8.11.1 Overview

Graceful restart is a function to reduce communication stop time by removing routes from the network when the master switch is switched during stack configuration or when the unicast routing program is restarted by an operation command.

<Structure of this section>

(1) Problem when graceful restart is not used

The Switch does not interrupt packet forwarding even when the master switch is switched while link aggregation is set for each member switch in the stack configuration or when the unicast routing program is restarted by an operation command. This is because the route of the previous routing program is held pending and the Switch continues to operate even if the routing program changes.

However, if the routing protocol is used, the neighboring router does not forward packets to the Switch, and communication may temporarily stop on the entire network. This is due to the following reasons:

When a newly started routing program initiates communication with a neighboring router, the neighboring router receives a new connection request. As a result, the neighboring router recognizes that the previous connection was disconnected and deletes the route through the switch.
The Switch does not advertise some routes. This is because, when the routing program that started the new operation starts route advertisement, route information learning has not been completed yet. On neighboring routers, routes that were not advertised by the Switch are deleted.

(2) Scope of support

The following table describes the support ranges for graceful restart.

Table 8-12: Graceful restart support ranges
Item		Availability of support
Target event	Restart all devices	NG
	Switching the master switch	OK^#1
	Unicast routing program restart	OK^#1
Target interface	VLAN interface	OK^{#2 #3}
Target interface	Management port	NG
Target routing protocol	OSPF	OK
	BGP	OK
	OSPFv3	OK
	BGP4+	OK

Legend: OK: Supported, NG: Not supported

#1

If the event occurs again during a graceful restart, the execution of the graceful restart is canceled. Alternatively, a graceful restart fails.

#2

Except for VLAN that use spanning tree.

#3

Unless LACP is used under the following conditions in link aggregation:

.LACPDU sending interval of the Switch or partner switch is used in short.