Virtual Router Redundancy Protocol
Electing the Master Router
VRRP uses an election algorithm to dynamically assign responsibility for the master router to one of the
VRRP routers on the network. A VRRP router is elected master if the router has the highest priority (the
range is 1 to 254; 255 is a reserved number).
If the master router becomes unavailable, the election process provides dynamic failover and the backup
router that has the highest priority assumes the role of master.
A new master is elected when one of the following things happen:
VRRP is disabled on the master router.
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Loss of communication occurs between master and backup router(s).
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Another VRRP router is attached to the VLAN, and the new router has the same priority as the
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current master.
When VRRP is disabled on the master interface, the master router sends an advertisement with the
priority set to 0 to all backup routers. This signals the backup routers that they do not need to wait for
the master down interval to expire, and the master election process for a new master can begin
immediately.
The master down interval is set as follows:
3 * advertisement interval + skew time
Where:
The advertisement interval is a user-configurable option.
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The skew time is (256-priority/256).
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NOTE
An extremely busy CPU can create a short dual master situation. To avoid this, increase the advertisement interval.
Additional VRRP Highlights
The following additional points pertain to VRRP:
VRRP packets are encapsulated IP packets.
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The VRRP multicast address is 224.0.0.18.
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The virtual router MAC address is
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Duplicate VRIDs are allowed on the router but not on the same interface.
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The maximum number of supported VRIDs per interface is seven.
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An interconnect link between VRRP routers should not be used, except when VRRP routers have
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hosts directly attached.
A maximum of 64 VRID instances are supported on the router.
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Up to seven unique VRIDs can be configured on the router. VRIDs can be re-used, but not on the
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same interface.
ExtremeWare XOS 11.1 Concepts Guide
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