
m - not in use, port not aggregated due to minimum links not met
u - unsuitable for bundling
w - waiting to be aggregated
d - default port
A - formed by Auto LAG

Number of channel-groups in use: 1
Number of aggregators: 1
Group Port-channel Protocol Ports
------+-------------+-----------+-----------------------------------------------
1 Po1(SU) LACP Gi0/0(H) Gi0/1(P)

 

Port Channel Member Interfaces Maximum Number

The maximum number of port channel member interfaces sets the active member interface number of multiple physical Ethernet links to a specific number. It also ensures that the number proceeds with powers of two, 2, 4, 8, etc. In this way, a better load-balancing algorithm is expected. There’s no need to set the command on both Cisco switches, however, it is recommended.

The maximum number of member interfaces is configured using the ‘lacp max-bundle <number>’ command under the port-channel interface. In our example below, we’ll set the maximum physical interface to one

Switch1(config)# interface port-channel1
Switch1(config-if)# lacp max-bundle 1
11:52:24.823: %LINEPROTO-5-UPDOWN: Line protocol on Interface Gigabit Ethernet0/0, changed state to down
11:52:24.830: %LINEPROTO-5-UPDOWN: Line protocol on Interface Gigabit Ethernet0/1, changed state to down
11:52:24.974: %LINEPROTO-5-UPDOWN: Line protocol on Interface Port-channel1, changed state to down
11:52:25.811: %LINEPROTO-5-UPDOWN: Line protocol on Interface Gigabit Ethernet0/0, changed state to up
11:52:25.853: %LINEPROTO-5-UPDOWN: Line protocol on Interface Port-channel1, changed state to up

 

Port Channel Member Interfaces Minimum Number

The minimum number of LACP port channel member interfaces can also be set. If the minimum number is not met, the port channel will not work. The minimum number of port channel physical interfaces is set under the interface port channel using the ‘port-channel min-links <number>’ command. There’s no need to set the command on both Cisco switches, however, it is recommended.

Let’s set the minimum number of port channel physical interfaces on Switch1 to two for our example.

Switch1(config)# interface port-channel 1
Switch1(config-if)# port-channel min-links 2
Switch1(config-if)#exit

 

Since we have two links already up, interfaces Gig0/0 and 0/1, let’s try shutting down interface Gig0/0 and see what will happen next.

SW1(config)# interface g0/0
SW1(config-if)# shutdown
11:35:50.489: %ETC-5-MINLINKS_NOTMET: Port-channel Po1 is down bundled ports (1) doesn't meet min-links
11:35:50.519: %LINEPROTO-5-UPDOWN: Line protocol on Interface Gigabit Ethernet0/1, changed state to down
11:35:51.486: %LINEPROTO-5-UPDOWN: Line protocol on Interface Port-channel1, changed state to down
11:35:51.490: %LINK-5-CHANGED: Interface GigabitEthernet0/0, changed state to administratively down
11:35:51.508: %LINK-3-UPDOWN: Interface Port-channel1, changed state to down

 

You can see the logging messages indicating that the port channel went down because it didn’t meet the minimum number of links. Another way to check the port channel status is by using the ‘show etherchannel summary’ command. You can see that the port channel status is SM, meaning “not in use, minimum links not met.”

Switch1#show etherchannel summary
Flags: D - down P - bundled in port-channel
I - stand-alone s - suspended
H - Hot-standby (LACP only)
R - Layer3 S - Layer2
U - in use N - not in use, no aggregation
f - failed to allocate aggregator
M - not in use, minimum links not met
m - not in use, port not aggregated due to minimum links not met
u - unsuitable for bundling
w - waiting to be aggregated
d - default port
A - formed by Auto LAG

Number of channel-groups in use: 1
Number of aggregators: 1
Group Port-channel Protocol Ports
------+-------------+-----------+-----------------------------------------------

1 Po1(SM) LACP Gi0/0(D) Gi0/1(P)

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