ENSGRPLACP is the IEEE (802.3ad) link aggregation standard. On The 3560 supports 8 active links with 8 standby links.
When using LACP, port channels are configured for either active or passive modes. Active will actively attempt to create a lacp link. Passive mode will respond to a LACP attempt from another device but attempt to create a LACP.
Rack1SW2(config-if)#channel-group 10 mode
When using LACP each switch has a system priority for the device and a port priority for each port. By default, these values are both 32768.
System priority is used to determine which switch makes the decision for adding links to the LACP bundle. Since both switches will by default have a system priority of 32768, the switch with the lowest mac address will make the decision.
Rack1SW1(config)#lacp system-priority <1-65535>
Port priority is used to determine what ports will be put in standby mode if the max active links limit is reached.Links with a LOWER priority are preferred. The port number is used as tiebreaker. This means that by default, interface gi0/1 will be prefered over gi0/2.
Rack1SW1(config-if)#lacp port-priority <0-65535>
For the following example I have two switches with 10 links between them. The 10 links are cabled as follows.
SW2——SW4
30——–40
31——–31
32——–32
33——–33
34——–34
35——–35
36——–36
37——–37
38——–38
39——–39
40——–30
We can see that with the default system priorities, SW4 will become the decision maker since it has a lower mac address.
Rack1SW4#sh lacp sys-id
32768, 0019.06b1.c180
Rack1SW2#sh lacp sys-id
32768, 001e.f634.e700
The 8 lowest priority links on SW4 will join the bundle with the others ( up to 8 ) being set to standby.
Rack1SW4#sh 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 10
LACP port Admin Oper Port Port
Port Flags State Priority Key Key Number State
Gi0/30 SA bndl 32768 0xA 0xA 0x1E 0x3D
Gi0/31 SA bndl 32768 0xA 0xA 0x1F 0x3D
Gi0/32 SA bndl 32768 0xA 0xA 0x20 0x3D
Gi0/33 SA bndl 32768 0xA 0xA 0x21 0x3D
Gi0/34 SA bndl 32768 0xA 0xA 0x22 0x3D
Gi0/35 SA bndl 32768 0xA 0xA 0x23 0x3D
Gi0/36 SA bndl 32768 0xA 0xA 0x24 0x3D
Gi0/37 SA bndl 32768 0xA 0xA 0x25 0x3D
Gi0/38 SA hot-sby 32768 0xA 0xA 0x26 0x5
Gi0/39 SA hot-sby 32768 0xA 0xA 0x27 0x5
Gi0/40 SA hot-sby 32768 0xA 0xA 0x28 0x5
If we look at SW2, we can see that gi0/30 is shut down because SW4 is the decision maker due to it’s lower priority and gi0/30 on SW2 is gi0/40 on SW4.
Rack1SW2#sh 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 10
LACP port Admin Oper Port Port
Port Flags State Priority Key Key Number State
Gi0/30 SA hot-sby 32768 0xA 0xA 0x1E 0x5
Gi0/31 SA bndl 32768 0xA 0xA 0x1F 0x3D
Gi0/32 SA bndl 32768 0xA 0xA 0x20 0x3D
Gi0/33 SA bndl 32768 0xA 0xA 0x21 0x3D
Gi0/34 SA bndl 32768 0xA 0xA 0x22 0x3D
Gi0/35 SA bndl 32768 0xA 0xA 0x23 0x3D
Gi0/36 SA bndl 32768 0xA 0xA 0x24 0x3D
Gi0/37 SA bndl 32768 0xA 0xA 0x25 0x3D
Gi0/38 SA hot-sby 32768 0xA 0xA 0x26 0x5
Gi0/39 SA hot-sby 32768 0xA 0xA 0x27 0x5
Gi0/40 SA bndl 32768 0xA 0xA 0x28 0x3D
Finally, if we make SW2 the decision maker by lowering it’s system priority, gi0/30 will become active and gi0/40 will go to standby on SW2.
Rack1SW2#sh 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 10
LACP port Admin Oper Port Port
Port Flags State Priority Key Key Number State
Gi0/30 SA bndl 32768 0xA 0xA 0x1E 0x3D
Gi0/31 SA bndl 32768 0xA 0xA 0x1F 0x3D
Gi0/32 SA bndl 32768 0xA 0xA 0x20 0x3D
Gi0/33 SA bndl 32768 0xA 0xA 0x21 0x3D
Gi0/34 SA bndl 32768 0xA 0xA 0x22 0x3D
Gi0/35 SA bndl 32768 0xA 0xA 0x23 0x3D
Gi0/36 SA bndl 32768 0xA 0xA 0x24 0x3D
Gi0/37 SA bndl 32768 0xA 0xA 0x25 0x3D
Gi0/38 SA hot-sby 32768 0xA 0xA 0x26 0x5
Gi0/39 SA - 32768 0xA 0xA 0x27 0x5
Gi0/40 SA - 32768 0xA 0xA 0x28 0x5
Note: Changing the lacp system priority dropped the entire bundle. Proceed with caution in production enviroments.
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