Virtual Interface Router; Vrrp Operation; Selecting The Master Vrrp Router - Lenovo Flex System Fabric CN4093 Application Manual

Virtual Interface Router

VRRP Operation

Selecting the Master VRRP Router

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At Layer 3, a Virtual Interface Router (VIR) allows two VRRP routers to share an IP
interface across the routers. VIRs provide a single Destination IPv4 (DIP) address
for upstream routers to reach various destination networks, and provide a virtual
default Gateway.
Note: Every VIR must be assigned to an IP interface, and every IP interface must
be assigned to a VLAN. If no port in a VLAN has link up, the IP interface of that
VLAN is down, and if the IP interface of a VIR is down, that VIR goes into INIT
state.
Only the virtual router master responds to ARP requests. Therefore, the upstream
routers only forward packets destined to the master. The master also responds to
ICMP ping requests. The backup does not forward any traffic, nor does it respond
to ARP requests.
If the master is not available, the backup becomes the master and takes over
responsibility for packet forwarding and responding to ARP requests.
Each VRRP router is configured with a priority between 1–254. A bidding process
determines which VRRP router is or becomes the master—the VRRP router with
the highest priority.
The master periodically sends advertisements to an IPv4 multicast address. As
long as the backups receive these advertisements, they remain in the backup state.
If a backup does not receive an advertisement for three advertisement intervals, it
initiates a bidding process to determine which VRRP router has the highest
priority and takes over as master. In addition to the three advertisement intervals, a
manually set holdoff time can further delay the backups from assuming the master
status.
If, at any time, a backup determines that it has higher priority than the current
master does, it can preempt the master and become the master itself, unless
configured not to do so. In preemption, the backup assumes the role of master and
begins to send its own advertisements. The current master sees that the backup has
higher priority and will stop functioning as the master.
A backup router can stop receiving advertisements for one of two reasons—the
master can be down, or all communications links between the master and the
backup can be down. If the master has failed, it is clearly desirable for the backup
(or one of the backups, if there is more than one) to become the master.
Note: If the master is healthy but communication between the master and the
backup has failed, there will then be two masters within the virtual router. To
prevent this from happening, configure redundant links to be used between the
switches that form a virtual router.
Chapter 34: Virtual Router Redundancy Protocol
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