No relevant resource is found in the selected language.

This site uses cookies. By continuing to browse the site you are agreeing to our use of cookies. Read our privacy policy>Search


To have a better experience, please upgrade your IE browser.


CLI-based Configuration Guide - IP Unicast Routing

AR100, AR120, AR150, AR160, AR200, AR1200, AR2200, AR3200, and AR3600 V200R010

This document describes the concepts and configuration procedures of IP Service features on the device, and provides the configuration examples.
Rate and give feedback:
Huawei uses machine translation combined with human proofreading to translate this document to different languages in order to help you better understand the content of this document. Note: Even the most advanced machine translation cannot match the quality of professional translators. Huawei shall not bear any responsibility for translation accuracy and it is recommended that you refer to the English document (a link for which has been provided).


Routers generally operate with separation of the control plane and forwarding plane. When the network topology remains stable, a restart of the control plane does not affect the forwarding plane, and the forwarding plane can still forward data properly. This separation ensures non-stop service forwarding.

In graceful restart (GR) mode, the forwarding plane continues to direct data forwarding after a restart occurs. The actions on the control plane, such as re-establishment of neighbor relationships and route calculation, do not affect the forwarding plane. Network reliability is improved because service interruption caused by route flapping is prevented.

Basic Concepts of OSPF GR

Graceful Restart (GR) is a technology used to ensure normal traffic forwarding and non-stop forwarding of key services during the restart of routing protocols.

Unless otherwise stated, GR described in this section refers to the GR technology defined in RFC 3623.

GR is one of the high availability (HA) technologies, which comprise a set of comprehensive technologies, such as fault-tolerant redundancy, link protection, faulty node recovery, and traffic engineering. As a fault-tolerant redundancy technology, GR is widely used to ensure non-stop forwarding of key services during master/slave switchover and system upgrade.

The following concepts are involved in GR:

  • Grace-LSA

    OSPF supports GR by flooding Grace-LSAs. Grace-LSAs are used to inform the neighbor of the GR time, cause, and interface address when the GR starts and ends.

  • Role of a router during GR

    • Restarter: is the router that restarts. The Restarter can be configured to support totally GR or partly GR.

    • Helper: is the router that helps the Restarter. The Helper can be configured to support planned GR or unplanned GR or to selectively support GR through the configured policies.

    In practical application, in order to realize that business forwarding is not affected by motherboard failure, it is usually possible to configure BGP GR in the hardware environment of dual motherboard to make sense.

    All the models support the GR Helper, and only AR3200 series support the GR Restarter.

  • Conditions that cause GR

    • Unknown: indicates that GR is triggered for an unknown reason.

    • Software restart: indicates that GR is triggered by commands.

    • Software reload/upgrade: indicates that GR is triggered by software restart or upgrade.

    • Switch to redundant control processor: indicates that GR is triggered by the abnormal master/slave switchover.

  • GR period

    The GR period cannot exceed 1800 seconds. OSPF routers can exit from GR regardless of whether GR succeeds or fails, without waiting for GR to expire.

Classification of OSPF GR

  • Totally GR: indicates that when a neighbor of a router does not support GR, the router exits from GR.

  • Partly GR: indicates that when a neighbor does not support GR, only the interface associated with this neighbor exits from GR, whereas the other interfaces perform GR normally.

  • Planned GR: indicates that a router restarts or performs the master/slave switchover using a command. The Restarter sends a Grace-LSA before restart or master/slave switchover.

  • Unplanned GR: indicates that a router restarts or performs the master/slave switchover because of faults. A router performs the master/slave switchover, without sending a Grace-LSA, and then enters GR after the slave board goes Up. The process of unplanned GR is the same as that of planned GR.

GR Process

  • A router starts GR.

    In planned GR mode, after master/slave switchover is triggered through a command, the Restarter sends a Grace-LSA to all neighbors to notify them of the start, period, and cause of GR, and then performs the master/slave switchover.

    In unplanned GR, the Restarter does not send the Grace-LSA.

    In unplanned GR mode, the Restarter sends a Grace-LSA immediately after the slave board goes Up, informing neighbors of the start, period, and cause of GR. The Restarter then sends a Grace-LSA to each neighbor five times consecutively. This ensures that neighbors receive the Grace-LSA. This operation is proposed by manufacturers but not defined by the OSPF protocol.

    The Restarter sends a Grace-LSA to notify neighbors that it enters GR. During GR, neighbors keep neighbor relationships with the Restarter so that other routers cannot detect the switchover of the Restarter.

  • The GR process runs, as shown in Figure 5-19.

    Figure 5-19 OSPF GR process

  • The router exits from GR.

    Table 5-14 Reasons that a router exits GR

    Execution of GR



    GR succeeds.

    Before GR expires, the Restarter re-establishes neighbor relationships with all neighbors before master/slave switchover.

    After the Helper receives the Grace-LSA with the Age being 3600s from the Restarter, the neighbor relationship between the Helper and Restarter enters the Full state.

    GR fails.

    • GR expires, and neighbor relationships do not recover completely.

    • Router-LSA or Network-LSA sent by the Helper causes Restarter to fail to perform bidirectional check.

    • Status of the interface that functions as the Restarter changes.

    • Restarter receives the one-way Hello packet from the Helper.

    • The Restarter receives the Grace-LSA that is generated by another router on the same network segment. Only one router can perform GR on the same network segment.

    • On the same network segment, neighbors of the Restarter have different DRs or BDRs because of the topology changes.

    • Helper does not receive the Grace-LSA from Restarter before the neighbor relationship expires.

    • Status of the interface that functions as the Helper changes.

    • Helper receives the LSA that is inconsistent with the LSA in the local LSDB from another router. This situation can be excluded after the Helper is configured not to perform strict LSA check.

    • Helper receives Grace-LSAs from two routers on the same network segment at the same time.

    • Neighbor relationships between Helper and other neighbors change.

Comparison Between GR Mode and Non-GR Mode

Table 5-15 Comparison of master/slave switchover in the GR mode and non-GR mode

Switchover in Non-GR Mode

Switchover in GR Mode

  • OSPF neighbor relationships are re-established.

  • Routes are recalculated.

  • Forwarding table changes.

  • Entire network detects route changes, and route flapping occurs for a short period of time.

  • Packets are lost during forwarding, and services are interrupted.

  • OSPF neighbor relationships are re-established.

  • Routes are recalculated.

  • Forwarding table remains unchanged.

  • Except for neighbors of the device where master/slave switchover occurs, other routers do not detect route changes.

  • No packets are lost during forwarding, and services are not affected.

Updated: 2019-08-12

Document ID: EDOC1100034072

Views: 114418

Downloads: 168

Average rating:
This Document Applies to these Products
Related Documents
Related Version
Previous Next