Which reserved 2-byte AS number is seen in the AS path when a BGP router is not capable of
understanding 4-byte AS numbers?

A.
00000

B.
12345

C.
23456

D.
65000

E.
99999

Explanation:

When BGP was under development, it was assumed that a 16 bit (2 bytes) number for the
Autonomous System (AS) was more than enough (65536 AS Numbers), but the same situation as
with IPv4 happened, we are running out of AS numbers “basically” at the same speed as IPv4
addresses. Strategies where put in place to keep the business rolling, like reusing old orphan AS
numbers, but something drastically had to be done. The solution was to create a bigger AS
number space with 32 bits (4 bytes) that provides a pool of 4, 294, 967, 296 BGP AS numbers.
The problem with this new strategy of a bigger AS space is that Old BGP systems that only
support 2 bytes AS number would have problems to deal with this new design. Some kind of
backward compatibility implementation was needed. In this post we are going to discuss the
details.
The 4 byte AS implementation and compatibility design is built around the following features:
New capability for the 32 bit (4 byte) AS numbers
New optional transitive attributes (AS4_PATH and AS4_AGGREGATOR)
New extended communities
A well known transit AS number “23456 to represent 4 bytes AS numbers to old BGP devices that
don’t understand the 4 byte as.