What is preserved by enabling graceful Routing Engine switchover (GRES)?

What is preserved by enabling graceful Routing Engine switchover (GRES)?

A.
control-plane state

B.
link-state protocol adjacencies

C.
interface and kernel information

D.
BGP peering relationships

Explanation:
Understanding Graceful Routing Engine Switchover in the JUNOS Software

Graceful Routing Engine Switchover Concepts

Graceful Routing Engine switchover (GRES) feature in the JUNOS software enables a routing platform with redundant Routing Engines to continue forwarding packets, even if one Routing Engine fails. Graceful Routing Engine switchover preserves interface and kernel information. Traffic is not interrupted. However, graceful Routing Engine switchover does not preserve the control plane. Neighboring routers detect that the router has experienced a restart and react to the event in a manner prescribed by individual routing protocol specifications. To preserve routing during a switchover, graceful Routing Engine switchover must be combined with either graceful restart protocol extensions or nonstop active routing. Any updates to the master Routing Engine are replicated to the backup Routing Engine as soon as they occur. If the kernel on the master Routing Engine stops operating, the master Routing Engine experiences a hardware failure, or the administrator initiates a manual switchover, mastership switches to the backup Routing Engine.

Note: To quickly restore or to preserve routing protocol state information during a switchover, graceful Routing Engine switchover must be combined with either graceful restart or nonstop active routing (NSR), respectively. For more information about graceful restart, see Graceful Restart Overview. For more information about nonstop active routing, see Nonstop Active Routing Overview.

If the backup Routing Engine does not receive a keepalive from the master Routing Engine after 2 seconds, it determines that the master Routing Engine has failed and takes mastership. The Packet Forwarding Engine seamlessly disconnects from the old master Routing Engine and reconnects to the new master Routing Engine. The Packet Forwarding Engine does not reboot, and traffic is not interrupted. The new master Routing Engine and the Packet Forwarding Engine then become synchronized. If the new master Routing Engine detects that the Packet Forwarding Engine state is not up to date, it resends state update messages.

Note: Successive Routing Engine switchover events must be a minimum of 240 seconds (4 minutes) apart after both Routing Engines have come up.
If the router displays a warning message similar to Standby Routing Engine is not ready for graceful switchover. Packet Forwarding Engines that are not ready for graceful switchover might be reset, do not attempt switchover. If you choose to proceed with switchover, only the Packet Forwarding Engines that were not ready for graceful switchover are reset. None of the FPCs should spontaneously restart. We recommend that you wait until the warning no longer appears and then proceed with the switchover.

Figure 1 shows the system architecture of graceful Routing Engine switchover and the process a routing platform follows to prepare for a switchover.

Figure 1: Preparing for a Graceful Routing Engine Switchover

The switchover preparation process for graceful Routing Engine switchover follows these steps:

1. The master Routing Engine starts.
2. The routing platform processes (such as the chassis process [chassisd]) start.
3. The Packet Forwarding Engine starts and connects to the master Routing Engine.
4. All state information is updated in the system.
5. The backup Routing Engine starts.
6. The system determines whether graceful Routing Engine switchover has been enabled.
7. The kernel synchronization process (ksyncd) synchronizes the backup Routing Engine with the master Routing Engine.
8. All state information is updated in the system.

Figure 2 shows the effects of a switchover on the routing platform.
Figure 2: Graceful Routing Engine Switchover Process

When a switchover occurs, the switchover process follows these steps:

1. When keepalives from the master Routing Engine are lost, the system switches over gracefully to the backup Routing Engine.
2. The Packet Forwarding Engine connects to the backup Routing Engine, which becomes the new master.
3. Routing platform processes that are not part of graceful Routing Engine switchover (such as the routing protocol process [rpd]) restart.
4. State information learned from the point of the switchover is updated in the system.
5. If configured, graceful restart protocol extensions collect and restore routing information from neighboring peer helper routers.

Effects of a Routing Engine Switchover

Table 4 describes the effects of a Routing Engine switchover when no high availability features are enabled and when graceful Routing Engine switchover, graceful restart, and nonstop active routing features are enabled.

Table 6: Effects of a Routing Engine Switchover