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    High Availability Features for EX4300 Switches Overview

    High availability refers to the hardware and software components that provide redundancy and reliability for network communications. This topic covers the following high availability features of Juniper Networks EX4300 Ethernet Switches:

    Redundant Routing Engines

    Redundant Routing Engines are two Routing Engines that are installed in a switch or a Virtual Chassis. When a switch has two Routing Engines, one functions as the master, while the other stands by as a backup in case the master Routing Engine fails. When a Virtual Chassis has two Routing Engines, the switch in the master role functions as the master Routing Engine and the switch in the backup role functions as the backup Routing Engine. Redundant Routing Engines are supported on Juniper Networks EX4300 Ethernet Switches configuring into a Virtual Chassis.

    The master Routing Engine receives and transmits routing information, builds and maintains routing tables, communicates with interfaces and Packet Forwarding Engine components of the switch, and has full control over the control plane of the switch.

    The backup Routing Engine 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.

    Network reconvergence takes place more quickly on switches and on Virtual Chassis with redundant Routing Engines than on switches and on Virtual Chassis with a single Routing Engine.

    Virtual Chassis

    A Virtual Chassis is multiple switches connected together that operate as a single network entity. The advantages of connecting multiple switches into a Virtual Chassis include better-managed bandwidth at a network layer, simplified configuration and maintenance because multiple devices can be managed as a single device, a simplified Layer 2 network topology that minimizes or eliminates the need for loop prevention protocols such as Spanning Tree Protocol (STP), and improved fault tolerance and high availability. A Virtual Chassis improves high availability for the following reasons:

    • Dual Routing Engine support. A Virtual Chassis automatically has two Routing Engines—the switches in the master and backup routing-engine roles—and, therefore, provides more high availability options than standalone switches. Many high availability features are available for an EX Series Virtual Chassis that are not available on standalone EX Series switches.
    • Increased fault tolerance. You increase your fault tolerance options when you configure your EX Series switches into a Virtual Chassis. You can, for instance, configure interfaces into a link aggregation group (LAG) with member interfaces on different member switches in the same Virtual Chassis to ensure network traffic is received by a Virtual Chassis even when a switch or physical interface in the Virtual Chassis fails.

    You can configure up to ten EX4300 switches into an EX4300 Virtual Chassis. See Understanding EX4300 Virtual Chassis.


    You can configure Virtual Router Redundancy Protocol (VRRP) for IP and IPv6 on most switch interfaces, including Gigabit Ethernet interfaces, high-speed uplink interfaces, and logical interfaces. When VRRP is configured, the switches act as virtual routing platforms. VRRP enables hosts on a LAN to make use of redundant routing platforms on that LAN without requiring more than the static configuration of a single default route on the hosts. The VRRP routing platforms share the IP address corresponding to the default route configured on the hosts. At any time, one of the VRRP routing platforms is the master (active) and the others are backups. If the master routing platform fails, one of the backup routing platforms becomes the new master, providing a virtual default routing platform and enabling traffic on the LAN to be routed without relying on a single routing platform. Using VRRP, a backup switch can take over a failed default switch within a few seconds. This is done with minimum loss of VRRP traffic and without any interaction with the hosts.

    Graceful Routing Engine Switchover

    You can configure graceful Routing Engine switchover (GRES) on a switch with redundant Routing Engines or on a Virtual Chassis, allowing control to switch from the master Routing Engine to the backup Routing Engine 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 the administrator initiates a manual switchover, mastership switches to the backup Routing Engine.

    When the backup Routing Engine assumes mastership in a redundant failover configuration (that is, when GRES is not enabled), the Packet Forwarding Engines initialize their state to the boot state before they connect to the new master Routing Engine. In contrast, in a GRES configuration, the Packet Forwarding Engines do not reinitialize their state, but resynchronize their state to that of the new master Routing Engine. The interruption to traffic is minimal.

    Link Aggregation

    You can combine multiple physical Ethernet ports to form a logical point-to-point link, known as a link aggregation group (LAG) or bundle. A LAG provides more bandwidth than a single Ethernet link can provide. Additionally, link aggregation provides network redundancy by load-balancing traffic across all available links. If one of the links should fail, the system automatically load-balances traffic across all remaining links. In a Virtual Chassis, LAGs can be used to load-balance network traffic between member switches, which increases high availability by ensuring that network traffic is received by the Virtual Chassis even if a single interface fails for any reason.

    The number of Ethernet interfaces you can include in a LAG and the number of LAGs you can configure on a switch depend on the switch model. For information about LAGs, see Understanding Aggregated Ethernet Interfaces and LACP.

    Nonstop Active Routing and Nonstop Bridging

    Nonstop active routing (NSR) provides high availability in a switch or Virtual Chassis with redundant Routing Engines by enabling transparent switchover of the Routing Engines without requiring restart of supported Layer 3 routing protocols. Both Routing Engines are fully active in processing protocol sessions, and so each can take over for the other. The switchover is transparent to neighbor routing devices, which do not detect that a change has occurred. The neighboring devices and other devices on the network do not, therefore, have to resynchronize their Layer 3 protocol states to respond to the downtime on the switch—a process that adds network overhead and risks disrupting network performance—when a Routing Engine switchover occurs when NSR is enabled.

    Nonstop bridging (NSB) provides the same mechanism for Layer 2 protocols. NSB operates by synchronizing all protocol information for NSB-supported Layer 2 protocols between the master and backup Routing Engines. If the switch has a Routing Engine switchover, the NSB-supported Layer 2 protocol sessions remain active because they are already synchronized on the backup Routing Engine. The Routing Engine switchover is transparent to neighbor devices, which do not detect any changes related to the Layer 2 protocol sessions. The neighboring devices and other devices on the network do not, therefore, have to resynchronize their Layer 2 protocol states to respond to the downtime on the switch—a process that adds network overhead and risks disrupting network performance—when a Routing Engine switchover occurs when NSB is enabled.

    To use NSR or NSB, you must also configure GRES.

    Modified: 2013-09-17