<s>
The	O
Virtual	B-Protocol
Router	I-Protocol
Redundancy	I-Protocol
Protocol	I-Protocol
(	O
VRRP	B-Protocol
)	O
is	O
a	O
computer	O
networking	O
protocol	O
that	O
provides	O
for	O
automatic	O
assignment	O
of	O
available	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	B-Protocol
)	O
routers	B-Protocol
to	O
participating	O
hosts	B-Device
.	O
</s>
<s>
This	O
increases	O
the	O
availability	O
and	O
reliability	O
of	O
routing	B-Protocol
paths	O
via	O
automatic	O
default	B-Protocol
gateway	I-Protocol
selections	O
on	O
an	O
IP	B-Protocol
subnetwork	B-Protocol
.	O
</s>
<s>
The	O
protocol	O
achieves	O
this	O
by	O
the	O
creation	O
of	O
virtual	O
routers	B-Protocol
,	O
which	O
are	O
an	O
abstract	O
representation	O
of	O
multiple	O
routers	B-Protocol
,	O
i.e.	O
</s>
<s>
primary/active	O
and	O
secondary/Standby	O
routers	B-Protocol
,	O
acting	O
as	O
a	O
group	O
.	O
</s>
<s>
The	O
virtual	O
router	B-Protocol
is	O
assigned	O
to	O
act	O
as	O
a	O
default	B-Protocol
gateway	I-Protocol
of	O
participating	O
hosts	B-Device
,	O
instead	O
of	O
a	O
physical	O
router	B-Protocol
.	O
</s>
<s>
If	O
the	O
physical	O
router	B-Protocol
that	O
is	O
routing	B-Protocol
packets	O
on	O
behalf	O
of	O
the	O
virtual	O
router	B-Protocol
fails	O
,	O
another	O
physical	O
router	B-Protocol
is	O
selected	O
to	O
automatically	O
replace	O
it	O
.	O
</s>
<s>
The	O
physical	O
router	B-Protocol
that	O
is	O
forwarding	O
packets	O
at	O
any	O
given	O
time	O
is	O
called	O
the	O
primary/active	O
router	B-Protocol
.	O
</s>
<s>
VRRP	B-Protocol
provides	O
information	O
on	O
the	O
state	O
of	O
a	O
router	B-Protocol
,	O
not	O
the	O
routes	O
processed	O
and	O
exchanged	O
by	O
that	O
router	B-Protocol
.	O
</s>
<s>
Each	O
VRRP	B-Protocol
instance	O
is	O
limited	O
,	O
in	O
scope	O
,	O
to	O
a	O
single	O
subnet	B-Protocol
.	O
</s>
<s>
It	O
does	O
not	O
advertise	O
IP	B-Protocol
routes	O
beyond	O
that	O
subnet	B-Protocol
or	O
affect	O
the	O
routing	B-Protocol
table	O
in	O
any	O
way	O
.	O
</s>
<s>
VRRP	B-Protocol
can	O
be	O
used	O
in	O
Ethernet	O
,	O
MPLS	B-Protocol
and	O
Token	B-Device
Ring	I-Device
networks	I-Device
with	O
Internet	B-Protocol
Protocol	I-Protocol
Version	I-Protocol
4	I-Protocol
(	O
IPv4	B-Protocol
)	O
,	O
as	O
well	O
as	O
IPv6	B-Protocol
.	O
</s>
<s>
A	O
virtual	O
router	B-Protocol
must	O
use	O
as	O
its	O
media	B-Protocol
access	I-Protocol
control	I-Protocol
(	O
MAC	O
)	O
address	O
.	O
</s>
<s>
The	O
last	O
byte	O
of	O
the	O
address	O
(	O
XX	O
)	O
is	O
the	O
virtual	O
router	B-Protocol
identifier	O
(	O
VRID	O
)	O
,	O
which	O
is	O
different	O
for	O
each	O
virtual	O
router	B-Protocol
in	O
the	O
network	O
.	O
</s>
<s>
This	O
address	O
is	O
used	O
by	O
only	O
one	O
physical	O
router	B-Protocol
at	O
a	O
time	O
,	O
and	O
it	O
will	O
reply	O
with	O
this	O
MAC	O
address	O
when	O
an	O
ARP	O
request	O
is	O
sent	O
for	O
the	O
virtual	O
router	B-Protocol
's	O
IP	B-Protocol
address	O
.	O
</s>
<s>
Physical	O
routers	B-Protocol
within	O
the	O
virtual	O
router	B-Protocol
must	O
communicate	O
within	O
themselves	O
using	O
packets	O
with	O
multicast	B-Protocol
IP	B-Protocol
address	O
and	O
IP	B-Protocol
protocol	I-Protocol
number	I-Protocol
112	O
.	O
</s>
<s>
Routers	B-Protocol
backing	O
up	O
a	O
virtual	O
router	B-Protocol
have	O
a	O
priority	O
between	O
1	O
and	O
254	O
,	O
and	O
the	O
router	B-Protocol
with	O
the	O
highest	O
priority	O
will	O
become	O
the	O
primary/active	O
.	O
</s>
<s>
A	O
failure	O
to	O
receive	O
a	O
multicast	B-Protocol
packet	O
from	O
the	O
primary/active	O
router	B-Protocol
for	O
a	O
period	O
longer	O
than	O
three	O
times	O
the	O
advertisement	O
timer	O
causes	O
the	O
secondary/standby	O
routers	B-Protocol
to	O
assume	O
that	O
the	O
primary/active	O
router	B-Protocol
is	O
dead	O
.	O
</s>
<s>
The	O
virtual	O
router	B-Protocol
then	O
transitions	O
into	O
an	O
unsteady	O
state	O
and	O
an	O
election	O
process	O
is	O
initiated	O
to	O
select	O
the	O
next	O
primary/active	O
router	B-Protocol
from	O
the	O
secondary/standby	O
routers	B-Protocol
.	O
</s>
<s>
This	O
is	O
fulfilled	O
through	O
the	O
use	O
of	O
multicast	B-Protocol
packets	O
.	O
</s>
<s>
Secondary/standby	O
router(s )	O
are	O
only	O
supposed	O
to	O
send	O
multicast	B-Protocol
packets	O
during	O
an	O
election	O
process	O
.	O
</s>
<s>
One	O
exception	O
to	O
this	O
rule	O
is	O
when	O
a	O
physical	O
router	B-Protocol
is	O
configured	O
with	O
a	O
higher	O
priority	O
than	O
the	O
current	O
primary/active	O
,	O
which	O
means	O
that	O
on	O
connection	O
to	O
the	O
network	O
it	O
will	O
pre-empt	O
the	O
primary/active	O
status	O
.	O
</s>
<s>
This	O
allows	O
a	O
system	O
administrator	O
to	O
force	O
a	O
physical	O
router	B-Protocol
to	O
the	O
primary/active	O
state	O
immediately	O
after	O
booting	B-Operating_System
,	O
for	O
example	O
when	O
that	O
particular	O
router	B-Protocol
is	O
more	O
powerful	O
than	O
others	O
within	O
the	O
virtual	O
router	B-Protocol
.	O
</s>
<s>
The	O
secondary/standby	O
router	B-Protocol
with	O
the	O
highest	O
priority	O
becomes	O
the	O
primary/active	O
router	B-Protocol
by	O
raising	O
its	O
priority	O
above	O
that	O
of	O
the	O
current	O
primary/active	O
.	O
</s>
<s>
It	O
will	O
then	O
take	O
responsibility	O
for	O
routing	B-Protocol
packets	O
sent	O
to	O
the	O
virtual	O
gateway	O
's	O
MAC	O
address	O
.	O
</s>
<s>
In	O
cases	O
where	O
secondary/standby	O
routers	B-Protocol
all	O
have	O
the	O
same	O
priority	O
,	O
the	O
secondary/standby	O
router	B-Protocol
with	O
the	O
highest	O
IP	B-Protocol
address	O
becomes	O
the	O
primary/active	O
router	B-Protocol
.	O
</s>
<s>
All	O
physical	O
routers	B-Protocol
acting	O
as	O
a	O
virtual	O
router	B-Protocol
must	O
be	O
in	O
the	O
same	O
local	B-General_Concept
area	I-General_Concept
network	I-General_Concept
(	O
LAN	B-General_Concept
)	O
segment	O
.	O
</s>
<s>
Communication	O
within	O
the	O
virtual	O
router	B-Protocol
takes	O
place	O
periodically	O
.	O
</s>
<s>
The	O
election	O
process	O
is	O
made	O
orderly	O
through	O
the	O
use	O
of	O
skew	O
time	O
,	O
derived	O
from	O
a	O
router	B-Protocol
's	O
priority	O
,	O
and	O
used	O
to	O
reduce	O
the	O
chance	O
of	O
the	O
thundering	B-Operating_System
herd	I-Operating_System
problem	I-Operating_System
occurring	O
during	O
the	O
election	O
.	O
</s>
<s>
Secondary/standby	O
router	B-Protocol
utilization	O
can	O
be	O
improved	O
by	O
load	O
sharing	O
.	O
</s>
<s>
VRRP	B-Protocol
is	O
described	O
in	O
Internet	O
Engineering	O
Task	O
Force	O
(	O
IETF	O
)	O
publication	O
RFC	O
5798	O
,	O
which	O
is	O
an	O
open	O
standard	O
,	O
but	O
Cisco	O
claims	O
that	O
Hot	B-Protocol
Standby	I-Protocol
Router	I-Protocol
Protocol	I-Protocol
,	O
a	O
similar	O
protocol	O
with	O
essentially	O
the	O
same	O
facility	O
,	O
is	O
patented	O
and	O
licensed	O
;	O
however	O
,	O
in	O
2001	O
,	O
in	O
reply	O
to	O
a	O
direct	O
request	O
,	O
Robert	O
Barr	O
of	O
Cisco	O
replied	O
that	O
they	O
will	O
not	O
assert	O
any	O
patent	O
claims	O
unless	O
someone	O
tried	O
to	O
assert	O
a	O
claim	O
against	O
Cisco	O
.	O
</s>
<s>
Mellanox	O
offers	O
MAGP	O
,	O
a	O
proprietary	O
protocol	O
based	O
on	O
VRRP	B-Protocol
that	O
allows	O
active-active	O
operation	O
.	O
</s>
