Extreme Networks ExtremeWare XOS 10.1, Manual

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168 ExtremeWare XOS 10.1 Concepts Guide
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 - 255).
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.
• Loss of communication occurs between master and backup router(s).
•
Another VRRP router is attached to the VLAN, and the new router has the same priority as the
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 in skew time
Where:
•
The advertisement interval is a user-configurable option.
• The skew time is (256-priority/256).
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.
• The VRRP multicast address is 224.0.0.18.
•
The virtual router MAC address is
00 00 5E 00 01 <vrid>
• Duplicate virtual router IDs are allowed on the router, but not on the same interface.
• The maximum number of supported VRIDs per interface is 7.
• An interconnect link between VRRP routers should not be used, except when VRRP routers have
hosts directly attached.
• A maximum of 64 VRID instances are supported on the router.
• Up to 7 unique VRIDs can be configured on the router. VRIDs can be re-used, but not on the same
interface.