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## Understanding Service Availability Features

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Service availability refers to ability of a network or a network component to provide uninterrupted delivery of services using highly available, redundant, and reliable components. This topic provides brief overviews of the benefits of using the following service availability features:

- Module Redundancy
- Stateful SRP Switchover
- Unified ISSU
- VRRP
- Interchassis Redundancy

### Module Redundancy

For hardware components, Juniper Networks provides redundancy solutions to ensure that the router continues to operate in the event of a hardware fault. Redundancy also enables you to hot-swap various components within your E Series router.

### Stateful SRP Switchover

Stateful SRP switchover (high availability) enables you to reduce or eliminate single points of failure in your network. Stateful SRP switchover provides both hardware-specific and software-specific methods to ensure minimal downtime and ultimately improve the performance of your network.

Stateful SRP switchover minimizes the impact to the router of a stateful switchover from the active SRP module to the standby SRP module. Stateful SRP switchover maintains user sessions and data forwarding through the router during the switchover, thus improving the overall availability of the router.

### Unified ISSU

A conventional software upgrade—one that does not use the unified in-service software upgrade (ISSU) process—causes a router-wide outage for all users. Only static configurations (stored on the flash card) are maintained across the upgrade; all dynamic configurations are lost. A conventional upgrade can take 30-40 minutes to complete, with additional time required to bring all users back online.

Unified ISSU enables you to upgrade the router to a higher-numbered software release without disconnecting user sessions or disrupting forwarding through the chassis.

When an application supports unified ISSU, you can configure the application on the router and proceed with the unified in-service software upgrade with no adverse effect on the upgrade.

When you perform a unified ISSU on a router that has one or more modules that do not support unified ISSU, these modules are upgraded by means of the legacy, conventional upgrade process. The unsupported modules undergo a cold reboot at the beginning of the unified ISSU process, and are held down until the ISSU process is completed.

### VRRP

Virtual Router Redundancy Protocol (VRRP) prevents loss of network connectivity to end hosts when the static default IP gateway fails. By implementing VRRP, you can designate a number of routers as backup routers in the event that the default master router fails. In case of a failure, VRRP dynamically shifts the packet-forwarding responsibility to a backup router. VRRP creates a redundancy scheme that enables hosts to keep a single IP address for the default gateway but maps the IP address to a well-known virtual MAC address. You can take advantage of the redundancy provided by VRRP without performing any special configuration on the end host systems.

Routers running VRRP dynamically elect master and backup routers. You can also force assignment of master and backup routers using priorities in the range 1– 255, with 255 being the highest priority.

VRRP supports virtual local area networks (VLANs), stacked VLANs (S-VLANs), and creation of interchassis redundancy (ICR) partitions.

### Interchassis Redundancy

ICR enables you to minimize subscriber downtime when the chassis or access interface on the edge router fails by re-creating subscriber sessions that were originally terminated on the failed router. It also enables you to track the failure of uplink interfaces. In this way, ICR enables you to completely recover from router failure. By using extended VRRP features, ICR enables you to track the failure of uplink interfaces, elect the master VRRP instance, and detect failure of a VRRP instance.

#### Related Topics

- Chapter 2, Managing Module Redundancy
- Chapter 3, Managing Stateful SRP Switchover
- Chapter 4, Configuring a Unified In-Service Software Upgrade
- Chapter 5, Configuring VRRP
- Chapter 6, Managing Interchassis Redundancy
- Service Availability Overview

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