Understand Graceful Routing Engine Switchover for Junos OS Evolved

Graceful Routing Engine Switchover Concepts

The graceful Routing Engine switchover (GRES) feature in Junos OS Evolved enables a router with redundant Routing Engines to continue forwarding packets, even if one Routing Engine fails. GRES preserves interface information. Traffic is not interrupted.

Note:

On PTX10004 and PTX10008 platforms running Junos OS Evolved, GRES is enabled by default and cannot be disabled.

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, GRES must be combined with either:

• Graceful restart protocol extensions

• Nonstop active routing (NSR)

Any updates to the primary Routing Engine during GRES are replicated to the backup Routing Engine as soon as they occur.

Note:

Because of its synchronization requirements and logic, NSR/GRES performance is limited by the slowest Routing Engine in the system.

The primary role switches to the backup Routing Engine if:

• The primary Routing Engine kernel stops operating.

• The primary Routing Engine experiences a hardware failure.

• The administrator initiates a manual switchover.

Note:

To quickly restore or to preserve routing protocol state information during a switchover, GRES must be combined with graceful restart or nonstop active routing, respectively. For more information about nonstop active routing, see Nonstop Active Routing Concepts.

If the backup Routing Engine does not receive a keepalive from the primary Routing Engine after 2 seconds, it determines that the primary Routing Engine has failed, and assumes the primary role.

The Packet Forwarding Engine:

• Seamlessly disconnects from the old primary Routing Engine

• Reconnects to the new primary Routing Engine

• Does not reboot

• Does not interrupt traffic

The new primary Routing Engine and the Packet Forwarding Engine then become synchronized. If the new primary Routing Engine detects that the Packet Forwarding Engine state is not up to date, it re-sends 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 or switch 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.

Note:

The hwdre application must be running for GRES to work properly.

Check GRES readiness by issuing both:

• The request chassis routing-engine master switch check command from the primary Routing Engine.

• The show system switchover command from the backup Routing Engine.

The switchover preparation process for GRES is as follows:

1. The primary Routing Engine starts.

2. The routing platform processes start.

3. The Packet Forwarding Engine starts and connects to the primary Routing Engine.

4. All state information is updated in the system.

5. The backup Routing Engine starts.

6. The system determines whether GRES has been enabled.

7. The backup Routing Engine is synchronized with the primary Routing Engine.

8. State information and the forwarding table are updated.

A switchover process comprises the following steps:

1. When keepalives from the primary 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 primary.

3. Routing platform processes that are not part of GRES (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 1 describes the effects of a Routing Engine switchover when different features are enabled:

• Graceful Routing Engine switchover only

• GRES plus nonstop active routing (NSR)

• GRES plus graceful restart

Table 1: Effects of a Routing Engine Switchover

Feature	Benefits	Considerations
GRES enabled	During the switchover, interface information is preserved. The switchover is faster because the Packet Forwarding Engines are not restarted.	The new primary Routing Engine restarts the routing protocol process (rpd). All adjacent systems are aware of the router's change in state.
GRES and NSR enabled	Traffic is not interrupted during the switchover. Interface information is preserved.	Unsupported protocols must be refreshed using the normal recovery mechanisms inherent in each protocol.
GRES and graceful restart enabled	Traffic is not interrupted during the switchover. Interface information is preserved. Graceful restart protocol extensions quickly collect and restore routing information from the neighboring routers.	Neighbors are required to support graceful restart, and a wait interval is required. The routing protocol process (rpd) restarts. For certain protocols, a significant change in the network can cause graceful restart to stop.