Alcatel-Lucent 7750 SR-OS, Configuration Manual

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Page 308 7750 SR OS Router Configuration Guide
Message Interval and Master Inheritance
Each virtual router is configured with a message interval per VRID within which it participates.
This parameter must be the same for every virtual router on the VRID.
For IPv4, the default advertisement interval is 1 second and can be configured between 100
milliseconds and 255 seconds 900 milliseconds. For IPv6, the default advertisement interval is 1
second and can be configured between 100 milliseconds and 40 seconds 950 milliseconds.
As specified in the RFC, the advertisement interval field in every received VRRP advertisement
message must match the locally configured advertisement interval. If a mismatch occurs,
depending on the inherit configuration, the current master's advertisement interval setting can be
used to operationally override the locally configured advertisement interval setting. If the current
master changes, the new master setting is used. If the local virtual router becomes master, the
locally configured advertisement interval is enforced.
If a VRRP advertisement message is received with an advertisement interval set to a value
different than the local value and the inherit parameter is disabled, the message is discarded
without processing.
The master virtual router on a VRID uses the advertisement interval to load the advertisement
timer, specifying when to send the next VRRP advertisement message. Each backup virtual router
on a VRID uses the advertisement interval (with the configured local priority) to derive the master
down timer value.
VRRP advertisements messages that are fragmented, contain IP options (IPv4), or contain
extension headers (IPv6) require a longer message interval to be configured.
Skew Time
The skew time is used to add a time period to the master down interval. This is not a configurable
parameter. It is derived from the current local priority of the virtual router’s VRID. To calculate the
skew time, the virtual router evaluates the following formula:
For IPv4:
Skew Time = ((256 - priority) / 256) seconds
For IPv6:
Skew Time = (((256 - priority) * Master_Adver_Interval) / 256) centiseconds
The higher priority value, the smaller the skew time will be. This means that virtual routers with a
lower priority will transition to master slower than virtual routers with higher priorities.