R0106-HP MSR Router Series High Availability Configuration Guide(V7)

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Strict active/standby mode
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, Serial 2/1/0 on Router A is the primary interface, and Serial 2/1/1 (with a
pr
iority of 30) and Serial 2/1/2 (with a priority of 20) are its backup interfaces.
• When Serial 2/1/0 is operating correctly, all traffic is transmitted through Serial 2/1/0.
• When Serial 2/1/0 fails, Serial 2/1/1 takes over because it has higher priority than Serial 2/1/2.
If Serial 2/1/1 also fails, Serial 2/1/2 takes over.
• When Serial 2/1/0 is recovered, it preempts the active backup interface because it is the primary
interface. If Serial 2/1/1 is recovered while Serial 2/1/0 is still down, Serial 2/1/1 cannot
preempt Serial 2/1/2 to forward traffic.
Figure 1 Strict active/backup mode
Load sharing mode
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.