Srrp Messaging - Alcatel-Lucent 7750 SR Manual

VPRN Features

SRRP Messaging

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.
Due to 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 due the explicit association
between the SRRP instance group IP interface, the associated redundant IP interface and the multi-
chassis synchronization (MCS) peering. Since only two nodes are participating, the VRRP skew
timer is not utilized when waiting to enter the master state. Also, SRRP always preempts when the
local priority is better than the current master and the backup SRRP instance always inherits the
master's 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 master initiates its becoming-backup state, stops routing with the SRRP
gateway MAC and sends an SRRP advertisement message with a priority set to zero. The new
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 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 new master either receives the old master's priority zero SRRP
advertisement message or the become-master state timer expires, it enters the 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 master. If an advertisement is
received with the current masters source IP address and the local priority is higher priority than the
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 master state. Priority zero is a special case and is sent
when an SRRP instance is relinquishing the master state.
Page 1066 7750 SR OS Services Guide