Juniper JUNOS OS 10.3 - SOFTWARE Manual page 117

Redundant Routing Engines
Graceful Routing Engine Switchover
Copyright © 2010, Juniper Networks, Inc.
On EX Series switches, graceful protocol restart can be applied to aggregate and static
routes and for routing protocols (BGP, IS-IS, OSPF, and RIP).
Graceful protocol restart works similarly for the different routing protocols. The main
benefits of graceful protocol restart are uninterrupted packet forwarding and temporary
suppression of all routing protocol updates. Graceful protocol restart thus allows a switch
to pass through intermediate convergence states that are hidden from the rest of the
network. Most graceful restart implementations define two types of switches—the
restarting switch and the helper switch. The restarting switch requires rapid restoration
of forwarding state information so that it can resume the forwarding of network traffic.
The helper switch assists the restarting switch in this process. Individual graceful restart
configuration statements typically apply to either the restarting switch or the helper
switch.
Two to ten EX4200 switches can be interconnected to create a Virtual Chassis
configuration that operates as a single network entity. Every Virtual Chassis configuration
has a master and a backup. The master acts as the master Routing Engine and the backup
acts as the backup Routing Engine. The Routing Engine provides the following
functionality:
Runs various routing protocols
Provides the forwarding table to the Packet Forwarding Engines (PFEs) in all the
member switches of the Virtual Chassis configuration
Runs other management and control processes for the entire Virtual Chassis
configuration
The master Routing Engine, which is in the master of the Virtual Chassis configuration,
runs Juniper Networks Junos operating system (Junos OS) in the master role. It receives
and transmits routing information, builds and maintains routing tables, communicates
with interfaces and Packet Forwarding Engine components of the member switches,
and has full control over the Virtual Chassis configuration.
The backup Routing Engine, which is in the backup of the Virtual Chassis configuration,
runs Junos OS in the backup role. It stays in sync with the master Routing Engine in terms
of protocol states, forwarding tables, and so forth. If the master becomes unavailable,
the backup Routing Engine takes over the functions that the master Routing Engine
performs.
You can configure graceful Routing Engine switchover (GRES) in a Virtual Chassis
configuration, allowing the configuration to switch from the master Routing Engine in
the master to the backup Routing Engine in the backup with minimal interruption to
network communications. When you configure GRES, the backup Routing Engine
automatically synchronizes with the master Routing Engine to preserve kernel state
information and forwarding state. 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
Chapter 1: Software Overview
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