4 Byte AS Number

You already know how IPv4 addresses are being depleted and how all Telecom Providers are looking at next Generation IP addressing scheme i.e. IPv6 for rescue. However there is one more resource which is depleting rapidly and that is AS Number (Autonomous System Number) or specifically 2 Byte AS Numbers.

As per official statement “An Autonomous System (AS) is a collection of connected Internet Protocol (IP) routing prefixes under the control of one or more network operators that presents a common, clearly defined routing policy to the Internet.” i.e. each Service provider or Enterprise network will have its own AS number where it can apply its own routing-policies and connect to other AS number using BGP (eBGP).

A 16-bit number (i.e. 2 Bytes) will give 65,536 possible numbers (2^16) (AS numbers 0 – 65535). Out of these, the IANA reserves 1,026 of them: 64512 – 65534 for private, reusable ASNs (similar to private RFC1918 IPv4 addresses) and a few others such as 0, 65535 and 23456. I will come back to 23456 AS number after short while. From total of 65536 ASs, around 63000 have already been allocated, 1026 are for private use and around 1500 are remaining for Public allocation. So you can estimate yourself, how much important is this resource and something needs to be done very quickly.

Fortunately, we have new 4 Byte AS number to rescue and this is the topic of my blog.

4-byte (32bit) AS Number provides 2^32 or 4,294,967,296 autonomous system numbers ranging from 0 to 4294967295. The first thing to notice about these numbers is that they include all of the older 2-byte ASNs, 0 through 65535. That greatly helps with interoperability between autonomous systems using 2-byte ASNs and those using 4-byte ASNs.

Now main thing about 4 Byte AS number is representation. How you will represent these lengthy AS Number in meaningful way (same like for IPv6 address we have some tricks). However unlike IPv6, AS number representation is not so much complex and easy to understand.

  1. asplain –> asplain is a simple decimal representation of the ASN, from 0 to 4294967295.
  2. asdot –> in asdot, any ASN in the 2-byte range i.e. between 0 – 65535 is written in asplain (so 65535 is written as “65535”) however any ASN above that range is written in different format. Suppose 65536 is ASN which you know is outside the range (0 – 65535) and it will be represented as 1.0. 65537 would be 1.1, 65680 is 1.144, and so on. So if you guessed it, basically what we are doing is subtracting multiples of 65,536 from the asplain representation of the ASN, with the high-order value representing the multiples of 65536. 
  3. So 134576 can be represented as 2.3504 because 134576 = 2*65536 + 3504

HDFC Bank in India has one 4 Byte AS number allocated to it and it is:

AS131283 –> HDFC Bank

I hope you know that in BGP, AS number is used to determine the shortest path to the destination and also as a loop avoidance mechanism. So how these new AS Number notation works in environment where both types of AS number exists i.e. 2 byte and 4 byte

Ok, so let’s define the BGP implementations supporting 4-byte ASNs as BGP-New, and legacy BGP implementations that only support 2-byte ASNs as BGP-Old.

The first requirement for a BGP-New implementation is to discover whether a neighbor is BGP-New or BGP-Old. It does this by using the BGP Capability Advertisement when starting a BGP session. In addition to advertising itself as BGP-New, it includes its 4-byte ASN in the Capability advertisement.

If a neighbor responds that it also is a BGP-NEW speaker, the neighbor includes its 4-byte ASN in its own Capability advertisement. Thus two BGP-New neighbors can inform each other of their 4-byte ASNs without using the 2-byte Autonomous System field in the Open message.

If a neighbor is BGP-Old, it either responds that it does not support the 4-byte ASN capability or does not respond to the Capability advertisement at all. In this case, the BGP-New neighbor can still bring up a session with the BGP-Old neighbor, but cannot advertise its 4-byte ASN. The neighbor wouldn’t understand it. Instead, BGP-New uses a reserved 2-byte ASN which I defined earlier i.e. 23456, called AS_TRANS. Router which is configured for 4 byte number will send the BGP Open message with 23456 AS Number so that neighbor Router can understand it. Because AS_TRANS is reserved, no BGP-Old speaker can use it as its own ASN; only BGP-New speakers can use it.

Interoperable peering, then, is achieved because the BGP-New speaker “knows” its neighbor is a BGP-Old speaker and adapts to it; the BGP-Old speaker simply continues using legacy BGP rules.

Cisco has started to include this functionality from IOS-XR 3.4 and Juniper Network has included this from Junos 9.1.

There is much more to 4 byte AS but I hope you will get some idea from this blog  🙂


Mohit Mittal


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