Prior to January
2009, BGP autonomous system numbers that were allocated to companies were
2-octet numbers in the range from 1 to 65535 as described in RFC 4271,
A Border Gateway
Protocol 4 (BGP-4). Due to increased demand for autonomous system numbers,
the Internet Assigned Number Authority (IANA) started in January 2009 will
allocate four-octet autonomous system numbers in the range from 65536 to
4294967295. RFC 5396,
Textual
Representation of Autonomous System (AS) Numbers, documents three methods
of representing autonomous system numbers. Cisco has implemented the following
two methods:
-
Asplain—Decimal
value notation where both 2-byte and 4-byte autonomous system numbers are
represented by their decimal value. For example, 65526 is a 2-byte autonomous
system number and 234567 is a 4-byte autonomous system number.
-
Asdot—Autonomous
system dot notation where 2-byte autonomous system numbers are represented by
their decimal value and 4-byte autonomous system numbers are represented by a
dot notation. For example, 65526 is a 2-byte autonomous system number and
1.169031 is a 4-byte autonomous system number (this is dot notation for the
234567 decimal number).
For details about the
third method of representing autonomous system numbers, see RFC 5396.
Asdot Only Autonomous System
Number Formatting
In Cisco IOS XE
Release 2.3, the 4-octet (4-byte) autonomous system numbers are entered and
displayed only in asdot notation, for example, 1.10 or 45000.64000. When using
regular expressions to match 4-byte autonomous system numbers the asdot format
includes a period, which is a special character in regular expressions. A
backslash must be entered before the period (for example, 1\.14) to ensure the
regular expression match does not fail. The table below shows the format in
which 2-byte and 4-byte autonomous system numbers are configured, matched in
regular expressions, and displayed in
show command
output in Cisco IOS images where only asdot formatting is available.
Table 1. Asdot Only 4-Byte Autonomous
System Number Format
Format
|
Configuration Format
|
Show
Command Output and Regular Expression Match Format
|
asdot
|
2-byte: 1
to 65535 4-byte: 1.0 to 65535.65535
|
2-byte: 1
to 65535 4-byte: 1.0 to 65535.65535
|
Asplain as Default Autonomous
System Number Formatting
In Cisco IOS XE
Release 2.4 and later releases, the Cisco implementation of 4-byte autonomous
system numbers uses asplain as the default display format for autonomous system
numbers, but you can configure 4-byte autonomous system numbers in both the
asplain and asdot format. In addition, the default format for matching 4-byte
autonomous system numbers in regular expressions is asplain, so you must ensure
that any regular expressions to match 4-byte autonomous system numbers are
written in the asplain format. If you want to change the default
show command
output to display 4-byte autonomous system numbers in the asdot format, use the
bgp
asnotation
dot command under router configuration mode. When
the asdot format is enabled as the default, any regular expressions to match
4-byte autonomous system numbers must be written using the asdot format, or the
regular expression match will fail. The tables below show that although you can
configure 4-byte autonomous system numbers in either asplain or asdot format,
only one format is used to display
show command
output and control 4-byte autonomous system number matching for regular
expressions, and the default is asplain format. To display 4-byte autonomous
system numbers in
show command
output and to control matching for regular expressions in the asdot format, you
must configure the
bgp
asnotation
dot command. After enabling the
bgp
asnotation
dot command, a hard reset must be initiated for
all BGP sessions by entering the
clear
ip
bgp
* command.
Note |
If you are
upgrading to an image that supports 4-byte autonomous system numbers, you can
still use 2-byte autonomous system numbers. The
show command
output and regular expression match are not changed and remain in asplain
(decimal value) format for 2-byte autonomous system numbers regardless of the
format configured for 4-byte autonomous system numbers.
|
Table 2. Default Asplain 4-Byte
Autonomous System Number Format
Format
|
Configuration Format
|
Show
Command Output and Regular Expression Match Format
|
asplain
|
2-byte: 1
to 65535 4-byte: 65536 to 4294967295
|
2-byte: 1
to 65535 4-byte: 65536 to 4294967295
|
asdot
|
2-byte: 1
to 65535 4-byte: 1.0 to 65535.65535
|
2-byte: 1
to 65535 4-byte: 65536 to 4294967295
|
Table 3. Asdot 4-Byte Autonomous
System Number Format
Format
|
Configuration Format
|
Show
Command Output and Regular Expression Match Format
|
asplain
|
2-byte: 1
to 65535 4-byte: 65536 to 4294967295
|
2-byte: 1
to 65535 4-byte: 1.0 to 65535.65535
|
asdot
|
2-byte: 1
to 65535 4-byte: 1.0 to 65535.65535
|
2-byte: 1
to 65535 4-byte: 1.0 to 65535.65535
|
Reserved and Private
Autonomous System Numbers
In Cisco IOS XE
Release 2.3 and later releases, the Cisco implementation of BGP supports RFC
4893. RFC 4893 was developed to allow BGP to support a gradual transition from
2-byte autonomous system numbers to 4-byte autonomous system numbers. A new
reserved (private) autonomous system number, 23456, was created by RFC 4893 and
this number cannot be configured as an autonomous system number in the Cisco
IOS CLI.
RFC 5398,
Autonomous
System (AS) Number Reservation for Documentation Use, describes new
reserved autonomous system numbers for documentation purposes. Use of the
reserved numbers allow configuration examples to be accurately documented and
avoids conflict with production networks if these configurations are literally
copied. The reserved numbers are documented in the IANA autonomous system
number registry. Reserved 2-byte autonomous system numbers are in the
contiguous block, 64496 to 64511 and reserved 4-byte autonomous system numbers
are from 65536 to 65551 inclusive.
Private 2-byte
autonomous system numbers are still valid in the range from 64512 to 65534 with
65535 being reserved for special use. Private autonomous system numbers can be
used for internal routing domains but must be translated for traffic that is
routed out to the Internet. BGP should not be configured to advertise private
autonomous system numbers to external networks. Cisco IOS software does not
remove private autonomous system numbers from routing updates by default. We
recommend that ISPs filter private autonomous system numbers.
Note |
Autonomous
system number assignment for public and private networks is governed by the
IANA. For information about autonomous-system numbers, including reserved
number assignment, or to apply to register an autonomous system number, see the
following URL: http://www.iana.org/.
|