Backup modes

The primary interface and its backup interfaces can operate in strict active/standby mode or load sharing mode.

• Strict active/standby mode—Only one interface transmits traffic. All the other interfaces are in STANDBY state.

• Load sharing mode—Backup interfaces participate in traffic forwarding when the amount of traffic on the primary interface reaches the upper threshold. They are activated and deactivated depending on the amount of traffic.

In strict active/standby mode, traffic loss occurs when the active interface is overloaded. Load sharing mode improves link efficiency and reduces the risk of packet loss.

In strict active/standby mode, the primary interface always has higher priority than all backup interfaces.

• When the primary interface is operating correctly, all traffic is transmitted through the primary interface.

• When the primary interface fails, the highest-priority backup interface takes over. If the highest-priority backup interface also fails, the second highest-priority backup interface takes over, and so forth.

	NOTE: If two backup interfaces have the same priority, the one configured first has preference.

An active backup interface is always preempted by the primary interface. However, a higher-priority backup interface cannot preempt a lower-priority backup interface that has taken over the primary interface.

• The primary interface takes over when it recovers from a failure condition.

• The higher-priority backup interface cannot take over when it recovers from a failure condition while the primary interface is still down.

As shown in Figure 1, GigabitEthernet 1/0/1 on Router A is the primary interface. GigabitEthernet 1/0/2 (with a priority of 30) and GigabitEthernet 1/0/3 (with a priority of 20) are its backup interfaces.

• When GigabitEthernet 1/0/1 is operating correctly, all traffic is transmitted through GigabitEthernet 1/0/1.

• When GigabitEthernet 1/0/1 fails, GigabitEthernet 1/0/2 takes over because it has higher priority than GigabitEthernet 1/0/3. If GigabitEthernet 1/0/2 also fails, GigabitEthernet 1/0/3 takes over.

• When GigabitEthernet 1/0/1 is recovered, it preempts the active backup interface because it is the primary interface. If GigabitEthernet 1/0/2 is recovered while GigabitEthernet 1/0/1 is still down, GigabitEthernet 1/0/2 cannot preempt GigabitEthernet 1/0/3 to forward traffic.

In load sharing mode, the backup interfaces are activated to transmit traffic depending on the traffic load on the primary interface.

• When the amount of traffic on the primary interface exceeds the upper threshold, the backup interfaces are activated in descending order of priority. This action continues until the traffic drops below the upper threshold.

• When the total amount of traffic on all load-shared interfaces decreases below the lower threshold, the backup interfaces are deactivated in ascending order of priority. This action continues until the total amount of traffic exceeds the lower threshold.

• When the primary interface fails (in DOWN state), the strict active/standby mode applies. Only one backup interface can forward traffic.

The upper and lower thresholds are user configurable.

	NOTE: "Traffic" on an interface refers to the amount of incoming or outgoing traffic, whichever is higher. If two backup interfaces have the same priority, the one configured first has preference.

As shown in Figure 2, GigabitEthernet 1/0/1 on Router A is the primary interface. GigabitEthernet 1/0/2 (with a priority of 30) and GigabitEthernet 1/0/3 (with a priority of 20) are its backup interfaces.

• When the amount of traffic on GigabitEthernet 1/0/1 exceeds the upper threshold, GigabitEthernet 1/0/2 is activated, because it has higher priority than GigabitEthernet 1/0/3. If the amount of traffic on GigabitEthernet 1/0/1 still exceeds the upper threshold, GigabitEthernet 1/0/3 is activated.

• When the total amount of traffic on all load-shared interfaces decreases below the lower threshold, GigabitEthernet 1/0/3 is first deactivated, because its priority is lower than GigabitEthernet 1/0/2. If the total amount of traffic on GigabitEthernet 1/0/1 and GigabitEthernet 1/0/2 is still below the lower threshold, GigabitEthernet 1/0/2 is deactivated.