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Platform Redundancy FEB Redundancy Support for High Availability of ACX7509 Devices

FEB redundancy is supported on the ACX7509 device. The Routing Control Board (RCB) and Forwarding Engine Board (FEB) mastership is tied and switchover together. The master Routing Engine (RE) software manages both FEBs. Graceful RE switchover (GRES) for RCB and FEB simultaneous switchover is supported with support for replication and restoration of statistics.

The graceful Routing Engine switchover (GRES) feature in Junos OS and Junos OS Evolved enables a router with redundant Routing Engines to continue forwarding packets, even if one Routing Engine fails. GRES preserves interface and kernel information. Traffic is not interrupted. However, GRES does not preserve the control plane.

When GRES mode is not enabled on the Junos OS, or Junos OS Evolved, it is considered as Non-GRES mode.

Routing Engine Switchover Conditions and Prerequisites

The prerequisite for a GRES is that, the backup RCB and FEB are online for 360 seconds. The back to back switchover time is more than 360 seconds.

The switchover conditions are as follows:

  • Master RCB power-failure.

  • Master RE rebooted,

  • Master RE plugged out,

  • Master RE offlined.

  • Linux kernel crash on master RE.

  • Critical application failure on master RE (including PFE management, PP management & packet input output applications).

  • Power fault on master FEB.

When the FEB redundancy configuration is active, the supported RCB-FEB configurations for ACX7509 are as follows:
Table 1: Supported Redundancy Modes in ACX7509
Supported Redundancy Modes Condition
Both RCBs and FEBs present Fully redundant system.
RCB0/FEB0 present and RCB1/FEB1 not present Non-redundant system.
RCB1/FEB1 present and RCB0/FEB0 not present Non-redundant system.

The unsupported redundancy modes for ACX7509 with related alarms are as follows:

Table 2: Unsupported Redundancy Modes in ACX7509
Unsupported Redundancy Modes Condition

RCB0/FEB1 present

The unmatched FEB1 does not come online.

RCB1/FEB0 present

The unmatched FEB0 does not come online.

Two RCBs with FEB0

Both RCBs and the FEB come online.

RCB0/FEB0 becomes master.

CLI based switchover is allowed.

Fault-triggered automatic switchover does not happen.

Support for Replication and Restoration of Statistics on RE Switchover (ACX7509)

The RCB and FEB mastership is tied and switchover together. The master RE software manages both FEBs.

Traffic Flow and Switchover

The WAN traffic entering the system is forwarded to and flows through both FEBs. The FEBs route the traffic internally to the egress FPC. The FPC transmits traffic from master FEB to WAN and drops the traffic from the backup FEB. The backup FEB is in hot standby and ready to forward traffic.

Junos software supports the replication and restoration of the following statistics for traffic flow and switchover:

  • Interface Statistics.
  • Sub-interface Statistics (Without Queue Statistics).

  • The replication for other PFE statistics entities, such as:

    • Interface per Queue Statistics

    • Firewall Statistics

    • uRPF Statistics

    • DDoS PFE Statistics (excluding the RE level DDoS counters)

Limitations for GRES mode

  • RCB and FEB are a host-subsystem and switchover due to one will cause complete host sub-system switch. Hence, the independent switchover of RCB and FEB is not supported.
  • For GRES, if there is sudden master PFE loss, the interface could go down briefly at the end router. The link comes back up immediately when the switchover takes effect.
  • The show interfaces command displays the restoration of the RE based statistics.

  • Ungraceful removal of FEB is not supported.

Traffic Management

When the WAN traffic is affected due to various fault scenarios, the traffic hit duration depends on:

  • The time taken to detect the fault condition.

  • The time taken for RCB-FEB switchover.

The RE manages the traffic switchover.

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