When a BGP router is not capable of understanding 4-byte AS numbers, it will see 4-byte AS
numbers as a special, reserved, 2-byte AS number in the AS path. Which 2-byte AS number is this
reserved one?
A.
00000
B.
12345
C.
23456
D.
65000
E.
99999
Explanation:
What’s New with 4-byte AS Number
The new AS number is 4-bytes and split into two 2-byte values, in X.Y syntax. The support for the
4-byte AS is advertised via BGP capability negotiation. In order to ensure interoperability with
existing BGP peers that do not support 4-byte AS, encoding of BGP OPEN message is reserved
and 4-byte AS support is exchanged between the BGP peers via the capability field.
In this whitepaper, we will refer to the BGP speaker that supports 4-byte AS as NEW speaker, and
the BGP speaker that does not support 4-byte AS as OLD speaker.
When BGP attempts to establish a session with its peer, the OPEN message may include an
optional parameter, called Capabilities. A NEW speaker will include the NEW (4-byte AS)
capability when it attempts to OPEN a session with its peer. An OLD speaker should simply ignore
the NEW capability advertised by its peer and continue to operate in OLD mode, as detailed in
RFC 3392.
If the NEW speaker advertises and receives the 4-byte AS capability from its peer, it will just
encode the 4-byte AS number in its AS_PATH or AGGREGATOR attributes when exchanging
information with this peer.
If the NEW speaker does not receive the 4-byte AS capability from a particular peer, it indicates
this peer is an OLD speaker. Two new attributes are introduced, namely AS4_PATH and
AS4_AGGREGATOR. Both attributes are optional transitive. These new attributes use the same
encoding as the original ASPATH and AGGREGATOR except the AS Number used is 4-bytes
instead of 2-bytes. The NEW speaker will substitute a reserved 2-byte AS number (called
AS_TRANS with AS # 23456) for each 4-byte AS so that ASPATH and AGGREGATOR is still 2-
byte in length and ASPATH length is still preserved, and at the same time insert the
new AS4_PATH and AS4_AGGREGATOR, which will contain the 4-byte encoded copy of the
attributes. The NEW speaker will then advertise ASPATH and/or AGGREGATOR together with the
AS4_PATH and/or AS4_AGGREGATOR. The OLD speaker that receives these new attributes will
preserve and blindly pass them along even though it does not understand them.http://www.cisco.com/web/about/security/intelligence/4byte-as.html