lacp system-priority [value]
Sets the LACP system priority to determine which switch controls the EtherChannel bundle when negotiating with a peer.
Definition: lacp system-priority [value] is a Cisco IOS global config command. Sets the LACP system priority to determine which switch controls the EtherChannel bundle when negotiating with a peer.
Overview
The `lacp system-priority` command is a global configuration command on Cisco IOS devices that sets the LACP system priority, a crucial parameter in Link Aggregation Control Protocol (LACP) negotiations. LACP is an IEEE 802.3ad standard protocol that dynamically bundles multiple physical links into a single logical EtherChannel, providing increased bandwidth and redundancy. The system priority determines which switch in a LACP negotiation has the higher authority to control the EtherChannel bundle.
When two switches negotiate an EtherChannel, they exchange LACP frames containing their system priorities. The switch with the lower numerical priority value (higher priority) becomes the 'controlling' switch, meaning it decides which ports are added to the bundle. If both switches have the same priority, the MAC address is used as a tie-breaker.
This command is essential in scenarios where you need to ensure a specific switch (e.g., a core switch) controls the EtherChannel, especially when connecting to a switch that might have a lower default priority. The default LACP system priority on Cisco IOS is 32768. You would use this command when you want to override the default to make a switch more or less likely to control the bundle.
For example, in a hierarchical network, you might set a lower priority on the distribution switch to ensure it manages the EtherChannel with access switches. Alternatives include using static EtherChannel (no negotiation) or PAgP (Cisco proprietary), but LACP is preferred for multi-vendor environments. This command fits into the broader configuration workflow after creating the port-channel interface and before configuring member ports.
It affects the running configuration immediately and is saved to startup config with `copy running-config startup-config`. The command requires global configuration mode (privilege level 15). There is no buffered output; the command simply sets a value.
Understanding system priority is critical for predictable EtherChannel formation and troubleshooting LACP issues.
lacp system-priority [value]When to Use This Command
- Ensuring a specific switch becomes the LACP active partner in a multi-vendor environment
- Load balancing across multiple EtherChannels by adjusting priority per switch
- Troubleshooting LACP negotiation failures due to priority mismatch
- Standardizing priority across all switches in a data center for predictable behavior
Parameters
| Parameter | Syntax | Description |
|---|---|---|
| value | <1-65535> | Sets the LACP system priority. Lower values indicate higher priority. Valid range is 1 to 65535. The default is 32768. Common mistake: setting the value too high (low priority) on the switch that should control the bundle, causing the peer to take control. Ensure consistency across all LACP configurations on the switch. |
Command Examples
Setting LACP system priority to 100
lacp system-priority 100Switch(config)# lacp system-priority 100 Switch(config)# end Switch# show lacp sys-id 32768, 00:1a:2b:3c:4d:5e
The command sets the system priority to 100. The 'show lacp sys-id' output displays the system priority (100) and the system ID (MAC address). The priority is used in LACP negotiation to elect the active side.
Verifying LACP system priority after configuration
show lacp sys-id100, 00:1a:2b:3c:4d:5e
The output shows the system priority (100) followed by the system MAC address. A lower priority value means higher precedence in LACP negotiations.
Understanding the Output
The 'show lacp sys-id' command displays the LACP system priority and the system identifier (MAC address). The system priority is a 16-bit value from 1 to 65535; the default is 32768. A lower number indicates higher priority.
In LACP negotiation, the switch with the lower system priority becomes the active partner (determines which ports are bundled). If priorities are equal, the lower MAC address breaks the tie. When troubleshooting, ensure both ends have compatible priorities; mismatches can cause negotiation failures or unexpected active roles.
Configuration Scenarios
Ensure Core Switch Controls EtherChannel with Access Switch
In a campus network, the core switch (Core1) connects to an access switch (Access1) via two Gigabit Ethernet links. The goal is to have Core1 control the LACP EtherChannel to ensure consistent load balancing and port selection.
Topology
Core1(Gi0/0, Gi0/1)---10.0.12.0/30---(Gi0/0, Gi0/1)Access1Steps
- 1.Step 1: Enter global configuration mode on Core1: Core1# configure terminal
- 2.Step 2: Set LACP system priority to a low value (e.g., 100) to make Core1 the controlling switch: Core1(config)# lacp system-priority 100
- 3.Step 3: Create the port-channel interface: Core1(config)# interface port-channel 1
- 4.Step 4: Assign an IP address to the port-channel: Core1(config-if)# ip address 10.0.12.1 255.255.255.252
- 5.Step 5: Configure member interfaces: Core1(config)# interface range gigabitethernet0/0-1
- 6.Step 6: Set them as LACP active: Core1(config-if-range)# channel-group 1 mode active
- 7.Step 7: Exit and verify: Core1(config-if-range)# end
! Core1 configuration lacp system-priority 100 ! interface Port-channel1 ip address 10.0.12.1 255.255.255.252 ! interface GigabitEthernet0/0 channel-group 1 mode active ! interface GigabitEthernet0/1 channel-group 1 mode active
Verify: Show LACP neighbor: Core1# show lacp neighbor Expected output: Flags: S - Device is sending Slow LACPDUs, F - Device is sending Fast LACPDUs. Partner details show system priority 32768 (default) for Access1, and system priority 100 for Core1. The 'Actor' side shows Core1's priority.
Watch out: If Access1 also has a lower priority (e.g., 50), it will control the bundle. Ensure only one switch has a significantly lower priority to avoid conflicts.
Override Default Priority for Multi-Vendor Interoperability
A Cisco switch connects to a Juniper switch via two 10G interfaces. The Juniper switch has a default LACP system priority of 32768. To ensure the Cisco switch controls the EtherChannel, set its priority lower than the Juniper's.
Topology
Cisco(Gi0/0, Gi0/1)---10.0.13.0/30---(xe-0/0/0, xe-0/0/1)JuniperSteps
- 1.Step 1: Enter global configuration mode on Cisco: Cisco# configure terminal
- 2.Step 2: Set LACP system priority to 100: Cisco(config)# lacp system-priority 100
- 3.Step 3: Create port-channel interface: Cisco(config)# interface port-channel 10
- 4.Step 4: Configure IP: Cisco(config-if)# ip address 10.0.13.1 255.255.255.252
- 5.Step 5: Configure member interfaces: Cisco(config)# interface range gigabitethernet0/0-1
- 6.Step 6: Set LACP active: Cisco(config-if-range)# channel-group 10 mode active
- 7.Step 7: End and save: Cisco(config-if-range)# end
! Cisco configuration lacp system-priority 100 ! interface Port-channel10 ip address 10.0.13.1 255.255.255.252 ! interface GigabitEthernet0/0 channel-group 10 mode active ! interface GigabitEthernet0/1 channel-group 10 mode active
Verify: Show LACP sys-id: Cisco# show lacp sys-id Expected output: 100, xxxx.xxxx.xxxx (system priority and MAC). The priority should be 100. Also check show etherchannel summary to see the bundle is up.
Watch out: The Juniper switch might have a different LACP implementation; ensure both sides use the same LACP mode (active/passive). If the Juniper is set to passive, the Cisco must be active.
Troubleshooting with This Command
When troubleshooting LACP EtherChannel issues, the `lacp system-priority` command is rarely changed after initial configuration, but understanding its effect is vital. Healthy output from `show lacp neighbor` shows consistent system priorities on both sides. For example, if you set priority 100 on your switch, the 'Actor' field should show system priority 100.
The 'Partner' field shows the peer's priority. If the EtherChannel fails to form, check that both switches have compatible priorities. A common symptom is that only one link comes up in the bundle, or the bundle forms but with unexpected port selection.
This can happen if the peer has a lower priority (higher numerical value) and thus controls the bundle, potentially excluding some ports. To diagnose, use `show lacp neighbor` to compare priorities. If the peer's priority is lower (e.g., 50) and yours is higher (e.g., 32768), the peer controls the bundle.
You can then decide to lower your priority further or accept the peer's control. Another symptom is that the EtherChannel does not form at all. Check `show etherchannel summary` for the state.
If ports are in 'independent' state, LACP negotiation failed. Verify that both sides are using the same LACP mode (active/active or active/passive). Also check `show lacp counters` for LACPDU errors.
If you see 'Illegal' frames, there might be a mismatch in system priority or other parameters. Correlate with `debug lacp events` to see negotiation details. In a multi-vendor environment, system priority mismatches can cause instability.
For example, if one vendor uses a different default priority, the controlling switch may change after a reload. To avoid this, explicitly set the priority on both sides. Also, note that changing the system priority resets all LACP negotiations, causing a brief traffic interruption.
Therefore, change it during a maintenance window. Finally, remember that the system priority is a global setting; it affects all LACP EtherChannels on the switch. If you need different priorities per bundle, consider using port-channel load-balancing or other methods, but LACP does not support per-channel priority.
In summary, when troubleshooting, always verify the system priority on both ends and ensure it aligns with your design intent.
CCNA Exam Tips
CCNA exam tip: The default LACP system priority is 32768; changing it can influence which switch controls the bundle.
CCNA exam tip: LACP uses system priority and MAC address to elect the active side; lower priority wins.
CCNA exam tip: You must configure the same LACP system priority on all ports of a switch for consistency.
CCNA exam tip: In exam scenarios, you may need to set a lower priority on the desired active switch.
Common Mistakes
Mistake 1: Forgetting to apply the command in global configuration mode; it will be rejected in interface mode.
Mistake 2: Setting a priority value outside the valid range (1-65535); the switch will reject it.
Mistake 3: Assuming the priority affects PAgP; it only applies to LACP EtherChannels.
lacp system-priority [value] vs channel-group [id] mode [mode]
Both commands are essential for LACP-based EtherChannel configuration, but they operate at different levels: lacp system-priority globally influences which switch becomes the active partner in LACP negotiation, while channel-group mode assigns interfaces to a channel group and sets the LACP mode on that interface.
| Aspect | lacp system-priority [value] | channel-group [id] mode [mode] |
|---|---|---|
| Scope | Global (entire switch) | Interface-specific |
| Configuration Mode | Global configuration | Interface configuration |
| Effect on LACP | Determines system priority for LACP system ID | Sets LACP mode (active/passive) and binds interface to channel |
| Persistence | Persists across all channel groups on switch | Specific to interface and channel group |
| Precedence or Interaction | Lower value means higher priority; used only with LACP | Channel-group mode must be active or passive for LACP; on for PAgP |
| Typical Use | Ensures desired switch controls EtherChannel when both ends have active LACP | Adds interfaces to a PortChannel and sets LACP negotiation behavior |
Use lacp system-priority [value] when you need to control which switch becomes the LACP partner with higher priority to influence link aggregation decisions.
Use channel-group [id] mode [mode] when you need to assign an interface to an EtherChannel and specify the LACP mode (active or passive) to initiate or respond to negotiation.
Platform Notes
In IOS-XE (e.g., Catalyst 9000 series), the `lacp system-priority` command syntax is identical to classic IOS. However, the output of `show lacp sys-id` may include additional fields like 'System Priority' and 'System ID (MAC)'. In NX-OS (e.g., Nexus switches), the equivalent command is `lacp system-priority` in global configuration mode, but the default priority is also 32768.
NX-OS also supports `lacp port-priority` per interface. On ASA firewalls, LACP is supported on hardware models with Ethernet interfaces; the command is `lacp system-priority` in global configuration mode, but ASA uses a different default (32768 as well). In IOS-XR (e.g., ASR 9000), LACP configuration is done under the bundle interface using `lacp system-priority` in the global configuration, but the syntax may vary slightly; for example, `lacp system-priority` is valid, but the default is 32768.
Note that in IOS-XR, the command is applied globally and affects all bundles. There are no significant differences between IOS 12.x, 15.x, and 16.x for this command; it has remained consistent. However, in older IOS versions (12.x), the command may not be available on all platforms; check platform support.
Always verify with `show version` and `show lacp sys-id` to confirm the command is supported.
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