<s>
Border	B-Protocol
Gateway	I-Protocol
Protocol	I-Protocol
(	O
BGP	B-Protocol
)	O
is	O
a	O
standardized	O
exterior	B-Protocol
gateway	I-Protocol
protocol	I-Protocol
designed	O
to	O
exchange	O
routing	B-Protocol
and	O
reachability	O
information	O
among	O
autonomous	B-Architecture
systems	I-Architecture
(	O
AS	O
)	O
on	O
the	O
Internet	O
.	O
</s>
<s>
BGP	B-Protocol
is	O
classified	O
as	O
a	O
path-vector	B-Protocol
routing	I-Protocol
protocol	I-Protocol
,	O
and	O
it	O
makes	O
routing	B-Protocol
decisions	O
based	O
on	O
paths	O
,	O
network	O
policies	O
,	O
or	O
rule-sets	O
configured	O
by	O
a	O
network	O
administrator	O
.	O
</s>
<s>
BGP	B-Protocol
used	O
for	O
routing	B-Protocol
within	O
an	O
autonomous	B-Architecture
system	I-Architecture
is	O
called	O
Interior	B-Protocol
Border	I-Protocol
Gateway	I-Protocol
Protocol	I-Protocol
,	O
Internal	B-Protocol
BGP	I-Protocol
(	O
iBGP	B-Protocol
)	O
.	O
</s>
<s>
In	O
contrast	O
,	O
the	O
Internet	O
application	O
of	O
the	O
protocol	O
is	O
called	O
Exterior	B-Protocol
Border	I-Protocol
Gateway	I-Protocol
Protocol	I-Protocol
,	O
External	B-Protocol
BGP	I-Protocol
(	O
eBGP	B-Protocol
)	O
.	O
</s>
<s>
The	O
Border	B-Protocol
Gateway	I-Protocol
Protocol	I-Protocol
was	O
sketched	O
out	O
in	O
1989	O
by	O
engineers	O
on	O
the	O
back	O
of	O
"	O
three	O
ketchup-stained	O
napkins	O
"	O
,	O
and	O
is	O
still	O
known	O
as	O
the	O
three-napkin	B-Protocol
protocol	I-Protocol
.	O
</s>
<s>
IPv6	B-Protocol
BGP	B-Protocol
was	O
first	O
defined	O
in	O
in	O
1994	O
,	O
and	O
it	O
was	O
improved	O
to	O
in	O
1998	O
.	O
</s>
<s>
The	O
current	O
version	O
of	O
BGP	B-Protocol
is	O
version	O
4	O
(	O
BGP4	B-Protocol
)	O
,	O
which	O
was	O
published	O
as	O
in	O
2006	O
.	O
</s>
<s>
The	O
major	O
enhancement	O
was	O
the	O
support	O
for	O
Classless	B-Protocol
Inter-Domain	I-Protocol
Routing	I-Protocol
(	O
CIDR	B-Protocol
)	O
and	O
use	O
of	O
route	B-Protocol
aggregation	I-Protocol
to	O
decrease	O
the	O
size	O
of	O
routing	B-Protocol
tables	I-Protocol
.	O
</s>
<s>
The	O
new	O
RFC	O
allows	O
BGP4	B-Protocol
to	O
carry	O
a	O
wide	O
range	O
of	O
IPv4	B-Protocol
and	O
IPv6	B-Protocol
"	O
address	O
families	O
"	O
.	O
</s>
<s>
It	O
is	O
also	O
called	O
the	O
Multiprotocol	B-Protocol
Extensions	I-Protocol
which	O
is	O
Multiprotocol	B-Protocol
BGP	I-Protocol
(	O
MP-BGP	B-Protocol
)	O
.	O
</s>
<s>
BGP	B-Protocol
neighbors	O
,	O
called	O
peers	O
,	O
are	O
established	O
by	O
manual	O
configuration	O
among	O
routers	B-Protocol
to	O
create	O
a	O
TCP	B-Protocol
session	O
on	O
port	B-Protocol
179	O
.	O
</s>
<s>
A	O
BGP	B-Protocol
speaker	O
sends	O
19-byte	O
keep-alive	O
messages	O
every	O
30	O
seconds	O
(	O
protocol	O
default	O
value	O
,	O
tunable	O
)	O
to	O
maintain	O
the	O
connection	O
.	O
</s>
<s>
Among	O
routing	B-Protocol
protocols	O
,	O
BGP	B-Protocol
is	O
unique	O
in	O
using	O
TCP	B-Protocol
as	O
its	O
transport	O
protocol	O
.	O
</s>
<s>
When	O
BGP	B-Protocol
runs	O
between	O
two	O
peers	O
in	O
the	O
same	O
autonomous	B-Architecture
system	I-Architecture
(	O
AS	O
)	O
,	O
it	O
is	O
referred	O
to	O
as	O
Internal	B-Protocol
BGP	I-Protocol
(	O
iBGP	B-Protocol
or	O
Interior	B-Protocol
Border	I-Protocol
Gateway	I-Protocol
Protocol	I-Protocol
)	O
.	O
</s>
<s>
When	O
it	O
runs	O
between	O
different	O
autonomous	B-Architecture
systems	I-Architecture
,	O
it	O
is	O
called	O
External	B-Protocol
BGP	I-Protocol
(	O
eBGP	B-Protocol
or	O
Exterior	B-Protocol
Border	I-Protocol
Gateway	I-Protocol
Protocol	I-Protocol
)	O
.	O
</s>
<s>
Routers	B-Protocol
on	O
the	O
boundary	O
of	O
one	O
AS	O
exchanging	O
information	O
with	O
another	O
AS	O
are	O
called	O
border	O
or	O
edge	B-Protocol
routers	I-Protocol
or	O
simply	O
eBGP	B-Protocol
peers	O
and	O
are	O
typically	O
connected	O
directly	O
,	O
while	O
iBGP	B-Protocol
peers	O
can	O
be	O
interconnected	O
through	O
other	O
intermediate	O
routers	B-Protocol
.	O
</s>
<s>
Other	O
deployment	O
topologies	B-Architecture
are	O
also	O
possible	O
,	O
such	O
as	O
running	O
eBGP	B-Protocol
peering	O
inside	O
a	O
VPN	B-Application
tunnel	O
,	O
allowing	O
two	O
remote	O
sites	O
to	O
exchange	O
routing	B-Protocol
information	I-Protocol
in	O
a	O
secure	O
and	O
isolated	O
manner	O
.	O
</s>
<s>
The	O
main	O
difference	O
between	O
iBGP	B-Protocol
and	O
eBGP	B-Protocol
peering	O
is	O
in	O
the	O
way	O
routes	O
that	O
were	O
received	O
from	O
one	O
peer	O
are	O
typically	O
propagated	O
by	O
default	O
to	O
other	O
peers	O
:	O
</s>
<s>
New	O
routes	O
learned	O
from	O
an	O
eBGP	B-Protocol
peer	O
are	O
re-advertised	O
to	O
all	O
iBGP	B-Protocol
and	O
eBGP	B-Protocol
peers	O
.	O
</s>
<s>
New	O
routes	O
learned	O
from	O
an	O
iBGP	B-Protocol
peer	O
are	O
re-advertised	O
to	O
all	O
eBGP	B-Protocol
peers	O
only	O
.	O
</s>
<s>
These	O
route-propagation	O
rules	O
effectively	O
require	O
that	O
all	O
iBGP	B-Protocol
peers	O
inside	O
an	O
AS	O
are	O
interconnected	O
in	O
a	O
full	O
mesh	O
with	O
iBGP	B-Protocol
sessions	O
.	O
</s>
<s>
The	O
action	O
could	O
be	O
to	O
drop	O
the	O
route	O
,	O
or	O
it	O
could	O
be	O
to	O
modify	O
some	O
attributes	O
of	O
the	O
route	O
before	O
inserting	O
it	O
in	O
the	O
routing	B-Protocol
table	I-Protocol
.	O
</s>
<s>
During	O
the	O
peering	O
handshake	O
,	O
when	O
OPEN	O
messages	O
are	O
exchanged	O
,	O
BGP	B-Protocol
speakers	O
can	O
negotiate	O
optional	O
capabilities	O
of	O
the	O
session	O
,	O
including	O
multiprotocol	B-Protocol
extensions	I-Protocol
and	O
various	O
recovery	O
modes	O
.	O
</s>
<s>
If	O
the	O
multiprotocol	B-Protocol
extensions	I-Protocol
to	O
BGP	B-Protocol
are	O
negotiated	O
at	O
the	O
time	O
of	O
creation	O
,	O
the	O
BGP	B-Protocol
speaker	O
can	O
prefix	O
the	O
Network	O
Layer	O
Reachability	O
Information	O
(	O
NLRI	O
)	O
it	O
advertises	O
with	O
an	O
address	O
family	O
prefix	O
.	O
</s>
<s>
These	O
families	O
include	O
the	O
IPv4	B-Protocol
(	O
default	O
)	O
,	O
IPv6	B-Protocol
,	O
IPv4/IPv6	O
Virtual	B-Application
Private	I-Application
Networks	I-Application
and	O
multicast	B-Protocol
BGP	I-Protocol
.	O
</s>
<s>
Increasingly	O
,	O
BGP	B-Protocol
is	O
used	O
as	O
a	O
generalized	O
signaling	O
protocol	O
to	O
carry	O
information	O
about	O
routes	O
that	O
may	O
not	O
be	O
part	O
of	O
the	O
global	O
Internet	O
,	O
such	O
as	O
VPNs	B-Application
.	O
</s>
<s>
In	O
order	O
to	O
make	O
decisions	O
in	O
its	O
operations	O
with	O
peers	O
,	O
a	O
BGP	B-Protocol
peer	O
uses	O
a	O
simple	O
finite	B-Architecture
state	I-Architecture
machine	I-Architecture
(	O
FSM	O
)	O
that	O
consists	O
of	O
six	O
states	O
:	O
Idle	O
;	O
Connect	O
;	O
Active	O
;	O
OpenSent	O
;	O
OpenConfirm	O
;	O
and	O
Established	O
.	O
</s>
<s>
For	O
each	O
peer-to-peer	O
session	O
,	O
a	O
BGP	B-Protocol
implementation	O
maintains	O
a	O
state	O
variable	O
that	O
tracks	O
which	O
of	O
these	O
six	O
states	O
the	O
session	O
is	O
in	O
.	O
</s>
<s>
The	O
BGP	B-Protocol
defines	O
the	O
messages	O
that	O
each	O
peer	O
should	O
exchange	O
in	O
order	O
to	O
change	O
the	O
session	O
from	O
one	O
state	O
to	O
another	O
.	O
</s>
<s>
In	O
the	O
Idle	O
state	O
,	O
BGP	B-Protocol
initializes	O
all	O
resources	O
,	O
refuses	O
all	O
inbound	O
BGP	B-Protocol
connection	O
attempts	O
and	O
initiates	O
a	O
TCP	B-Protocol
connection	O
to	O
the	O
peer	O
.	O
</s>
<s>
In	O
the	O
Connect	O
state	O
,	O
the	O
router	B-Protocol
waits	O
for	O
the	O
TCP	B-Protocol
connection	O
to	O
complete	O
and	O
transitions	O
to	O
the	O
OpenSent	O
state	O
if	O
successful	O
.	O
</s>
<s>
In	O
the	O
Active	O
state	O
,	O
the	O
router	B-Protocol
resets	O
the	O
ConnectRetry	O
timer	O
to	O
zero	O
and	O
returns	O
to	O
the	O
Connect	O
state	O
.	O
</s>
<s>
In	O
the	O
OpenSent	O
state	O
,	O
the	O
router	B-Protocol
sends	O
an	O
Open	O
message	O
and	O
waits	O
for	O
one	O
in	O
return	O
in	O
order	O
to	O
transition	O
to	O
the	O
OpenConfirm	O
state	O
.	O
</s>
<s>
Keepalive	O
messages	O
are	O
exchanged	O
and	O
,	O
upon	O
successful	O
receipt	O
,	O
the	O
router	B-Protocol
is	O
placed	O
into	O
the	O
Established	O
state	O
.	O
</s>
<s>
In	O
the	O
Established	O
state	O
,	O
the	O
router	B-Protocol
can	O
send	O
and	O
receive	O
:	O
Keepalive	O
;	O
Update	O
;	O
and	O
Notification	O
messages	O
to	O
and	O
from	O
its	O
peer	O
.	O
</s>
<s>
Refuse	O
all	O
incoming	O
BGP	B-Protocol
connections	O
.	O
</s>
<s>
Initiates	O
a	O
TCP	B-Protocol
connection	O
with	O
its	O
configured	O
BGP	B-Protocol
peer	O
.	O
</s>
<s>
Listens	O
for	O
a	O
TCP	B-Protocol
connection	O
from	O
its	O
peer	O
.	O
</s>
<s>
If	O
an	O
error	O
occurs	O
at	O
any	O
state	O
of	O
the	O
FSM	O
process	O
,	O
the	O
BGP	B-Protocol
session	I-Protocol
is	O
terminated	O
immediately	O
and	O
returned	O
to	O
the	O
Idle	O
state	O
.	O
</s>
<s>
Some	O
of	O
the	O
reasons	O
why	O
a	O
router	B-Protocol
does	O
not	O
progress	O
from	O
the	O
Idle	O
state	O
are	O
:	O
</s>
<s>
TCP	B-Protocol
port	I-Protocol
179	O
is	O
not	O
open	O
.	O
</s>
<s>
A	O
random	O
TCP	B-Protocol
port	I-Protocol
over	O
1023	O
is	O
not	O
open	O
.	O
</s>
<s>
Peer	O
address	O
configured	O
incorrectly	O
on	O
either	O
router	B-Protocol
.	O
</s>
<s>
AS	B-Architecture
number	I-Architecture
configured	O
incorrectly	O
on	O
either	O
router	B-Protocol
.	O
</s>
<s>
Waits	O
for	O
successful	O
TCP	B-Protocol
negotiation	O
with	O
peer	O
.	O
</s>
<s>
BGP	B-Protocol
does	O
not	O
spend	O
much	O
time	O
in	O
this	O
state	O
if	O
the	O
TCP	B-Protocol
session	O
has	O
been	O
successfully	O
established	O
.	O
</s>
<s>
If	O
an	O
error	O
occurs	O
,	O
BGP	B-Protocol
moves	O
to	O
the	O
Active	O
state	O
.	O
</s>
<s>
TCP	B-Protocol
port	I-Protocol
179	O
is	O
not	O
open	O
.	O
</s>
<s>
A	O
random	O
TCP	B-Protocol
port	I-Protocol
over	O
1023	O
is	O
not	O
open	O
.	O
</s>
<s>
Peer	O
address	O
configured	O
incorrectly	O
on	O
either	O
router	B-Protocol
.	O
</s>
<s>
AS	B-Architecture
number	I-Architecture
configured	O
incorrectly	O
on	O
either	O
router	B-Protocol
.	O
</s>
<s>
If	O
the	O
router	B-Protocol
was	O
unable	O
to	O
establish	O
a	O
successful	O
TCP	B-Protocol
session	O
,	O
then	O
it	O
ends	O
up	O
in	O
the	O
Active	O
state	O
.	O
</s>
<s>
BGP	B-Protocol
FSM	O
tries	O
to	O
restart	O
another	O
TCP	B-Protocol
session	O
with	O
the	O
peer	O
and	O
,	O
if	O
successful	O
,	O
then	O
it	O
sends	O
an	O
Open	O
message	O
to	O
the	O
peer	O
.	O
</s>
<s>
Repeated	O
failures	O
may	O
result	O
in	O
a	O
router	B-Protocol
cycling	O
between	O
the	O
Idle	O
and	O
Active	O
states	O
.	O
</s>
<s>
TCP	B-Protocol
port	I-Protocol
179	O
is	O
not	O
open	O
.	O
</s>
<s>
A	O
random	O
TCP	B-Protocol
port	I-Protocol
over	O
1023	O
is	O
not	O
open	O
.	O
</s>
<s>
BGP	B-Protocol
configuration	O
error	O
.	O
</s>
<s>
BGP	B-Protocol
FSM	O
listens	O
for	O
an	O
Open	O
message	O
from	O
its	O
peer	O
.	O
</s>
<s>
Once	O
the	O
message	O
has	O
been	O
received	O
,	O
the	O
router	B-Protocol
checks	O
the	O
validity	O
of	O
the	O
Open	O
message	O
.	O
</s>
<s>
If	O
there	O
is	O
an	O
error	O
it	O
is	O
because	O
one	O
of	O
the	O
fields	O
in	O
the	O
Open	O
message	O
does	O
not	O
match	O
between	O
the	O
peers	O
,	O
e.g.	O
,	O
BGP	B-Protocol
version	O
mismatch	O
,	O
the	O
peering	O
router	B-Protocol
expects	O
a	O
different	O
My	O
AS	O
,	O
etc	O
.	O
</s>
<s>
The	O
router	B-Protocol
then	O
sends	O
a	O
Notification	O
message	O
to	O
the	O
peer	O
indicating	O
why	O
the	O
error	O
occurred	O
.	O
</s>
<s>
If	O
a	O
Keepalive	O
message	O
is	O
received	O
and	O
no	O
timer	O
has	O
expired	O
before	O
reception	O
of	O
the	O
Keepalive	O
,	O
BGP	B-Protocol
transitions	O
to	O
the	O
Established	O
state	O
.	O
</s>
<s>
If	O
a	O
timer	O
expires	O
before	O
a	O
Keepalive	O
message	O
is	O
received	O
,	O
or	O
if	O
an	O
error	O
condition	O
occurs	O
,	O
the	O
router	B-Protocol
transitions	O
back	O
to	O
the	O
Idle	O
state	O
.	O
</s>
<s>
In	O
this	O
state	O
,	O
the	O
peers	O
send	O
Update	O
messages	O
to	O
exchange	O
information	O
about	O
each	O
route	O
being	O
advertised	O
to	O
the	O
BGP	B-Protocol
peer	O
.	O
</s>
<s>
If	O
there	O
is	O
any	O
error	O
in	O
the	O
Update	O
message	O
then	O
a	O
Notification	O
message	O
is	O
sent	O
to	O
the	O
peer	O
,	O
and	O
BGP	B-Protocol
transitions	O
back	O
to	O
the	O
Idle	O
state	O
.	O
</s>
<s>
In	O
the	O
simplest	O
arrangement	O
,	O
all	O
routers	B-Protocol
within	O
a	O
single	O
AS	O
and	O
participating	O
in	O
BGP	B-Protocol
routing	B-Protocol
must	O
be	O
configured	O
in	O
a	O
full	O
mesh	O
:	O
each	O
router	B-Protocol
must	O
be	O
configured	O
as	O
a	O
peer	O
to	O
every	O
other	O
router	B-Protocol
.	O
</s>
<s>
This	O
causes	O
scaling	O
problems	O
,	O
since	O
the	O
number	O
of	O
required	O
connections	O
grows	O
quadratically	O
with	O
the	O
number	O
of	O
routers	B-Protocol
involved	O
.	O
</s>
<s>
To	O
alleviate	O
the	O
problem	O
,	O
BGP	B-Protocol
implements	O
two	O
options	O
:	O
route	O
reflectors	O
(	O
RFC	O
4456	O
)	O
and	O
BGP	B-Protocol
confederations	O
(	O
RFC	O
5065	O
)	O
.	O
</s>
<s>
The	O
following	O
discussion	O
of	O
basic	O
update	O
processing	O
assumes	O
a	O
full	O
iBGP	B-Protocol
mesh	O
.	O
</s>
<s>
A	O
given	O
BGP	B-Protocol
router	B-Protocol
may	O
accept	O
network-layer	O
reachability	O
information	O
(	O
NLRI	O
)	O
updates	O
from	O
multiple	O
neighbors	O
and	O
advertise	O
NLRI	O
to	O
the	O
same	O
,	O
or	O
a	O
different	O
set	O
,	O
of	O
neighbors	O
.	O
</s>
<s>
The	O
BGP	B-Protocol
process	O
maintains	O
several	O
routing	B-Protocol
information	I-Protocol
base	I-Protocol
:	O
</s>
<s>
RIB	O
:	O
routers	B-Protocol
main	O
routing	B-Protocol
information	I-Protocol
base	I-Protocol
table	O
.	O
</s>
<s>
Loc-RIB	O
:	O
local	O
routing	B-Protocol
information	I-Protocol
base	I-Protocol
BGP	B-Protocol
maintains	O
its	O
own	O
master	O
routing	B-Protocol
table	I-Protocol
separate	O
from	O
the	O
main	O
routing	B-Protocol
table	I-Protocol
of	O
the	O
router	B-Protocol
.	O
</s>
<s>
Adj-RIB-In	O
:	O
For	O
each	O
neighbor	O
,	O
the	O
BGP	B-Protocol
process	O
maintains	O
a	O
conceptual	O
adjacent	O
routing	B-Protocol
information	I-Protocol
base	I-Protocol
,	O
incoming	O
,	O
containing	O
the	O
NLRI	O
received	O
from	O
the	O
neighbor	O
.	O
</s>
<s>
Adj-RIB-Out	O
:	O
For	O
each	O
neighbor	O
,	O
the	O
BGP	B-Protocol
process	O
maintains	O
a	O
conceptual	O
adjacent	O
routing	B-Protocol
information	I-Protocol
base	I-Protocol
,	O
outgoing	O
,	O
containing	O
the	O
NLRI	O
send	O
to	O
the	O
neighbor	O
.	O
</s>
<s>
The	O
physical	O
storage	O
and	O
structure	O
of	O
these	O
conceptual	O
tables	O
are	O
decided	O
by	O
the	O
implementer	O
of	O
the	O
BGP	B-Protocol
code	O
.	O
</s>
<s>
Their	O
structure	O
is	O
not	O
visible	O
to	O
other	O
BGP	B-Protocol
routers	B-Protocol
,	O
although	O
they	O
usually	O
can	O
be	O
interrogated	O
with	O
management	O
commands	O
on	O
the	O
local	O
router	B-Protocol
.	O
</s>
<s>
The	O
additional	O
information	O
tells	O
the	O
BGP	B-Protocol
process	O
such	O
things	O
as	O
whether	O
individual	O
entries	O
belong	O
in	O
the	O
Adj-RIBs	O
for	O
specific	O
neighbors	O
,	O
whether	O
the	O
peer-neighbor	O
route	O
selection	O
process	O
made	O
received	O
policies	O
eligible	O
for	O
the	O
Loc-RIB	O
,	O
and	O
whether	O
Loc-RIB	O
entries	O
are	O
eligible	O
to	O
be	O
submitted	O
to	O
the	O
local	O
router	B-Protocol
's	O
routing	B-Protocol
table	I-Protocol
management	O
process	O
.	O
</s>
<s>
BGP	B-Protocol
submits	O
the	O
routes	O
that	O
it	O
considers	O
best	O
to	O
the	O
main	O
routing	B-Protocol
table	I-Protocol
process	I-Protocol
.	O
</s>
<s>
Depending	O
on	O
the	O
implementation	O
of	O
that	O
process	O
,	O
the	O
BGP	B-Protocol
route	O
is	O
not	O
necessarily	O
selected	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
directly	O
connected	O
prefix	O
,	O
learned	O
from	O
the	O
router	B-Protocol
's	O
own	O
hardware	O
,	O
is	O
usually	O
most	O
preferred	O
.	O
</s>
<s>
As	O
long	O
as	O
that	O
directly	O
connected	O
route	O
's	O
interface	O
is	O
active	O
,	O
the	O
BGP	B-Protocol
route	O
to	O
the	O
destination	O
will	O
not	O
be	O
put	O
into	O
the	O
routing	B-Protocol
table	I-Protocol
.	O
</s>
<s>
Once	O
the	O
interface	O
goes	O
down	O
,	O
and	O
there	O
are	O
no	O
more	O
preferred	O
routes	O
,	O
the	O
Loc-RIB	O
route	O
would	O
be	O
installed	O
in	O
the	O
main	O
routing	B-Protocol
table	I-Protocol
.	O
</s>
<s>
BGP	B-Protocol
carries	O
the	O
information	O
with	O
which	O
rules	O
inside	O
BGP-speaking	O
routers	B-Protocol
can	O
make	O
policy	O
decisions	O
.	O
</s>
<s>
The	O
BGP	B-Protocol
standard	O
specifies	O
a	O
number	O
of	O
decision	O
factors	O
,	O
more	O
than	O
the	O
ones	O
that	O
are	O
used	O
by	O
any	O
other	O
common	O
routing	B-Protocol
process	O
,	O
for	O
selecting	O
NLRI	O
to	O
go	O
into	O
the	O
Loc-RIB	O
.	O
</s>
<s>
Another	O
way	O
of	O
saying	O
the	O
next-hop	O
must	O
be	O
reachable	O
is	O
that	O
there	O
must	O
be	O
an	O
active	O
route	O
,	O
already	O
in	O
the	O
main	O
routing	B-Protocol
table	I-Protocol
of	O
the	O
router	B-Protocol
,	O
to	O
the	O
prefix	O
in	O
which	O
the	O
next-hop	O
address	O
is	O
reachable	O
.	O
</s>
<s>
Next	O
,	O
for	O
each	O
neighbor	O
,	O
the	O
BGP	B-Protocol
process	O
applies	O
various	O
standard	O
and	O
implementation-dependent	O
criteria	O
to	O
decide	O
which	O
routes	O
conceptually	O
should	O
go	O
into	O
the	O
Adj-RIB-In	O
.	O
</s>
<s>
Whenever	O
a	O
conceptual	O
Adj-RIB-In	O
changes	O
,	O
the	O
main	O
BGP	B-Protocol
process	O
decides	O
if	O
any	O
of	O
the	O
neighbor	O
's	O
new	O
routes	O
are	O
preferred	O
to	O
routes	O
already	O
in	O
the	O
Loc-RIB	O
.	O
</s>
<s>
If	O
a	O
given	O
route	O
is	O
withdrawn	O
by	O
a	O
neighbor	O
,	O
and	O
there	O
is	O
no	O
other	O
route	O
to	O
that	O
destination	O
,	O
the	O
route	O
is	O
removed	O
from	O
the	O
Loc-RIB	O
and	O
no	O
longer	O
sent	O
by	O
BGP	B-Protocol
to	O
the	O
main	O
routing	B-Protocol
table	I-Protocol
manager	O
.	O
</s>
<s>
If	O
the	O
router	B-Protocol
does	O
not	O
have	O
a	O
route	O
to	O
that	O
destination	O
from	O
any	O
non-BGP	O
source	O
,	O
the	O
withdrawn	O
route	O
will	O
be	O
removed	O
from	O
the	O
main	O
routing	B-Protocol
table	I-Protocol
.	O
</s>
<s>
+	O
Steps	O
to	O
determine	O
best	O
path	O
,	O
in	O
order	O
of	O
tiebreaker	O
:	O
Step	O
Scope	O
Name	O
Default	O
Preferred	O
BGP	B-Protocol
field	O
NOTE	O
1	O
Local	O
to	O
router	B-Protocol
local	O
Weight	O
Higher	O
Cisco-specific	O
parameter	O
2	O
Internal	O
to	O
AS	O
Local	O
preference	O
Higher	O
LOCAL_PREF	O
If	O
there	O
are	O
several	O
iBGP	B-Protocol
routes	O
from	O
the	O
neighbor	O
,	O
the	O
one	O
with	O
the	O
highest	O
local	O
preference	O
is	O
selected	O
unless	O
there	O
are	O
several	O
routes	O
with	O
the	O
same	O
local	O
preference	O
.	O
</s>
<s>
3	O
Accumulated	O
Interior	O
Gateway	O
Protocol	O
(	O
AIGP	O
)	O
Lowest	O
AIGP	O
rfc7311	O
4	O
External	O
to	O
AS	O
Autonomous	B-Architecture
system	I-Architecture
(	O
AS	O
)	O
jumps	O
Lowest	O
AS-path	O
AS	O
jumps	O
is	O
the	O
number	O
of	O
AS	B-Architecture
numbers	I-Architecture
that	O
must	O
be	O
traversed	O
to	O
reach	O
the	O
advertised	O
destination	O
.	O
</s>
<s>
5	O
origin	O
type	O
"	O
IGP	O
"	O
Lowest	O
ORIGIN	O
0	O
=	O
IGP1	O
=	O
EGP2	O
=	O
Incomplete	O
6	O
multi-exit	O
discriminator	O
(	O
MED	O
)	O
Lowest	O
MULTI_EXIT_DISC	O
By	O
default	O
only	O
route	O
with	O
the	O
same	O
autonomous	B-Architecture
system	I-Architecture
(	O
AS	O
)	O
is	O
compared	O
.	O
</s>
<s>
Can	O
be	O
set	O
to	O
ignore	O
same	O
autonomous	B-Architecture
system	I-Architecture
(	O
AS	O
)	O
.	O
</s>
<s>
Before	O
the	O
most	O
recent	O
edition	O
of	O
the	O
BGP	B-Protocol
standard	O
,	O
if	O
an	O
update	O
had	O
no	O
MED	O
value	O
,	O
several	O
implementations	O
created	O
a	O
MED	O
with	O
the	O
highest	O
possible	O
value	O
.	O
</s>
<s>
Since	O
the	O
current	O
rule	O
may	O
cause	O
different	O
behavior	O
than	O
the	O
vendor	O
interpretations	O
,	O
BGP	B-Protocol
implementations	O
that	O
used	O
the	O
nonstandard	O
default	O
value	O
have	O
a	O
configuration	O
feature	O
that	O
allows	O
the	O
old	O
or	O
standard	O
rule	O
to	O
be	O
selected	O
.	O
</s>
<s>
7	O
Local	O
to	O
router	B-Protocol
(	O
Loc-RIB	O
)	O
eBGP	B-Protocol
over	O
iBGP	B-Protocol
paths	O
"	O
on	O
"	O
Directly	O
connected	O
,	O
over	O
indirectly	O
8	O
IGP	O
metric	O
to	O
BGP	B-Protocol
next	O
hop	O
"	O
on	O
"	O
,	O
imported	O
from	O
IGP	O
Lowest	O
Continue	O
,	O
even	O
if	O
bestpath	O
is	O
already	O
selected	O
.	O
</s>
<s>
Prefer	O
the	O
route	O
with	O
the	O
lowest	O
interior	O
cost	O
to	O
the	O
next	O
hop	O
,	O
according	O
to	O
the	O
main	O
routing	B-Protocol
table	I-Protocol
.	O
</s>
<s>
If	O
two	O
neighbors	O
advertised	O
the	O
same	O
route	O
,	O
but	O
one	O
neighbor	O
is	O
reachable	O
via	O
a	O
low-bitrate	O
link	O
and	O
the	O
other	O
by	O
a	O
high-bitrate	O
link	O
,	O
and	O
the	O
interior	O
routing	B-Protocol
protocol	O
calculates	O
lowest	O
cost	O
based	O
on	O
highest	O
bitrate	O
,	O
the	O
route	O
through	O
the	O
high-bitrate	O
link	O
would	O
be	O
preferred	O
and	O
other	O
routes	O
dropped	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
community	O
attribute	O
(	O
see	O
below	O
)	O
is	O
not	O
directly	O
used	O
by	O
the	O
BGP	B-Protocol
selection	O
process	O
.	O
</s>
<s>
The	O
BGP	B-Protocol
neighbor	O
process	O
can	O
have	O
a	O
rule	O
to	O
set	O
local	O
preference	O
or	O
another	O
factor	O
based	O
on	O
a	O
manually	O
programmed	O
rule	O
to	O
set	O
the	O
attribute	O
if	O
the	O
community	O
value	O
matches	O
some	O
pattern-matching	O
criterion	O
.	O
</s>
<s>
If	O
the	O
route	O
was	O
learned	O
from	O
an	O
external	O
peer	O
the	O
per-neighbor	O
BGP	B-Protocol
process	O
computes	O
a	O
local	O
preference	O
value	O
from	O
local	O
policy	O
rules	O
and	O
then	O
compares	O
the	O
local	O
preference	O
of	O
all	O
routes	O
from	O
the	O
neighbor	O
.	O
</s>
<s>
BGP	B-Protocol
communities	O
are	O
attribute	O
tags	O
that	O
can	O
be	O
applied	O
to	O
incoming	O
or	O
outgoing	O
prefixes	O
to	O
achieve	O
some	O
common	O
goal	O
.	O
</s>
<s>
While	O
it	O
is	O
common	O
to	O
say	O
that	O
BGP	B-Protocol
allows	O
an	O
administrator	O
to	O
set	O
policies	O
on	O
how	O
prefixes	O
are	O
handled	O
by	O
ISPs	O
,	O
this	O
is	O
generally	O
not	O
possible	O
,	O
strictly	O
speaking	O
.	O
</s>
<s>
For	O
instance	O
,	O
BGP	B-Protocol
natively	O
has	O
no	O
concept	O
to	O
allow	O
one	O
AS	O
to	O
tell	O
another	O
AS	O
to	O
restrict	O
advertisement	O
of	O
a	O
prefix	O
to	O
only	O
North	O
American	O
peering	O
customers	O
.	O
</s>
<s>
+	O
Well-known	O
BGP	B-Protocol
communities	O
Attribute	O
value	O
Attribute	O
Description	O
Reference	O
0x00000000-0x0000FFFF	O
Reserved	O
0x00010000-0xFFFEFFFF	O
Reserved	O
for	O
private	O
use	O
0xFFFF0000	O
GRACEFUL_SHUTDOWN	O
At	O
neighbor	O
AS-peer	O
,	O
set	O
LOCAL_PREF	O
,	O
lower	O
to	O
route	O
away	O
from	O
source	O
.	O
</s>
<s>
0xFFFF0001	O
ACCEPT_OWN	O
Used	O
to	O
modify	O
how	O
a	O
route	O
originated	O
within	O
one	O
VRF	O
is	O
imported	O
into	O
other	O
VRFs	O
0xFFFF0002	O
ROUTE_FILTER_TRANSLATED_v4	O
RFC	O
draft-l3vpn-legacy-rtc	O
0xFFFF0003	O
ROUTE_FILTER_v4	O
RFC	O
draft-l3vpn-legacy-rtc	O
0xFFFF0004	O
ROUTE_FILTER_TRANSLATED_v6	O
RFC	O
draft-l3vpn-legacy-rtc	O
0xFFFF0005	O
ROUTE_FILTER_v6	O
RFC	O
draft-l3vpn-legacy-rtc	O
0xFFFF0006	O
LLGR_STALE	O
RFC	O
draft-uttaro-idr-bgp-persistence	O
0xFFFF0007	O
NO_LLGR	O
RFC	O
draft-uttaro-idr-bgp-persistence	O
0xFFFF0008	O
accept-own-nexthop	O
RFC	O
draft-agrewal-idr-accept-own-nexthop	O
0xFFFF0009	O
Standby	O
PE	O
Allow	O
for	O
faster	O
recovery	O
of	O
connectivity	O
on	O
different	O
types	O
of	O
failures	O
,	O
with	O
multicast	O
in	O
BGP/MPLS	O
VPNs	B-Application
.	O
</s>
<s>
0xFFFF000A-0xFFFF0299	O
Unassigned	O
0xFFFF029A	O
BLACKHOLE	O
To	O
temporary	O
protect	O
against	O
denial-of-service	O
attack	O
,	O
by	O
enable	O
blackhole	O
at	O
neighbour	O
AS-peer	O
0xFFFF029B-0xFFFFFF00	O
Unassigned	O
0xFFFFFF01	O
NO_EXPORT	O
limit	O
to	O
a	O
BGP	B-Protocol
confederation	O
boundary	O
0xFFFFFF02	O
NO_ADVERTISE	O
limit	O
to	O
a	O
BGP	B-Protocol
peer	O
0xFFFFFF03	O
NO_EXPORT_SUBCONFED	O
limit	O
to	O
the	O
autonomous	B-Architecture
system	I-Architecture
0xFFFFFF04	O
NOPEER	O
to	O
limited	O
the	O
number	O
of	O
specific	O
routes	O
to	O
all	O
of	O
internet	O
.	O
</s>
<s>
It	O
is	O
a	O
common	O
tactic	O
for	O
end	O
customers	O
to	O
use	O
BGP	B-Protocol
communities	O
(	O
usually	O
ASN:70	O
,	O
80,90,100	O
)	O
to	O
control	O
the	O
local	O
preference	O
the	O
ISP	O
assigns	O
to	O
advertised	O
routes	O
instead	O
of	O
using	O
MED	O
(	O
the	O
effect	O
is	O
similar	O
)	O
.	O
</s>
<s>
The	O
BGP	B-Protocol
Extended	O
Community	O
Attribute	O
was	O
added	O
in	O
2006	O
,	O
in	O
order	O
to	O
extend	O
the	O
range	O
of	O
such	O
attributes	O
and	O
to	O
provide	O
a	O
community	O
attribute	O
structuring	O
by	O
means	O
of	O
a	O
type	O
field	O
.	O
</s>
<s>
The	O
IANA	O
administers	O
the	O
registry	O
for	O
BGP	B-Protocol
Extended	O
Communities	O
Types	O
.	O
</s>
<s>
The	O
Extended	O
Communities	O
Attribute	O
itself	O
is	O
a	O
transitive	O
optional	O
BGP	B-Protocol
attribute	O
.	O
</s>
<s>
RFC	O
4360	O
exemplarily	O
defines	O
the	O
"	O
Two-Octet	O
AS	O
Specific	O
Extended	O
Community	O
"	O
,	O
the	O
"	O
IPv4	B-Protocol
Address	O
Specific	O
Extended	O
Community	O
"	O
,	O
the	O
"	O
Opaque	O
Extended	O
Community	O
"	O
,	O
the	O
"	O
Route	O
Target	O
Community	O
"	O
,	O
and	O
the	O
"	O
Route	O
Origin	O
Community	O
"	O
.	O
</s>
<s>
A	O
number	O
of	O
BGP	B-Protocol
QoS	O
drafts	O
also	O
use	O
this	O
Extended	O
Community	O
Attribute	O
structure	O
for	O
inter-domain	O
QoS	O
signalling	O
.	O
</s>
<s>
With	O
the	O
introduction	O
of	O
32-bit	O
AS	B-Architecture
numbers	I-Architecture
,	O
some	O
issues	O
were	O
immediately	O
obvious	O
with	O
the	O
community	O
attribute	O
that	O
only	O
defines	O
a	O
16-bit	O
ASN	O
field	O
,	O
which	O
prevents	O
the	O
matching	O
between	O
this	O
field	O
and	O
the	O
real	O
ASN	O
value	O
.	O
</s>
<s>
MEDs	O
,	O
defined	O
in	O
the	O
main	O
BGP	B-Protocol
standard	O
,	O
were	O
originally	O
intended	O
to	O
show	O
to	O
another	O
neighbor	O
AS	O
the	O
advertising	O
AS	O
's	O
preference	O
as	O
to	O
which	O
of	O
several	O
links	O
are	O
preferred	O
for	O
inbound	O
traffic	O
.	O
</s>
<s>
Another	O
application	O
of	O
MEDs	O
is	O
to	O
advertise	O
the	O
value	O
,	O
typically	O
based	O
on	O
delay	O
,	O
of	O
multiple	O
ASs	O
that	O
have	O
a	O
presence	O
at	O
an	O
IXP	B-Protocol
,	O
that	O
they	O
impose	O
to	O
send	O
traffic	O
to	O
some	O
destination	O
.	O
</s>
<s>
Some	O
routers	B-Protocol
(	O
like	O
Juniper	O
)	O
will	O
use	O
the	O
Metric	O
from	O
OSPF	B-Protocol
to	O
set	O
MED	O
.	O
</s>
<s>
Topology	B-Architecture
default	B-Protocol
Route	I-Protocol
Table	O
:	O
</s>
<s>
Type	O
:	O
Type	O
of	O
BGP	B-Protocol
message	O
.	O
</s>
<s>
Version	O
(	O
8bit	O
)	O
Version	O
of	O
BGP	B-Protocol
used	O
.	O
</s>
<s>
My	O
AS	O
(	O
16bit	O
)	O
Senders	O
autonomous	B-Architecture
system	I-Architecture
number	I-Architecture
.	O
</s>
<s>
BGP	B-Protocol
Identifier	O
(	O
32bit	O
)	O
IP-address	O
of	O
sender	O
.	O
</s>
<s>
BGP	B-Protocol
Identifier	O
:	O
</s>
<s>
If	O
there	O
is	O
an	O
error	O
it	O
is	O
because	O
one	O
of	O
the	O
fields	O
in	O
the	O
OPEN	O
or	O
UPDATE	O
message	O
does	O
not	O
match	O
between	O
the	O
peers	O
,	O
e.g.	O
,	O
BGP	B-Protocol
version	O
mismatch	O
,	O
the	O
peering	O
router	B-Protocol
expects	O
a	O
different	O
My	O
AS	O
,	O
etc	O
.	O
</s>
<s>
The	O
router	B-Protocol
then	O
sends	O
a	O
Notification	O
message	O
to	O
the	O
peer	O
indicating	O
why	O
the	O
error	O
occurred	O
.	O
</s>
<s>
3	O
Bad	O
BGP	B-Protocol
Identifier	O
.	O
</s>
<s>
6	O
Invalid	O
ORIGIN	O
Attribute	O
7	O
AS	O
Routing	B-Protocol
Loop	O
.	O
</s>
<s>
BGP	B-Protocol
is	O
"	O
the	O
most	O
scalable	O
of	O
all	O
routing	B-Protocol
protocols.	O
"	O
</s>
<s>
An	O
autonomous	B-Architecture
system	I-Architecture
with	O
internal	B-Protocol
BGP	I-Protocol
(	O
iBGP	B-Protocol
)	O
must	O
have	O
all	O
of	O
its	O
iBGP	B-Protocol
peers	O
connect	O
to	O
each	O
other	O
in	O
a	O
full	O
mesh	O
(	O
where	O
everyone	O
speaks	O
to	O
everyone	O
directly	O
)	O
.	O
</s>
<s>
This	O
full-mesh	O
configuration	O
requires	O
that	O
each	O
router	B-Protocol
maintain	O
a	O
session	O
with	O
every	O
other	O
router	B-Protocol
.	O
</s>
<s>
In	O
large	O
networks	O
,	O
this	O
number	O
of	O
sessions	O
may	O
degrade	O
the	O
performance	O
of	O
routers	B-Protocol
,	O
due	O
to	O
either	O
a	O
lack	O
of	O
memory	O
,	O
or	O
high	O
CPU	O
process	O
requirements	O
.	O
</s>
<s>
A	O
single	O
router	B-Protocol
(	O
or	O
two	O
for	O
redundancy	O
)	O
can	O
be	O
made	O
an	O
RR	O
:	O
other	O
routers	B-Protocol
in	O
the	O
AS	O
need	O
only	O
be	O
configured	O
as	O
peers	O
to	O
them	O
.	O
</s>
<s>
An	O
RR	O
offers	O
an	O
alternative	O
to	O
the	O
logical	O
full-mesh	O
requirement	O
of	O
iBGP	B-Protocol
.	O
</s>
<s>
Multiple	O
BGP	B-Protocol
routers	B-Protocol
can	O
peer	O
with	O
a	O
central	O
point	O
,	O
the	O
RR	O
acting	O
as	O
an	O
RR	O
server	O
rather	O
than	O
peer	O
with	O
every	O
other	O
router	B-Protocol
in	O
a	O
full	O
mesh	O
.	O
</s>
<s>
All	O
the	O
other	O
iBGP	B-Protocol
routers	B-Protocol
become	O
RR	O
clients	O
.	O
</s>
<s>
This	O
approach	O
,	O
similar	O
to	O
OSPF	B-Protocol
's	O
DR/BDR	O
feature	O
,	O
provides	O
large	O
networks	O
with	O
added	O
iBGP	B-Protocol
scalability	O
.	O
</s>
<s>
In	O
a	O
fully	O
meshed	O
iBGP	B-Protocol
network	O
of	O
10	O
routers	B-Protocol
,	O
90	O
individual	O
CLI	O
statements	O
(	O
spread	O
throughout	O
all	O
routers	B-Protocol
in	O
the	O
topology	B-Architecture
)	O
are	O
needed	O
just	O
to	O
define	O
the	O
remote-AS	O
of	O
each	O
peer	O
:	O
this	O
quickly	O
becomes	O
a	O
headache	O
to	O
manage	O
.	O
</s>
<s>
An	O
RR	O
topology	B-Architecture
can	O
cut	O
these	O
90	O
statements	O
down	O
to	O
18	O
,	O
offering	O
a	O
viable	O
solution	O
for	O
the	O
larger	O
networks	O
administered	O
by	O
ISPs	O
.	O
</s>
<s>
An	O
RR	O
is	O
a	O
single	B-Architecture
point	I-Architecture
of	I-Architecture
failure	I-Architecture
,	O
therefore	O
at	O
least	O
a	O
second	O
RR	O
may	O
be	O
configured	O
in	O
order	O
to	O
provide	O
redundancy	O
.	O
</s>
<s>
As	O
it	O
is	O
an	O
additional	O
peer	O
for	O
the	O
other	O
10	O
routers	B-Protocol
,	O
it	O
approximately	O
doubles	O
the	O
number	O
of	O
CLI	O
statements	O
,	O
requiring	O
an	O
additional	O
statements	O
in	O
this	O
case	O
.	O
</s>
<s>
In	O
a	O
BGP	B-Protocol
multipath	O
environment	O
the	O
additional	O
RR	O
also	O
can	O
benefit	O
the	O
network	O
by	O
adding	O
local	O
routing	B-Protocol
throughput	O
if	O
the	O
RRs	O
are	O
acting	O
as	O
traditional	O
routers	B-Protocol
instead	O
of	O
just	O
a	O
dedicated	O
RR	O
server	O
role	O
.	O
</s>
<s>
RRs	O
and	O
confederations	O
both	O
reduce	O
the	O
number	O
of	O
iBGP	B-Protocol
peers	O
to	O
each	O
router	B-Protocol
and	O
thus	O
reduce	O
processing	O
overhead	O
.	O
</s>
<s>
Routes	O
are	O
always	O
reflected	O
to	O
eBGP	B-Protocol
peers	O
.	O
</s>
<s>
A	O
cluster	O
ID	O
is	O
a	O
cumulative	O
,	O
non-transitive	O
BGP	B-Protocol
attribute	O
,	O
and	O
every	O
RR	O
must	O
prepend	O
the	O
local	O
cluster	O
ID	O
to	O
the	O
cluster	O
list	O
to	O
avoid	O
routing	B-Protocol
loops	O
.	O
</s>
<s>
Confederations	O
are	O
sets	O
of	O
autonomous	B-Architecture
systems	I-Architecture
.	O
</s>
<s>
In	O
common	O
practice	O
,	O
only	O
one	O
of	O
the	O
confederation	O
AS	B-Architecture
numbers	I-Architecture
is	O
seen	O
by	O
the	O
Internet	O
as	O
a	O
whole	O
.	O
</s>
<s>
Each	O
confederated	O
AS	O
alone	O
has	O
iBGP	B-Protocol
fully	O
meshed	O
and	O
has	O
connections	O
to	O
other	O
ASs	O
inside	O
the	O
confederation	O
.	O
</s>
<s>
Even	O
though	O
these	O
ASs	O
have	O
eBGP	B-Protocol
peers	O
to	O
ASs	O
within	O
the	O
confederation	O
,	O
the	O
ASs	O
exchange	O
routing	B-Protocol
as	O
if	O
they	O
used	O
iBGP	B-Protocol
.	O
</s>
<s>
With	O
this	O
solution	O
,	O
iBGP	B-Protocol
transit	O
AS	O
problems	O
can	O
be	O
resolved	O
as	O
iBGP	B-Protocol
requires	O
a	O
full	O
mesh	O
between	O
all	O
BGP	B-Protocol
routers	B-Protocol
:	O
large	O
number	O
of	O
TCP	B-Protocol
sessions	O
and	O
unnecessary	O
duplication	O
of	O
routing	B-Protocol
traffic	O
.	O
</s>
<s>
Both	O
confederations	O
and	O
route	O
reflectors	O
can	O
be	O
subject	O
to	O
persistent	O
oscillation	O
unless	O
specific	O
design	O
rules	O
,	O
affecting	O
both	O
BGP	B-Protocol
and	O
the	O
interior	O
routing	B-Protocol
protocol	O
,	O
are	O
followed	O
.	O
</s>
<s>
Additionally	O
,	O
route	O
reflectors	O
and	O
BGP	B-Protocol
confederations	O
were	O
not	O
designed	O
to	O
ease	O
BGP	B-Protocol
router	B-Protocol
configuration	O
.	O
</s>
<s>
Nevertheless	O
,	O
these	O
are	O
common	O
tools	O
for	O
experienced	O
BGP	B-Protocol
network	O
architects	O
.	O
</s>
<s>
The	O
routing	B-Protocol
tables	I-Protocol
managed	O
by	O
a	O
BGP	B-Protocol
implementation	O
are	O
adjusted	O
continually	O
to	O
reflect	O
actual	O
changes	O
in	O
the	O
network	O
,	O
such	O
as	O
links	O
or	O
routers	B-Protocol
going	O
down	O
and	O
coming	O
back	O
up	O
.	O
</s>
<s>
In	O
the	O
network	O
as	O
a	O
whole	O
,	O
it	O
is	O
normal	O
for	O
these	O
changes	O
to	O
happen	O
almost	O
continuously	O
,	O
but	O
for	O
any	O
particular	O
router	B-Protocol
or	O
link	O
,	O
changes	O
are	O
expected	O
to	O
be	O
relatively	O
infrequent	O
.	O
</s>
<s>
If	O
a	O
router	B-Protocol
is	O
misconfigured	O
or	O
mismanaged	O
then	O
it	O
may	O
get	O
into	O
a	O
rapid	O
cycle	O
between	O
down	O
and	O
up	O
states	O
.	O
</s>
<s>
This	O
pattern	O
of	O
repeated	O
withdrawal	O
and	O
re-announcement	O
known	O
as	O
route	B-Protocol
flapping	I-Protocol
can	O
cause	O
excessive	O
activity	O
in	O
all	O
the	O
other	O
routers	B-Protocol
that	O
know	O
about	O
the	O
cycling	O
entity	O
,	O
as	O
the	O
same	O
route	O
is	O
continually	O
injected	O
and	O
withdrawn	O
from	O
the	O
routing	B-Protocol
tables	I-Protocol
.	O
</s>
<s>
The	O
BGP	B-Protocol
design	O
is	O
such	O
that	O
delivery	O
of	O
traffic	O
may	O
not	O
function	O
while	O
routes	O
are	O
being	O
updated	O
.	O
</s>
<s>
On	O
the	O
Internet	O
,	O
a	O
BGP	B-Protocol
routing	B-Protocol
change	O
may	O
cause	O
outages	O
for	O
several	O
minutes	O
.	O
</s>
<s>
A	O
feature	O
known	O
as	O
route	O
flap	O
damping	O
(	O
)	O
is	O
built	O
into	O
many	O
BGP	B-Protocol
implementations	O
in	O
an	O
attempt	O
to	O
mitigate	O
the	O
effects	O
of	O
route	B-Protocol
flapping	I-Protocol
.	O
</s>
<s>
Without	O
damping	O
,	O
the	O
excessive	O
activity	O
can	O
cause	O
a	O
heavy	O
processing	O
load	O
on	O
routers	B-Protocol
,	O
which	O
may	O
in	O
turn	O
delay	O
updates	O
on	O
other	O
routes	O
,	O
and	O
so	O
affect	O
overall	O
routing	B-Protocol
stability	O
.	O
</s>
<s>
At	O
the	O
first	O
instance	O
when	O
a	O
route	O
becomes	O
unavailable	O
and	O
quickly	O
reappears	O
,	O
damping	O
does	O
not	O
take	O
effect	O
,	O
so	O
as	O
to	O
maintain	O
the	O
normal	O
fail-over	O
times	O
of	O
BGP	B-Protocol
.	O
</s>
<s>
At	O
the	O
second	O
occurrence	O
,	O
BGP	B-Protocol
shuns	O
that	O
prefix	O
for	O
a	O
certain	O
length	O
of	O
time	O
;	O
subsequent	O
occurrences	O
are	O
ignored	O
exponentially	O
longer	O
.	O
</s>
<s>
It	O
is	O
also	O
suggested	O
in	O
RFC	O
2439	O
that	O
route	O
flap	O
damping	O
is	O
a	O
feature	O
more	O
desirable	O
if	O
implemented	O
to	O
Exterior	B-Protocol
Border	I-Protocol
Gateway	I-Protocol
Protocol	I-Protocol
Sessions	O
(	O
eBGP	B-Protocol
sessions	O
or	O
simply	O
called	O
exterior	O
peers	O
)	O
and	O
not	O
on	O
Interior	B-Protocol
Border	I-Protocol
Gateway	I-Protocol
Protocol	I-Protocol
Sessions	O
(	O
iBGP	B-Protocol
sessions	O
or	O
simply	O
called	O
internal	O
peers	O
)	O
.	O
</s>
<s>
With	O
this	O
approach	O
when	O
a	O
route	O
flaps	O
inside	O
an	O
autonomous	B-Architecture
system	I-Architecture
,	O
it	O
is	O
not	O
propagated	O
to	O
the	O
external	O
ASs	O
flapping	O
a	O
route	O
to	O
an	O
eBGP	B-Protocol
will	O
cause	O
a	O
chain	O
of	O
flapping	O
for	O
the	O
particular	O
route	O
throughout	O
the	O
backbone	O
.	O
</s>
<s>
This	O
method	O
also	O
successfully	O
avoids	O
the	O
overhead	O
of	O
route	O
flap	O
damping	O
for	O
iBGP	B-Protocol
sessions	O
.	O
</s>
<s>
Moreover	O
,	O
as	O
backbone	O
links	O
and	O
router	B-Protocol
processors	O
have	O
become	O
faster	O
,	O
some	O
network	O
architects	O
have	O
suggested	O
that	O
flap	O
damping	O
may	O
not	O
be	O
as	O
important	O
as	O
it	O
used	O
to	O
be	O
,	O
since	O
changes	O
to	O
the	O
routing	B-Protocol
table	I-Protocol
can	O
be	O
handled	O
much	O
faster	O
by	O
routers	B-Protocol
.	O
</s>
<s>
This	O
has	O
led	O
the	O
RIPE	O
Routing	B-Protocol
Working	O
Group	O
to	O
write	O
,	O
"	O
With	O
the	O
current	O
implementations	O
of	O
BGP	B-Protocol
flap	O
damping	O
,	O
the	O
application	O
of	O
flap	O
damping	O
in	O
ISP	O
networks	O
is	O
NOT	O
recommended	O
.	O
</s>
<s>
One	O
of	O
the	O
largest	O
problems	O
faced	O
by	O
BGP	B-Protocol
,	O
and	O
indeed	O
the	O
Internet	O
infrastructure	O
as	O
a	O
whole	O
,	O
is	O
the	O
growth	O
of	O
the	O
Internet	O
routing	B-Protocol
table	I-Protocol
.	O
</s>
<s>
If	O
the	O
global	O
routing	B-Protocol
table	I-Protocol
grows	O
to	O
the	O
point	O
where	O
some	O
older	O
,	O
less	O
capable	O
routers	B-Protocol
cannot	O
cope	O
with	O
the	O
memory	O
requirements	O
or	O
the	O
CPU	O
load	O
of	O
maintaining	O
the	O
table	O
,	O
these	O
routers	B-Protocol
will	O
cease	O
to	O
be	O
effective	O
gateways	O
between	O
the	O
parts	O
of	O
the	O
Internet	O
they	O
connect	O
.	O
</s>
<s>
In	O
addition	O
,	O
and	O
perhaps	O
even	O
more	O
importantly	O
,	O
larger	O
routing	B-Protocol
tables	I-Protocol
take	O
longer	O
to	O
stabilize	O
after	O
a	O
major	O
connectivity	O
change	O
,	O
leaving	O
network	O
service	O
unreliable	O
,	O
or	O
even	O
unavailable	O
,	O
in	O
the	O
interim	O
.	O
</s>
<s>
Until	O
late	O
2001	O
,	O
the	O
global	O
routing	B-Protocol
table	I-Protocol
was	O
growing	O
exponentially	O
,	O
threatening	O
an	O
eventual	O
widespread	O
breakdown	O
of	O
connectivity	O
.	O
</s>
<s>
In	O
an	O
attempt	O
to	O
prevent	O
this	O
,	O
ISPs	O
cooperated	O
in	O
keeping	O
the	O
global	O
routing	B-Protocol
table	I-Protocol
as	O
small	O
as	O
possible	O
,	O
by	O
using	O
Classless	B-Protocol
Inter-Domain	I-Protocol
Routing	I-Protocol
(	O
CIDR	B-Protocol
)	O
and	O
route	B-Protocol
aggregation	I-Protocol
.	O
</s>
<s>
While	O
this	O
slowed	O
the	O
growth	O
of	O
the	O
routing	B-Protocol
table	I-Protocol
to	O
a	O
linear	O
process	O
for	O
several	O
years	O
,	O
with	O
the	O
expanded	O
demand	O
for	O
multihoming	B-Protocol
by	O
end-user	O
networks	O
the	O
growth	O
was	O
once	O
again	O
superlinear	O
by	O
the	O
middle	O
of	O
2004	O
.	O
</s>
<s>
A	O
Y2K-like	O
overflow	O
triggered	O
in	O
2014	O
for	O
those	O
models	O
that	O
were	O
not	O
appropriately	O
updated	O
.	O
</s>
<s>
While	O
a	O
full	O
IPv4	B-Protocol
BGP	B-Protocol
table	O
(	O
512k	O
day	O
)	O
was	O
in	O
excess	O
of	O
512,000	O
prefixes	O
,	O
many	O
older	O
routers	B-Protocol
had	O
a	O
limit	O
of	O
512k	O
(	O
512,000	O
–	O
524,288	O
)	O
routing	B-Protocol
table	I-Protocol
entries	O
.	O
</s>
<s>
On	O
August	O
12	O
,	O
2014	O
,	O
outages	O
resulting	O
from	O
full	O
tables	O
hit	O
eBay	B-Application
,	O
LastPass	B-Operating_System
and	O
Microsoft	B-Application
Azure	I-Application
among	O
others	O
.	O
</s>
<s>
A	O
number	O
of	O
Cisco	O
routers	B-Protocol
commonly	O
in	O
use	O
had	O
TCAM	O
,	O
a	O
form	O
of	O
high-speed	O
content-addressable	B-Data_Structure
memory	I-Data_Structure
,	O
for	O
storing	O
BGP	B-Protocol
advertised	O
routes	O
.	O
</s>
<s>
On	O
impacted	O
routers	B-Protocol
,	O
the	O
TCAM	O
was	O
by	O
default	O
allocated	O
as	O
512k	O
IPv4	B-Protocol
routes	O
and	O
256k	O
IPv6	B-Protocol
routes	O
.	O
</s>
<s>
While	O
the	O
reported	O
number	O
of	O
IPv6	B-Protocol
advertised	O
routes	O
was	O
only	O
about	O
20k	O
,	O
the	O
number	O
of	O
advertised	O
IPv4	B-Protocol
routes	O
reached	O
the	O
default	O
limit	O
,	O
causing	O
a	O
spillover	O
effect	O
as	O
routers	B-Protocol
attempted	O
to	O
compensate	O
for	O
the	O
issue	O
by	O
using	O
slow	O
software	O
routing	B-Protocol
(	O
as	O
opposed	O
to	O
fast	O
hardware	O
routing	B-Protocol
via	O
TCAM	O
)	O
.	O
</s>
<s>
The	O
main	O
method	O
for	O
dealing	O
with	O
this	O
issue	O
involves	O
operators	O
changing	O
the	O
TCAM	O
allocation	O
to	O
allow	O
more	O
IPv4	B-Protocol
entries	O
,	O
by	O
reallocating	O
some	O
of	O
the	O
TCAM	O
reserved	O
for	O
IPv6	B-Protocol
routes	O
,	O
which	O
requires	O
a	O
reboot	O
on	O
most	O
routers	B-Protocol
.	O
</s>
<s>
Almost	O
all	O
of	O
these	O
routes	O
were	O
to	O
Verizon	O
Autonomous	B-Architecture
Systems	I-Architecture
701	O
and	O
705	O
,	O
created	O
as	O
a	O
result	O
of	O
deaggregation	O
of	O
larger	O
blocks	O
,	O
introducing	O
thousands	O
of	O
new	O
routes	O
,	O
and	O
making	O
the	O
routing	B-Protocol
table	I-Protocol
reach	O
515,000	O
entries	O
.	O
</s>
<s>
Even	O
if	O
Verizon	O
had	O
not	O
caused	O
the	O
routing	B-Protocol
table	I-Protocol
to	O
exceed	O
512k	O
entries	O
in	O
the	O
short	O
spike	O
,	O
it	O
would	O
have	O
soon	O
happened	O
through	O
natural	O
growth	O
.	O
</s>
<s>
Route	B-Protocol
summarization	I-Protocol
is	O
often	O
used	O
to	O
improve	O
aggregation	O
of	O
the	O
BGP	B-Protocol
global	O
routing	B-Protocol
table	I-Protocol
,	O
thereby	O
reducing	O
the	O
necessary	O
table	O
size	O
in	O
routers	B-Protocol
of	O
an	O
AS	O
.	O
</s>
<s>
AS2	O
will	O
see	O
the	O
four	O
routes	O
from	O
AS1	O
(	O
,	O
,	O
,	O
and	O
)	O
and	O
it	O
is	O
up	O
to	O
the	O
routing	B-Protocol
policy	O
of	O
AS2	O
to	O
decide	O
whether	O
or	O
not	O
to	O
take	O
a	O
copy	O
of	O
the	O
four	O
routes	O
or	O
,	O
as	O
overlaps	O
all	O
the	O
other	O
specific	O
routes	O
,	O
to	O
just	O
store	O
the	O
summary	O
,	O
.	O
</s>
<s>
If	O
AS2	O
wants	O
to	O
send	O
data	O
to	O
prefix	O
,	O
it	O
will	O
be	O
sent	O
to	O
the	O
routers	B-Protocol
of	O
AS1	O
on	O
route	O
.	O
</s>
<s>
At	O
AS1	O
,	O
it	O
will	O
either	O
be	O
dropped	O
or	O
a	O
destination	O
unreachable	O
ICMP	B-Protocol
message	O
will	O
be	O
sent	O
back	O
,	O
depending	O
on	O
the	O
configuration	O
of	O
AS1	O
's	O
routers	B-Protocol
.	O
</s>
<s>
Depending	O
on	O
the	O
routing	B-Protocol
policy	O
of	O
AS2	O
,	O
it	O
will	O
store	O
a	O
copy	O
of	O
the	O
three	O
routes	O
,	O
or	O
aggregate	O
and	O
to	O
,	O
thereby	O
reducing	O
the	O
number	O
of	O
routes	O
AS2	O
stores	O
to	O
two	O
(	O
and	O
)	O
.	O
</s>
<s>
If	O
AS2	O
now	O
wants	O
to	O
send	O
data	O
to	O
prefix	O
,	O
it	O
will	O
be	O
dropped	O
or	O
a	O
destination	O
unreachable	O
ICMP	B-Protocol
message	O
will	O
be	O
sent	O
back	O
at	O
the	O
routers	B-Protocol
of	O
AS2	O
(	O
not	O
AS1	O
as	O
before	O
)	O
,	O
because	O
is	O
not	O
in	O
the	O
routing	B-Protocol
table	I-Protocol
.	O
</s>
<s>
The	O
RFC	O
1771	O
(	O
A	O
Border	B-Protocol
Gateway	I-Protocol
Protocol	I-Protocol
4	O
(	O
BGP-4	O
)	O
)	O
planned	O
the	O
coding	O
of	O
AS	B-Architecture
numbers	I-Architecture
on	O
16	O
bits	O
,	O
for	O
64,510	O
possible	O
public	O
AS	O
,	O
since	O
ASN	O
64512	O
to	O
65534	O
were	O
reserved	O
for	O
private	O
use	O
(	O
0	O
and	O
65535	O
being	O
forbidden	O
)	O
.	O
</s>
<s>
In	O
2011	O
,	O
only	O
15,000	O
AS	B-Architecture
numbers	I-Architecture
were	O
still	O
available	O
,	O
and	O
projections	O
were	O
envisioning	O
a	O
complete	O
depletion	O
of	O
available	O
AS	B-Architecture
numbers	I-Architecture
in	O
September	O
2013	O
.	O
</s>
<s>
RFC	O
6793	O
extends	O
AS	O
coding	O
from	O
16	O
to	O
32	O
bits	O
(	O
keeping	O
the	O
16-bit	O
AS	O
range	O
0	O
to	O
65535	O
,	O
and	O
its	O
reserved	O
AS	B-Architecture
numbers	I-Architecture
)	O
,	O
which	O
now	O
allows	O
up	O
to	O
4	O
billion	O
available	O
AS	O
.	O
</s>
<s>
To	O
allow	O
the	O
traversal	O
of	O
router	B-Protocol
groups	O
not	O
able	O
to	O
manage	O
those	O
new	O
ASNs	O
,	O
the	O
new	O
attribute	O
AS4_PATH	O
(	O
optional	O
transitive	O
)	O
is	O
used	O
.	O
</s>
<s>
Another	O
factor	O
causing	O
this	O
growth	O
of	O
the	O
routing	B-Protocol
table	I-Protocol
is	O
the	O
need	O
for	O
load	O
balancing	O
of	O
multi-homed	B-Protocol
networks	I-Protocol
.	O
</s>
<s>
It	O
is	O
not	O
a	O
trivial	O
task	O
to	O
balance	O
the	O
inbound	O
traffic	O
to	O
a	O
multi-homed	B-Protocol
network	I-Protocol
across	O
its	O
multiple	O
inbound	O
paths	O
,	O
due	O
to	O
limitation	O
of	O
the	O
BGP	B-Protocol
route	O
selection	O
process	O
.	O
</s>
<s>
For	O
a	O
multi-homed	B-Protocol
network	I-Protocol
,	O
if	O
it	O
announces	O
the	O
same	O
network	O
blocks	O
across	O
all	O
of	O
its	O
BGP	B-Protocol
peers	O
,	O
the	O
result	O
may	O
be	O
that	O
one	O
or	O
several	O
of	O
its	O
inbound	O
links	O
become	O
congested	O
while	O
the	O
other	O
links	O
remain	O
under-utilized	O
,	O
because	O
external	O
networks	O
all	O
picked	O
that	O
set	O
of	O
congested	O
paths	O
as	O
optimal	O
.	O
</s>
<s>
Like	O
most	O
other	O
routing	B-Protocol
protocols	O
,	O
BGP	B-Protocol
does	O
not	O
detect	O
congestion	O
.	O
</s>
<s>
To	O
work	O
around	O
this	O
problem	O
,	O
BGP	B-Protocol
administrators	O
of	O
that	O
multihomed	B-Protocol
network	O
may	O
divide	O
a	O
large	O
contiguous	O
IP	B-Protocol
address	O
block	O
into	O
smaller	O
blocks	O
and	O
tweak	O
the	O
route	O
announcement	O
to	O
make	O
different	O
blocks	O
look	O
optimal	O
on	O
different	O
paths	O
,	O
so	O
that	O
external	O
networks	O
will	O
choose	O
a	O
different	O
path	O
to	O
reach	O
different	O
blocks	O
of	O
that	O
multi-homed	B-Protocol
network	I-Protocol
.	O
</s>
<s>
Such	O
cases	O
will	O
increase	O
the	O
number	O
of	O
routes	O
as	O
seen	O
on	O
the	O
global	O
BGP	B-Protocol
table	O
.	O
</s>
<s>
One	O
method	O
growing	O
in	O
popularity	O
to	O
address	O
the	O
load	O
balancing	O
issue	O
is	O
to	O
deploy	O
BGP/LISP	O
(	O
Locator/Identifier	B-Protocol
Separation	I-Protocol
Protocol	I-Protocol
)	O
gateways	O
within	O
an	O
Internet	B-Protocol
exchange	I-Protocol
point	I-Protocol
to	O
allow	O
ingress	O
traffic	O
engineering	O
across	O
multiple	O
links	O
.	O
</s>
<s>
This	O
technique	O
does	O
not	O
increase	O
the	O
number	O
of	O
routes	O
seen	O
on	O
the	O
global	O
BGP	B-Protocol
table	O
.	O
</s>
<s>
By	O
design	O
,	O
routers	B-Protocol
running	O
BGP	B-Protocol
accept	O
advertised	O
routes	O
from	O
other	O
BGP	B-Protocol
routers	B-Protocol
by	O
default	O
.	O
</s>
<s>
This	O
allows	O
for	O
automatic	O
and	O
decentralized	O
routing	B-Protocol
of	O
traffic	O
across	O
the	O
Internet	O
,	O
but	O
it	O
also	O
leaves	O
the	O
Internet	O
potentially	O
vulnerable	O
to	O
accidental	O
or	O
malicious	O
disruption	O
,	O
known	O
as	O
BGP	B-Protocol
hijacking	I-Protocol
.	O
</s>
<s>
Due	O
to	O
the	O
extent	O
to	O
which	O
BGP	B-Protocol
is	O
embedded	O
in	O
the	O
core	O
systems	O
of	O
the	O
Internet	O
,	O
and	O
the	O
number	O
of	O
different	O
networks	O
operated	O
by	O
many	O
different	O
organizations	O
which	O
collectively	O
make	O
up	O
the	O
Internet	O
,	O
correcting	O
this	O
vulnerability	O
(	O
such	O
as	O
by	O
introducing	O
the	O
use	O
of	O
cryptographic	O
keys	O
to	O
verify	O
the	O
identity	O
of	O
BGP	B-Protocol
routers	B-Protocol
)	O
is	O
a	O
technically	O
and	O
economically	O
challenging	O
problem	O
.	O
</s>
<s>
An	O
extension	O
to	O
BGP	B-Protocol
is	O
the	O
use	O
of	O
multipathing	O
this	O
typically	O
requires	O
identical	O
MED	O
,	O
weight	O
,	O
origin	O
,	O
and	O
AS-path	O
although	O
some	O
implementations	O
provide	O
the	O
ability	O
to	O
relax	O
the	O
AS-path	O
checking	O
to	O
only	O
expect	O
an	O
equal	O
path	O
length	O
rather	O
than	O
the	O
actual	O
AS	B-Architecture
numbers	I-Architecture
in	O
the	O
path	O
being	O
expected	O
to	O
match	O
too	O
.	O
</s>
<s>
Multiprotocol	B-Protocol
Extensions	I-Protocol
for	I-Protocol
BGP	I-Protocol
(	O
MBGP	B-Protocol
)	O
,	O
sometimes	O
referred	O
to	O
as	O
Multiprotocol	B-Protocol
BGP	I-Protocol
or	O
Multicast	B-Protocol
BGP	I-Protocol
and	O
defined	O
in	O
IETF	O
RFC	O
4760	O
,	O
is	O
an	O
extension	O
to	O
(	O
BGP	B-Protocol
)	O
that	O
allows	O
different	O
types	O
of	O
addresses	O
(	O
known	O
as	O
address	O
families	O
)	O
to	O
be	O
distributed	O
in	O
parallel	O
.	O
</s>
<s>
Whereas	O
standard	O
BGP	B-Protocol
supports	O
only	O
IPv4	B-Protocol
unicast	O
addresses	O
,	O
Multiprotocol	B-Protocol
BGP	I-Protocol
supports	O
IPv4	B-Protocol
and	O
IPv6	B-Protocol
addresses	O
and	O
it	O
supports	O
unicast	O
and	O
multicast	O
variants	O
of	O
each	O
.	O
</s>
<s>
Multiprotocol	B-Protocol
BGP	I-Protocol
allows	O
information	O
about	O
the	O
topology	B-Architecture
of	O
IP	B-Protocol
multicast-capable	O
routers	B-Protocol
to	O
be	O
exchanged	O
separately	O
from	O
the	O
topology	B-Architecture
of	O
normal	O
IPv4	B-Protocol
unicast	O
routers	B-Protocol
.	O
</s>
<s>
Thus	O
,	O
it	O
allows	O
a	O
multicast	O
routing	B-Protocol
topology	B-Architecture
different	O
from	O
the	O
unicast	O
routing	B-Protocol
topology	B-Architecture
.	O
</s>
<s>
Although	O
MBGP	B-Protocol
enables	O
the	O
exchange	O
of	O
inter-domain	O
multicast	O
routing	B-Protocol
information	I-Protocol
,	O
other	O
protocols	O
such	O
as	O
the	O
Protocol	O
Independent	O
Multicast	O
family	O
are	O
needed	O
to	O
build	O
trees	O
and	O
forward	O
multicast	O
traffic	O
.	O
</s>
<s>
Multiprotocol	B-Protocol
BGP	I-Protocol
is	O
also	O
widely	O
deployed	O
in	O
case	O
of	O
MPLS	B-Protocol
L3	O
VPN	B-Application
,	O
to	O
exchange	O
VPN	B-Application
labels	O
learned	O
for	O
the	O
routes	O
from	O
the	O
customer	O
sites	O
over	O
the	O
MPLS	B-Protocol
network	O
,	O
in	O
order	O
to	O
distinguish	O
between	O
different	O
customer	O
sites	O
when	O
the	O
traffic	O
from	O
the	O
other	O
customer	O
sites	O
comes	O
to	O
the	O
Provider	O
Edge	B-Protocol
router	I-Protocol
(	O
PE	O
router	B-Protocol
)	O
for	O
routing	B-Protocol
.	O
</s>
<s>
BGP4	B-Protocol
is	O
standard	O
for	O
Internet	O
routing	B-Protocol
and	O
required	O
of	O
most	O
Internet	O
service	O
providers	O
(	O
ISPs	O
)	O
to	O
establish	O
routing	B-Protocol
between	O
one	O
another	O
.	O
</s>
<s>
Very	O
large	O
private	B-Protocol
IP	I-Protocol
networks	O
use	O
BGP	B-Protocol
internally	O
.	O
</s>
<s>
An	O
example	O
is	O
the	O
joining	O
of	O
a	O
number	O
of	O
large	O
Open	B-Protocol
Shortest	I-Protocol
Path	I-Protocol
First	I-Protocol
(	O
OSPF	B-Protocol
)	O
networks	O
,	O
when	O
OSPF	B-Protocol
by	O
itself	O
does	O
not	O
scale	O
to	O
the	O
size	O
required	O
.	O
</s>
<s>
Another	O
reason	O
to	O
use	O
BGP	B-Protocol
is	O
multihoming	B-Protocol
a	O
network	O
for	O
better	O
redundancy	O
,	O
either	O
to	O
multiple	O
access	O
points	O
of	O
a	O
single	O
ISP	O
or	O
to	O
multiple	O
ISPs	O
.	O
</s>
<s>
Routers	B-Protocol
,	O
especially	O
small	O
ones	O
intended	O
for	O
small	O
office/home	O
office	O
(	O
SOHO	O
)	O
use	O
,	O
may	O
not	O
include	O
BGP	B-Protocol
software	O
.	O
</s>
<s>
Some	O
SOHO	O
routers	B-Protocol
simply	O
are	O
not	O
capable	O
of	O
running	O
BGP	B-Protocol
or	O
using	O
BGP	B-Protocol
routing	B-Protocol
tables	I-Protocol
of	O
any	O
size	O
.	O
</s>
<s>
Other	O
commercial	O
routers	B-Protocol
may	O
need	O
a	O
specific	O
software	O
executable	O
image	O
that	O
contains	O
BGP	B-Protocol
,	O
or	O
a	O
license	O
that	O
enables	O
it	O
.	O
</s>
<s>
Devices	O
marketed	O
as	O
Layer	O
3	O
switches	O
are	O
less	O
likely	O
to	O
support	O
BGP	B-Protocol
than	O
devices	O
marketed	O
as	O
routers	B-Protocol
,	O
but	O
high-end	O
Layer	O
3	O
switches	O
usually	O
can	O
run	O
BGP	B-Protocol
.	O
</s>
<s>
Products	O
marketed	O
as	O
switches	O
may	O
or	O
may	O
not	O
have	O
a	O
size	O
limitation	O
on	O
BGP	B-Protocol
tables	O
,	O
such	O
as	O
20,000	O
routes	O
,	O
far	O
smaller	O
than	O
a	O
full	O
Internet	O
table	O
plus	O
internal	O
routes	O
.	O
</s>
<s>
These	O
devices	O
may	O
be	O
perfectly	O
reasonable	O
and	O
useful	O
when	O
used	O
for	O
BGP	B-Protocol
routing	B-Protocol
of	O
some	O
smaller	O
part	O
of	O
the	O
network	O
,	O
such	O
as	O
a	O
confederation-AS	O
representing	O
one	O
of	O
several	O
smaller	O
enterprises	O
that	O
are	O
linked	O
,	O
by	O
a	O
BGP	B-Protocol
backbone	O
of	O
backbones	O
,	O
or	O
a	O
small	O
enterprise	O
that	O
announces	O
routes	O
to	O
an	O
ISP	O
but	O
only	O
accepts	O
a	O
default	B-Protocol
route	I-Protocol
and	O
perhaps	O
a	O
small	O
number	O
of	O
aggregated	O
routes	O
.	O
</s>
<s>
A	O
BGP	B-Protocol
router	B-Protocol
used	O
only	O
for	O
a	O
network	O
with	O
a	O
single	O
point	O
of	O
entry	O
to	O
the	O
Internet	O
may	O
have	O
a	O
much	O
smaller	O
routing	B-Protocol
table	I-Protocol
size	O
(	O
and	O
hence	O
RAM	O
and	O
CPU	O
requirement	O
)	O
than	O
a	O
multihomed	B-Protocol
network	O
.	O
</s>
<s>
Even	O
simple	O
multihoming	B-Protocol
can	O
have	O
modest	O
routing	B-Protocol
table	I-Protocol
size	O
.	O
</s>
<s>
See	O
RFC	O
4098	O
for	O
vendor-independent	O
performance	O
parameters	O
for	O
single-BGP-router	O
convergence	O
in	O
the	O
control	O
plane	O
.	O
</s>
<s>
The	O
actual	O
amount	O
of	O
memory	O
required	O
in	O
a	O
BGP	B-Protocol
router	B-Protocol
depends	O
on	O
the	O
amount	O
of	O
BGP	B-Protocol
information	O
exchanged	O
with	O
other	O
BGP	B-Protocol
speakers	O
and	O
the	O
way	O
in	O
which	O
the	O
particular	O
router	B-Protocol
stores	O
BGP	B-Protocol
information	O
.	O
</s>
<s>
The	O
router	B-Protocol
may	O
have	O
to	O
keep	O
more	O
than	O
one	O
copy	O
of	O
a	O
route	O
,	O
so	O
it	O
can	O
manage	O
different	O
policies	O
for	O
route	O
advertising	O
and	O
acceptance	O
to	O
a	O
specific	O
neighboring	O
AS	O
.	O
</s>
<s>
The	O
term	O
view	O
is	O
often	O
used	O
for	O
these	O
different	O
policy	O
relationships	O
on	O
a	O
running	O
router	B-Protocol
.	O
</s>
<s>
If	O
one	O
router	B-Protocol
implementation	O
takes	O
more	O
memory	O
per	O
route	O
than	O
another	O
implementation	O
,	O
this	O
may	O
be	O
a	O
legitimate	O
design	O
choice	O
,	O
trading	O
processing	O
speed	O
against	O
memory	O
.	O
</s>
<s>
A	O
full	O
IPv4	B-Protocol
BGP	B-Protocol
table	O
is	O
in	O
excess	O
of	O
590,000	O
prefixes	O
.	O
</s>
<s>
Notable	O
free	O
and	O
open-source	B-Application
implementations	O
of	O
BGP	B-Protocol
include	O
:	O
</s>
<s>
BIRD	B-Protocol
,	O
a	O
GPL	B-License
routing	B-Protocol
package	O
for	O
Unix-like	B-Operating_System
systems	I-Operating_System
.	O
</s>
<s>
FRRouting	B-Protocol
,	O
a	O
fork	O
of	O
Quagga	B-Protocol
for	O
Unix-like	B-Operating_System
systems	I-Operating_System
;	O
and	O
its	O
ancestors	O
:	O
</s>
<s>
Quagga	B-Protocol
,	O
a	O
fork	O
of	O
GNU	B-Application
Zebra	I-Application
for	O
Unix-like	B-Operating_System
systems	I-Operating_System
(	O
no	O
longer	O
developed	O
)	O
.	O
</s>
<s>
GNU	B-Application
Zebra	I-Application
,	O
a	O
GPL	B-License
routing	B-Protocol
suite	O
supporting	O
BGP4	B-Protocol
(	O
decommissioned	O
)	O
.	O
</s>
<s>
OpenBGPD	B-Application
,	O
a	O
BSD-licensed	B-Operating_System
implementation	O
by	O
the	O
OpenBSD	B-Operating_System
team	O
.	O
</s>
<s>
XORP	B-Protocol
,	O
the	O
eXtensible	O
Open	O
Router	B-Protocol
Platform	O
,	O
a	O
BSD-licensed	B-Operating_System
suite	O
of	O
routing	B-Protocol
protocols	O
.	O
</s>
<s>
Systems	O
for	O
testing	O
BGP	B-Protocol
conformance	O
,	O
load	O
or	O
stress	O
performance	O
come	O
from	O
vendors	O
such	O
as	O
:	O
</s>
