# Understanding Service Availability Features

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
- Stateful Line Module 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.

## Stateful Line Module Switchover

High availability of line modules increases the overall availability of the router by ensuring that all the subscribers who were connected during a line module recovery continue to remain logged in and can access network resources during the switchover from the primary line module to the secondary line module. Forwarding of data through the fabric slice for those subscribers continues with a brief disruption of two minutes. If you configured stateful line module switchover on a router, when a switchover occurs, a message is displayed on the active SRP module after the secondary line module successfully takes over the role of the previously configured primary line module. If the primary line module fails, the secondary line module takes the role of the primary line module. Mirrored configuration data and any mirrored volatile data are already resident in memory. The protocols and other applications re-initialize from the mirrored data and resynchronize communications with the line modules (non-volatile configuration and volatile state). Data forwarding operation continues to function normally with the secondary line module operating on behalf of the primary line module (with a small loss of packets when the fabric is switched from the formerly active line module to the newly active line module). When resynchronization is completed, the router resumes normal operations, including updates of any routing tables that result from changes that occurred during the warm restart.

## 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 router or access interface on the edge router fails. ICR accomplishes this by re-creating subscriber sessions on the backup router 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. ICR uses Virtual Router Redundancy Protocol (VRRP) to detect failures. ICR also enables you to track the failure of uplink interfaces. ICR currently supports only PPPoE subscribers.

#### Related Documentation

- Managing Module Redundancy
- Managing Stateful SRP Switchover
- Configuring a Unified In-Service Software Upgrade
- Configuring VRRP
- Managing Interchassis Redundancy
- Service Availability Overview