Each Autonomous System (AS) must have its own unique number. Because the 2-byte AS numbering scheme is unable to meet the increasing demand, the switch supports 4-byte AS numbers. This feature is enabled by supporting RFC 4893, BGP Support for 4-octet AS Number Space.
The switch supports the following three types of peer relationships as a result of 4 byte AS support:
Old peer to old peer
Old peer to new peer
New peer to new peer
An old peer is the one that supports 2–byte AS numbers only and new peer is the one that supports both 2–byte AS numbers and 4–byte AS numbers.
RFC4893 supports two new path attributes:
AS4_PATH contains the AS path encoded with a 4-octet AS number.
AS4-AGGR is a new aggregator attribute that carries a 4-octet AS number.
The preceding figure shows an example of how the switch uses the AS4_PATH attribute in a mixed environment. The figure illustrates how a 2-byte BGP speaker interoperates with a 4-byte BGP speaker.
Router B is a 2-byte BGP speaker. Router A substitutes AS_PATH with the AS_TRANS, a 2-octet AS number defined by RFC4893 for backward compatibility, and encodes the 4-byte AS into AS4_PATH in BGP updates it sends to router B.
Router B does not understand the AS4_PATH but does preserve the information and sends it to router C.
Router C is a 4-byte BGP speaker. Router C merges the information received in AS_PATH and AS4_PATH, and encodes the 4-byte AS when it sends the AS_PATH information to router D.
When the peer relationship between an old peer and another old peer is established, 4 byte AS numbers contained in the AS4_PATH and AS4_AGGREGATOR are transited to other peers.
Important
Do not assign 23456 as an AS number. The Internet Assigned Numbers Authority (IANA) reserved this number for the AS_TRANS attribute and BGP uses it to facilitate communication between peer modes. AS_TRANS uses a 2-byte AS format to represent a 4-byte AS number. The switch interprets the AS_TRANS attribute and propagates it to other peers.
The new BGP speaker establishes its 4 byte AS support through BGP capability advertisement. A BGP speaker that announces such capability and receives it from its peer, uses 4 byte AS numbers in AS_PATH and AGGREGATOR attributes and assumes these attributes received from its peer are encoded in 4 byte AS numbers.
The new BGP attributes AS4_PATH and AS4_AGGREGATOR received from the new BGP speaker between the new BGP peers in the update message is discarded.
An old BGP speaker and a new BGP speaker can form peering relationship only if the new BGP speaker is assigned a 2 byte AS number. This 2 byte number can be any global unique AS number or AS_TRANS.
New BGP speaker sends AS path information to the old BGP speaker in AS_PATH attribute as well as AS4_PATH attribute. If the entire AS_PATH consists of only 2 byte AS numbers then the new BGP speaker does not send AS4_PATH information.
The 4-byte AS number feature does not in any way restrict the use or change the way you configure 2-byte AS numbers. You can also configure 2-byte AS or 4 byte AS numbers in AS path lists, community lists, and route policies.
BGP 4–byte AS numbers are represented in two ways: AS Plain and AS dot. The default form of representing the AS numbers is AS Plain while you have an option to configure AS dot. AS Plain form of representation is preferred over AS dot representation as a large amount of network providers find the AS dot notation incompatible with the regular expressions used by them. In case of any issues, troubleshooting and analyzing also gets difficult with AS dot notation.
|
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 |
|
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 |
For more information on configuring 4 byte AS numbers, see Configure 4-byte AS numbers.