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Understanding Graceful Routing Engine Switchover in the Junos OS

This topic contains the following sections:

Graceful Routing Engine Switchover Concepts

The graceful Routing Engine switchover (GRES) feature in Junos OS enables a routing platform with redundant Routing Engines to continue forwarding packets, even if one Routing Engine fails. GRES preserves interface and kernel information. Traffic is not interrupted. However, GRES does not preserve the control plane.

Note: On T Series routers, TX Matrix routers, and TX Matrix Plus routers, the control plane is preserved in case of GRES with nonstop active routing (NSR), and nearly 75 percent of line rate worth of traffic per Packet Forwarding Engine remains uninterrupted during GRES.

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:

Any updates to the master Routing Engine are replicated to the backup Routing Engine as soon as they occur.

Mastership switches to the backup Routing Engine if:

  • The master Routing Engine kernel stops operating.
  • The master 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 either graceful restart or nonstop active routing, respectively. For more information about graceful restart, see Graceful Restart Concepts. For more information about nonstop active routing, see Nonstop Active Routing Concepts.

If the backup Routing Engine does not receive a keepalive from the master Routing Engine after 2 seconds (4 seconds on M20 routers), it determines that the master Routing Engine has failed and: takes mastership.

The Packet Forwarding Engine:

  • Seamlessly disconnects from the old master Routing Engine
  • Reconnects to the new master Routing Engine
  • Does not reboot
  • Does not interrupt traffic

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: If adjacencies between the restarting router and the neighboring peer 'helper' routers time out, graceful restart protocol extensions are unable to notify the peer 'helper' routers about the impending restart. Graceful restart can then stop and cause interruptions in traffic.

To ensure that these adjacencies are kept, change the hold-time for IS-IS protocols from the default of 27 seconds to a value higher than 40 seconds.

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 , od 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: In a routing matrix with TX Matrix Plus router with 3D SIBs, for successive Routing Engine switchover, events must be a minimum of 900 seconds (15 minutes) apart after both Routing Engines have come up.

GRES must be performed on one line-card chassis (LCC) (of a TX Matrix router with 3D SIBs) at a time to avoid synchronization issues.

Note:

  • We do not recommend performing a commit operation on the backup Routing Engine when GRES is enabled on the router.
  • We do not recommend enabling GRES on the backup Routing Engine in any scenario.

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

Preparing for a Graceful Routing Engine Switchover

Note: Check GRES readiness by executing both:

  • The request chassis routing-engine master switch check command from the master Routing Engine
  • The show system switchover command from the Backup Routing Engine

The switchover preparation process for GRES is as follows:

  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 GRES has been enabled.
  7. The kernel synchronization process (ksyncd) synchronizes the backup Routing Engine with the master Routing Engine.
  8. After ksyncd completes the synchronization, all state information and the forwarding table are updated.

Figure 2 shows the effects of a switchover on the routing platform.

Figure 2: Graceful Routing Engine Switchover Process

Graceful
Routing Engine Switchover Process

When a switchover occurs, the switchover process is as follows:

  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 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.

Note: On T Series and M320 routers during GRES, the Switch Interface Boards (SIBs) are taken offline and restarted one by one. This is done to provide the Switch Processor Mezzanine Board (SPMB) that manages the SIB enough time to populate state information for its associated SIB. However, on a fully populated chassis where all FPCs are sending traffic at full line rate, there might be momentary packet loss during the switchover.

Note: When GRES is configured and the restart chassis-control command is executed on a TX Matrix Plus router with 3D SIBs, we cannot ascertain which Routing Engine becomes the master. This is because the chassisd process restarts with the execution of the restart chassis-control command. The chassisd process is responsible for maintaining and retaining mastership and when it is restarted, the new chassisd is processed based on the router load. As a result, any one of the Routing Engines is made the master.

Effects of a Routing Engine Switchover

Table 1 describes the effects of a Routing Engine switchover when different features are enabled:

  • No high availability features
  • Graceful Routing Engine switchover
  • Graceful restart
  • Nonstop active routing

Table 1: Effects of a Routing Engine Switchover

Feature

Benefits

Considerations

Dual Routing Engines only (no features enabled)

  • When the switchover to the new master Routing Engine is complete, routing convergence takes place and traffic is resumed.
  • All physical interfaces are taken offline.
  • Packet Forwarding Engines restart.
  • The standby Routing Engine restarts the routing protocol process (rpd).
  • All hardware and interfaces are discovered by the new master Routing Engine.
  • The switchover takes several minutes.
  • All of the router's adjacencies are aware of the physical (interface alarms) and routing (topology) changes.

GRES enabled

  • During the switchover, interface and kernel information is preserved.
  • The switchover is faster because the Packet Forwarding Engines are not restarted.
  • The new master Routing Engine restarts the routing protocol process (rpd).
  • All hardware and interfaces are acquired by a process that is similar to a warm restart.
  • All adjacencies are aware of the router's change in state.

GRES and nonstop active routing enabled

  • Traffic is not interrupted during the switchover.
  • Interface and kernel information are 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 and kernel information are 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.
  • If adjacencies between the restarting router and the neighboring peer 'helper' routers time out, graceful restart can stop and cause interruptions in traffic.

Published: 2014-07-25