Configuring Unequal Cost Multiple Path
Context
ECMP evenly load balances traffic over multiple equal-cost links, regardless of the bandwidth. Consequently, traffic congestion may occur on low-speed links and bandwidth of high-speed links cannot be used efficiently. To solve this problem, you can configure Unequal Cost Multiple Path (UCMP) on an interface so that proportional traffic can be loaded over equal-cost links based on the bandwidth. This configuration can achieve proper load balancing.
You are advised to configure the bandwidth of each link participating in UCMP to an integral multiple of the minimum bandwidth. If the bandwidth of each link is not configured to an integral multiple of the minimum bandwidth, the system still uses an integral multiple of the minimum bandwidth to process. For example, if the bandwidths of four links participating in UCMP are configured to 2 kbps, 3 kbps, 4 kbps, and 5 kbps, the final bandwidths of the links are 2 kbps, 2 kbps, 4 kbps, and 4 kbps.
Procedure
- Run system-view
The system view is displayed.
- Run interface interface-type interface-number [.subinterface-number ]
The interface or sub-interface view is displayed.
Among all subinterfaces, only the Ethernet sub-interfaces (Dot1q and QinQ termination sub-interfaces) and Eth-trunk sub-interfaces (Dot1q termination sub-interfaces) support configuration of unequal cost multiple path.
- (Optional) Run load-balance bandwidth bandwidth
The bandwidth is manually configured for the interface.
For a logical interface, the interface bandwidth is not configured by default; for a physical interface, the actual interface bandwidth is used by default. Therefore, this step is mandatory for the logical interface.
Perform this step if you need to adjust the bandwidth of equal-cost links so that the equal-cost links load balance traffic based on the configured bandwidth.
- Run load-balance unequal-cost enable
UCMP is enabled on the interface.
By default, UCMP is disabled on an interface.
- Run shutdown
The interface is shut down.
- Run undo shutdown
The interface is started.
- Run quit
Return to the system view.
To configure UCMP on other interfaces, repeat steps 2 through 7.
Equal-cost links load balance proportional traffic based on the configured bandwidth only when UCMP is enabled on the outbound interfaces of all equal-cost links and the shutdown and undo shutdown commands are executed on the outbound interfaces in sequence to trigger FIB entry update. If UCMP is not enabled on any outbound interface, the equal-cost links evenly load balance traffic even though FIB entry update is triggered.
