© Copyright Lenovo 2018
Chapter 32: Virtual Router Redundancy Protocol
447
Virtual Interface Router
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.
VRRP Operation
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.
Selecting the Master VRRP Router
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.
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