Subscriber Routed Redundancy Protocol (SRRP) uses the same messaging format as VRRP with slight modifications. The source IP address is derived from the system IP address assigned to the local router. The destination IP address and IP protocol are the same as VRRP (224.0.0.18 and 112, respectively).
The message type field is set to 1 (advertisement) and the protocol version is set to 8 to differentiate SRRP message processing from VRRP message processing.
The vr-id field has been expanded to support an SRRP instance ID of 32 bits.
Because of the large number of subnets backed up by SRRP, only one message every minute carries the gateway IP addresses associated with the SRRP instance. These gateway addresses are stored by the local SRRP instance and are compared with the gateway addresses associated with the local subscriber IP interface.
Unlike VRRP, only two nodes may participate in an SRRP instance because of the explicit association between the SRRP instance group IP interface, the associated redundant IP interface and the multi-chassis synchronization (MCS) peering. Because only two nodes are participating, the VRRP skew timer is not used when waiting to enter the SRRP master state. Also, SRRP always preempts when the local priority is better than the current SRRP master instance and the backup SRRP instance always inherits the SRRP master’s instance advertisement interval from the SRRP advertisement messaging.
SRRP advertisement messages carry a becoming-master indicator flag. The becoming-master flag is set by a node that is attempting to usurp the master state from an existing SRRP master router. When receiving an SRRP advertisement message with a better priority and with the becoming-master flag set, the local SRRP master initiates the becoming-backup state, stops routing with the SRRP gateway MAC and sends an SRRP advertisement message with a priority set to zero. The new SRRP master continues to send SRRP advertisement messages with the becoming-master flag set until it either receives a return priority zero SRRP advertisement message from the previous SRRP master or its becoming-master state timer expires. The new backup node continues to send zero priority SRRP advertisement messages every time it receives an SRRP advertisement message with the becoming-master flag set. After the SRRP new master either receives the old SRRP master’s priority zero SRRP advertisement message or the become-master state timer expires, it enters the SRRP master state. The become-master state timer is set to 10 seconds upon entering the become-master state.
The SRRP advertisement message is always evaluated to see if it has higher priority than the SRRP advertisement that would be sent by the local node. If the advertised priority is equal to the current local priority, the source IP address of the received SRRP advertisement is used as a tie breaker. The node with the lowest IP address is considered to have the highest priority.
The SRRP instance maintains the source IP address of the current SRRP master. If an advertisement is received with the current SRRP master’s source IP address and the local priority is higher priority than the SRRP masters advertised priority, the local node immediately enters the becoming-master state unless the advertised priority is zero. If the advertised priority is zero, the local node bypasses the becoming-master state and immediately enters the SRRP master state. Priority zero is a special case and is sent when an SRRP instance is relinquishing the SRRP master state.