Parallel Redundancy Protocol (PRP)

PRP uses the end nodes to implement redundancy, instead of network elements, by connecting two network interfaces to two independent, disjointed, parallel networks (LAN-A and LAN-B). These end nodes are Dual Attached Node (DAN). PRP is a layer 2 protocol.

How PRP Works:

• The DAN sends two packets simultaneously through its two network interfaces to the destination node.
• A redundancy control trailer (RCT), which includes a sequence number, is added to each frame to help the destination node distinguish between duplicate packets.
• When the destination DAN receives the first packet successfully, it removes the RCT and consumes the packet.
• If the second packet arrives successfully, it is discarded.
• If a failure occurs in one of the paths, traffic continues to flow over the other path uninterrupted, and zero recovery time is required.

A simple network topology with DANs

Redundancy Control Trailer (RCT)

Ethernet Frame with an RCT insert:

Example of an RCT added to the Ethernet Frame in Packet Tracer:

Incomping frame:

Outgoing frame with an RCT:

RedBox (Redundancy Box)

When an end node that does not have two network ports to implement redundancy in PRP, such an end node can connect to a RedBox (Redundancy Box), which provides connectivity to the two different networks on behalf of the device. The end node is called a VDAN (Virtual DAN), which is behind a Redbox.

Example of how a RexBox duplicates an ARP picket to send out to its two connected network

PRP added an RCT to its orignal packet and replicates another packet:

PRP Command Configuration in Packet Tracer

On the IE9300

• PRP channel group 1 uses Gi1/0/21 (LAN-A) and Gi1/0/22 (LAN-B)
• PRP channel group 2 uses Gi1/0/23 (LAN-A) and Gi1/0/24 (LAN-B)

Switch(config)#interface prp-channel 1
Switch(config-if)#spanning-tree bpdufilter enable

Switch(config)#interface range gig 1/0/21-22
Switch(config-if-range)#prp-channel-group 1

!
interface PRP-channel 1
 spanning-tree bpdufilter enable
!
interface GigabitEthernet1/0/21
 prp-channel-group 1
!
interface GigabitEthernet1/0/22  prp-channel-group 1
!

On the IE3400:

• PRP channel group 1 uses Gig1/1 (LAN-A) and Gig1/ 2 (LAN-B) or Gig1/3 (LAN-A) and Gig1/4 (LAN-B)
• PRP channel group 2 uses Gig2/1 (LAN-A) and Gig2/2 (LAN-B)

Switch(config)#interface prp-channel 1
Switch(config-if)#spanning-tree bpdufilter enable
Switch(config)#interface range gig 1/3-4
Switch(config-if-range)#prp-channel-group 1

!
interface PRP-channel 1
 spanning-tree bpdufilter enable
!
interface GigabitEthernet1/3
 prp-channel-group 1
!
interface GigabitEthernet1/4
 prp-channel-group 1
!

!
interface PRP-channel 1
 spanning-tree bpdufilter enable
!
interface GigabitEthernet1/0/21
 switchport access vlan 20
 switchport mode access
 prp-channel-group 1
!
interface GigabitEthernet1/0/22
 switchport access vlan 20
 switchport mode access
 prp-channel-group 1
!

!
interface PRP-channel 1
 ip address 192.168.2.2 255.255.255.0
 spanning-tree bpdufilter enable
!
interface GigabitEthernet1/0/21
 prp-channel-group 1
interface GigabitEthernet1/0/22
 prp-channel-group 1
!